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Starting with the BBC in 1941 - Reminiscences about Daventry

by Don Bowman

Previously published in Contact, the World DX Club magazine, in August 1997.

Published on
March 2007
with help from Jeff Cant.
Colour pictures below by Russell Barnes.

Don Bowman


'H' Group Transmitters

Maida Vale, then Daventry


Senders 1 & 2

Senders 4 and 5 in the immediate post war years

Aerials in the immediate post war years

The ‘DAVENTRY Pattern’ - a study in RF Interference

Senders 8, 9, 10 & 11 - ‘Boy’s Town’

Senders 6 and 7

Memories of GS Engineers

T3, the Third Programme transmitters

Re-engineering in the 1960s

Closing Down 1992

List of important dates
Questions and answers related to the above article

'H' Group Transmitters

On July 13th 1941, I joined the BBC at Sheffield ‘H’ Group transmitter at Manor Lane School, or what was left of the school after the blitz the previous November, with the grand title of ‘Youth Transmitters’. The transmitter was located in the former cloak room of the abandoned school where the roof had been reinforced and for an air raid shelter we had a steel ‘Bell Shelter’ standing outside. The mast for the aerial was stuck in the middle of the playground. The transmitter, as I remember, was designed by a man called Webb and had an output of 100 watts, sufficient power to reach all-round the city. The drive unit was the old faithful CP17E using a ‘Post Office’ crystal, the operating frequency was 1474 KHz or 203.5 metres.

I am told that there were 60 ‘H’ group stations throughout the country and they radiated the Home Service. One of these ‘H’ Group stations in Reading was hit by a bomb during the war. As lads our main duty was monitoring the programme and conferring with ‘Control’ in Manchester by private telephone line if there were are clicks or interruptions to the programme. We also were required to regularly contact the local police in order that they knew we were all right.

The idea of the ‘H’ groups was, in war time, to provide communication to the people in the large cities in the event of an invasion or the appearance of German Paratroops. So each station had a microphone and some means of creating a makeshift studio so that instructions could be broadcast by well known local people or politicians should the need arise. I remember one night testing the microphone as we were required to do but failed to disconnect a feed to a Radio Relay Company which was used to distribute BBC programmes by wire. Perhaps all the subscribers had gone to bed as there were no repercussions afterwards.

The BBC management were very keen that we should be proficient in morse code, both sending and receiving, up to 25 words per minute. We had a gramophone and morse practice recordings which we had to use frequently. The shift hours were long, within a few days of joining, I spent a week on night shift starting at 10.00 p.m. and ending at 9.00 a.m. plus a double tram journey on each end and seven, eleven hour long night shifts in a row! I still remember coming home after my first week at work and gave my mother about 30 shillings (£1.50p). She couldn’t believe they would pay a lad of sixteen years old all that money and she wanted to know where I had obtained it.

One incident for which I was very unpopular was having Diphtheria within a week or so of getting a ‘flu injection. I was in an isolation hospital for three weeks and both the house and the transmitting station had to be fumigated. In those days Diphtheria was still a serious illness. Within a year we were faced with interviews in Manchester to become Junior Transmitter Engineers (JME) and this meant the transfer to a large station. I suppose this was in an effort to replace staff from the large stations who had been called up into the armed forces.

Maida Vale, then Daventry

In June 1942, I was sent on a course at Maida Vale studios in London followed by a week’s course at Daventry. I have no memory of air attacks whilst we were in London so it was probably a quiet period of the war. During the Daventry based course we were billeted in Long Buckby and ferried daily the six miles to Daventry by bus.

At the end of the course my three colleagues, Peter Dyson, Ken Jackson and Alan Slater and myself (all ex-Sheffield Junior Technical School) were posted to Daventry transmitting station. At that time the Engineer in Charge was Douglas Birkinshaw who previously had been on the engineering staff at Alexandra Palace from where the pre-war Television had been broadcast. The Assistant Engineer in Charge (AEiC) was L. F. Ivin, who seven years later, was back at Daventry involved in Television Interference research. In those 1942 early days the station, on Borough Hill, had fields all the way down to the town. Across a railway line which ran from Northampton to Blisworth, via Weedon and Daventry to Leamington Spa, a railway line which no longer exists.

I worked on the two short wave Senders 1 & 2 as I can clearly recall taking my oral TA1 examination, during the war, on Senders 1 & 2 with D.C. Birkinshaw. By the way, the Borough Hill site was bought in 1925 for £2,670.


I spent quite a lot of the next two years on shift on the old 1925 5XX transmitter. My contact with 5XX started in 1942 when the 17 year old 25 kW transmitter was in use once more, transmitting to Europe on I500 metres. 5XX had the distinction of being the first transmitter designed to transmit to a national audience, compared to the few low power 1kW local transmitters currently in use in major towns with a range of 25 miles. 5XX radiated the National programme, said to reach about 85% of the population.


The red brick building housing 5XX was not quite at the summit of Borough Hill, Daventry, and in those days a metal concentric feeder, which tended to arcle and sparkle a bit, ran above ground, except where it went under the concrete drive, and connected the RF output to an ATH (Aerial Tuning Hut) and thence to a ‘T’ aerial supported by two 500 foot masts. Memories of walking up to the ATH carrying the hut key on a large ring made of copper to take the two hourly ATH meter readings. Notwithstanding the size of the key ring I managed one day to get it into my pocket and inadvertently take it home.

5XX was very spread out and the transmitter works’ were surrounded by a protective wire netting fence about six feet high with an interlocked access gate. Photographs taken in the early days of 5XX showed only a waist high rail around the transmitter enclosure, showing safely wasn’t that important in 1925,

The Radio Frequency side consisted of one air cooled valve ACT9, followed by the Penultimate (Pen) RF stage using two Marconi MR9 valves, which glowed cherry red, and, for the Final RF stage, four Marconi water cooled CAT1 valves operating in parallel.

By the time I arrived, the tuning fork which had been used to control the carrier frequency, had been replaced by the CP17E crystal drive unit. There was a clever gadget in the drive room which received a very accurate 1000 cycle tone which was used to check our carrier frequency was the same as the ‘L’ Group of transmitters which included the very powerful Ottringham (Yorkshire) transmitters. Unlike operating a short wave transmitter tuning the 5XX transmitter called for the Senior Maintenance Engineer (SME) at least or even the Assistant Engineer in Charge (AEiC)! The Audio Frequency side ended up with eight water cooled Marconi CAM 1 (cooled anode modulator) valves in parallel.

The EHT dc supplies (11 kV) were derived initially from two motor alternators, each producing about 3000 volts at 300 cycles, then via oil filled step up transformers to water cooled CAR2 valve rectifiers. EHT smoothing used oil filled chokes and a line of 0.1 mfd condensers in pot jars connected to a copper HT rail by thin wire fuses. Touching a condenser terminal where the fuse wire was broken could be a very ‘moving’ experience. (Failure to use ‘earthing wands’ and getting a nasty shock comes to mind).

Motor-Generator Sets

Filament supplies were produced from two 10 kW motor generator sets with a third 25kW MG set producing auxiliary supplies. Starting the Motor Generator sets was a lengthy and very noisy operation usually performed about 5.30 a.m.. in preparation for 6.00 a.m. service. I recall the noisy job of ‘hand starting’ all the 5XX rotating machinery, the 10kW filament machines, then the 25kW and finally the two 70 kW machines for high tension. You were about deafened after 15 minutes in the MG room. Ringing in the ears afterwards was an understatement!

The control desk with red bulbs in glass domes on top as the only indication the ‘excitation’ on the machines had been established. Winding up the transmitter filaments on the rectifiers, valve filaments and other motor generator derived supplies kept one busy and hopeful that it would all work. Then during the war hearing the Trumpet Voluntary blaring out about 6.00 a.m. at the start of a transmission in German.

Valve cooling, compared to modern standards, was a bit of a joke, nevertheless very effective with good insulation figures. None of your rubber hose coils but instead spray units to insulate the valves using hard tap water!

Outside were two ponds, one with cold water the other one containing the hot water discharged from the valves. Cold water was pumped up to a large header tank inside the building from whence water flowed by gravity to the water cooled units. Each unit had a rectangular ‘top’ tank into which the water was sprayed, water then was admitted to the bottom of the valve jackets by rubber hoses. Warm water left the top of the valve jackets and via individual insulating sprays into a lower tank and thence out to the ‘hot’ pond. Finally water found its way from the hot to the cold ponds. One daily maintenance chore, which was carried out during an afternoon break in transmission, was dipping valve copper anodes in acid to remove hard tap water scale.

I don’t recall many breakdowns, generally once you had got 5XX going it stayed on the air the main tasks were recording meter readings and adjusting voltages following variations in the mains power supply. However I remember one day after the water header tank had been cleaned out, one by one steam issued from valve jackets as dirt, dislodged from the tank, had blocked the valve spray units. This involved taking the transmitter off the air several times in order to disconnect valve filaments!

Being in 5XX during a thunderstorm was a rather frightening experience, as well as the transmitter tripping, we often watched lightning dancing down the control panel. 5XX had its own electrician, maintenance and cleaning staff and a coke boiler which had to be tended correctly by the shift staff after office hours otherwise the night shift, in particular, would be cold.


During the war I got the nickname ‘Bobby’ from the 5XX staff, a name that stuck with me for many years. One drawback was the absence of a flush toilet just a smelly ‘Elsan’.

In wartime there was a shift of three, but one had to man an Air Ministry transmitter, known as AM1, located in an adjoining room. This spent its odd hours on the air as required by signals received in morse code from some unidentified place! On night shift we awaited the welcome signal ‘GNOM’ which was an acronym for ‘Good night old man’.

One plus, particularly on evening and night duty, was the ‘billiard room’ along the main entrance passage, where the Club snooker table resided. With the 5XX building being perched on top of a 600 foot high hill the lights from its windows could of course be seen at night. Quite often in the war, during hours of darkness, we were told to douse the lights at the time of an air raid warning. Sometimes we also received urgent messages to take the transmitter off the air to prevent its use by aircraft for direction finding. One night we lost the aerial due to a low flying aircraft clipping it with its gun turret (whose aircraft I don’t know) and we were unable to radiate our programme in German at 5.45a.m..

There is also the apocryphal story (during a period when paratroops were expected) that the military guard, consisting of ‘Red Caps’ - Corps of Military Police, whose guard hut was close to 5XX, turned out in force one night when loud voices were heard from the aerial field in German! Only to discover that it was a corona on a feeder carrying a German language broadcast!

One tale I must tell concerning the 5XX communal ‘Lilo’ we used during the war for a rest during a night shift. There wasn’t a pump so one had to blow it up. Unfortunately there was a small hole in it and if you didn’t drop off to sleep within a few minutes you found yourself on the hard concrete floor! Often one could spend the whole rest hour blowing it up. I never discovered why someone hadn’t the sense to mend the leak with a bike puncture outfit!

I also remember the guards stopping the oncoming shift bus, near their guard house, and coming on board to check that we all had our BBC passes. Never mind us vouching for a colleague - no pass - he was sent back to get it! The shift bus was a Morris with the entrance door at the rear, with long side facing seats . It was always crowded but sometimes we had some light relief as a man called Batty used to entertain us with his fiddle.

Not only 5XX but what about Sender 3 in another building on the top of the hill, with its demountable output valves? Only one story comes to mind about Sender 3. One night a demountable valve had been stripped, possibly to replace an electrode and very close to transmission time there was the usual problem of removing air and establishing the necessary internal vacuum. One youngster (not me) was shouted at for suggesting to the SME that a cup of tea was in order! For many years afterwards there was a cartoon depicting the incident displayed on the wall in the building. Sender 3 building was later used for the ‘Ampliphase’ transmitter to start the Third Programme, the building later became the station workshop and in the early 1960’s was demolished to make way for aerial feeders. Post War 5XX was replaced by T3, a modern twin air cooled I00kW transmitter operating to Third Programme, but more of that later. Finally, in 1992, 5XX building was home to the Tape Reclamation Unit. My wartime Daventry experiences were cut short by the war and papers calling me up for army service in June 1944.

Senders 1 & 2

These two Standard Telephone & Cables 15kW ‘senders’ date from 1932 and opened the Empire Service of the BBC, and were made famous by King George 5th.’s historical Broadcast to the Empire at Christmas 1932. I operated Senders 1 & 2 ( later and after the war) on shift, three evenings, finishing at 11.00 p.m. and three days starting the next day at 9.00 a.m. After finishing at 5.30 p.m. on the third day, we started nights the same night. After the third night shift we had a rest day and one whole day off then back on evening shift. The second ‘time round’ we had two days off, this happened every three weeks.

Sender 1

The building was located further up the hill than 5XX, at the top of the drive, and to the right. It was really self contained as there were four rooms off the entrance hall, some used for offices and others as for technical purposes. Either side of the sender hall were Motor Generator and Valve Cooling and water distiller equipment rooms with red quarry tiled floors. At the rear of the latter was a toilet and boiler room. The Motor Generator sets were remotely started, and the output controlled, from a wide seven feet tall black polished control panel which ran the entire width of the sender hall. This was a luxury compared with starting the machines in 5XX..

There were three complete Motor Generator sets, one set being required per sender. The sets were colour coded ‘Red’, ‘White’ and ‘Blue’ and were mounted on plinths, about 18” high, again covered with red quarry tiles. Mounted along the inside wall of the Motor Generator room were black cabinets with the Castel key interlocked switchgear for assigning an Motor Generator set to a sender. It was a daily maintenance task to changeover the Motor Generator sets. In the centre of the long black panel, in the sender hall, was a white faced half minute clock and either side two glass windows through which one could view the induction regulator and six water cooled rectifiers for Extra High Tension, plus the other oiled filled apparatus.

In a trench under the sender units were mercury tilt switches which were a protection against failure of the valve cooling water flow. At the rear of the panel were the contactors controlling the Motor Generator’s outputs. There was a metal framed wire netting gate to give access to the rectifier and smoothing enclosure with a Castel key interlock system to safeguard access. The sender hall had cork floor tiles, each sender comprising four individual units standing in a row, with space in between them, down each side of the sender hall. These were: Osc. Mod., (Oscillator Modulator), IA (Intermediate Amplifier), PA1 and PA2 (Power Amplifiers 1 & 2). Again finished in polished black. (Was the type of insulating panel called St. Danio?) [Ed: there was a material called SINDANYO, perhaps this was it] Each sender unit had its own individual mechanical interlocks with a common electrical interlock circuit linking all four units. At the rear of every unit were double aluminium mesh doors and some of the units had side aluminium mess removable panels where access was required for coil changing or padders.

The technology was rather unusual as Lower Power Modulation was carried out in the Osc. Mod. (Oscillator Modulator) unit the remaining units being modulated RF (Radio Frequency) carrier amplifiers. Drive cabinets were no longer used as the drive was obtained from the central drive room in the main building, from which, in war time days, was in a room leading off the west end of Sender 5’s Motor Generator room.

In those very early 1930’s days the operating schedule consisted of several transmissions of two hourly duration these reflected the five different time zones around Empire to which the signals were directed. and consisted mainly of repeats of domestic programmes. This meant that artists had to be in the studios even during night times as sound recording as we know it today had not been invented. A London studio I visited in 1942 only possessed a Marconi Stille steel tape recording machine. When the tape broke someone had to join the ends by spot welding!

During the war the operating schedules were divided into world reception zones, I think some of the names were: Eastern, Asian, African and Pacific, instead of, as it was later, when the schedule was split into Evening, Day and Night shifts. During wartime Senders 1 & 2 were staffed by two lads, and I think we were called Junior Maintenance Engineers (JME’s). The two senders therefore got the name of the ‘JME’s Corner’. This leads on the subject of wave band changing, quite involved on Senders 1 & 2. The Osc. Mod. had pre-amp, plus output amplifier grid and anode coils to be changed plus sundry padders. One rather advanced feature (for 1932) was the anode coils in two units, Intermediate Amplifier & Power Amplifier 1, were water cooled hollow copper tubes. However there were problems on band changing as these water cooled anode coils, had tapping positions, unlike the grid coils, which could be replaced, were fixed, and some fiddly copper shorting straps had to be fitted, shorting out some of the turns, depending on the operating band, to obtain the correct inductance.

I recall the grid coils, colour coded representing the wave band, mounted on rectangular pieces of ‘Mycalex’ insulating material, and stored in wall mounted, brown varnished coil boxes mounted on the rear walls. The two anode coils in the Power Amplifier 2 unit were even worse to deal with as they consisted of a series of part rings of copper, placed one above each other, mounted on Mycalex formers. The two ends of each copper ring were connected in a variety of combinations, by using copper strips, to obtain the correct inductance. Also there was a ‘Spade’, a disc of copper mounted at the base of each coil which, depending on its angle of rotation, could modify the inductance and tuning. For 11, 14, 16 and 19 metre bands alternative pieces of copper inductances were used, and I remember we called one anode inductance a ‘Sine Wave’ as it had that shape. It will be appreciated that carrying out a wavechange in the 15 minutes allowed took a lot of practice.

There were many hazards, a common one was dropping a copper connecting strip into the trench which ran under all the units, made worse if it was the one and only one left! That meant getting the steel keys, and someone else to help, to lift the heavy trench cover and retrieve the lost strip. Another problem was the strong Radio Frequency pick up which came down the aerial feeder from other senders, particularly in the Power Amplifier 2 unit, and the resulting burns to the hands, the shock and pain of the burn often causing shorting straps to be dropped. In addition to changing the coils there were condenser and ‘neutralising’ settings to be placed on each unit. ‘Hot Neutralising’ the Output stage in the Oscillator Modulator unit was necessary and so was checking the entire sender was stable when the drive was removed.

One schedule required a 15 minute wavechange on both senders right at the commencement of the evening shift at 5.45 p.m.! And it also meant a ‘Crash start’ at 6.00 p.m. I recall once an SME (Senior Maintenance Engineer) taking me to task for radiating the incorrect frequency (which turned out to be the fault of the drive room operator) and I told him that I couldn’t tell the difference between two close frequencies on my dials. During the war sender operating frequencies were referred to as call signs and I have a very clear memory of a new call sign ‘GWC’ otherwise known as 15.070 MHz.

Maintenance was usually carried out at night and a very dirty job was replacing the water cooled grid load resistors or changing one of the four SS 1971 valves in Power Amplifier 2. It was so easy to become covered in brown dirty water. One indication that the sender was radiating was the Power Amplifier 2 valves singing. like a kettle coming to the boil. Sometimes this would get more and more pronounced sometimes due to the Automatic Mains Supply Voltage Regulator in the adjoining Sub-station becoming jammed in the ‘raise’ position.

In the centre of the sender hall were two wooden ‘control’ desks, one for each sender, containing a master valve cooling water flow meter, a row of warning lamps and an ‘emergency’ stop button. I mention this as a favourite trick was to press this button, thus tripping the sender, and seeing the rightful sender operator dash round to the power board in order to restore transmission. Putting the sender on the air again quickly meant running to the control panel, pressing ‘HT On’ buttons for the Osc. Mod., IA and PA1 units then winding down the EHT rectifier filaments pressing ‘HT On’ button for the PA2 then winding up the rectifier filaments again. Having operated Senders 1 & 2 both during the war and post war, afterwards in the engineering in the early 1960’s, I was involved in taking the two senders finally out of service when the role of Senders 1 & 2 building changed to that of workshops for maintenance and rigging staff and also a stores.

I always remember ‘old’ Pop on Senders 1 & 2, as he didn’t go by the clock in the middle of the switchboard but by a great ‘turnip’ pocket watch. And if his watch said it was time, OFF the Sender went! When he clipped a broadcast of The Koran we thought the Arabs would have him one day!

Senders 4 and 5 in the immediate post war years

Following service in the Far East and my demobilisation, I returned to Daventry in January 1948. Having passed an examination before ‘call up’ to be promoted to TA1 (Technical Assistant) I wasn’t best pleased on being downgraded to TA3 on my return thus having to start all over again. One of my first duties was to learn how to operate Senders 4 & 5. So here are some memories of them.

Originally in 1925 there was only the 5XX building, then 5GB and the Empire Building was added. In 1932 the BBC started a regular short wave service to the Empire. Then in the mid 1930’s Daventry was considerably extended by the construction of the Main Building, initially half was built to accommodate Senders 4,5,6 & 7 plus the associated crypt, Motor Generator and Mercury Arc Rectifier rooms, the programme control room & the Radio Frequency drive room, plus an office block all on the west end. At the same time extra directional aerials plus switching gantry were constructed. Out on a limb, so to speak, was the diesel engine room, main battery room and Low Tension switch room.

The first two senders to be installed were the Standard Telephone & Cables 4 & 5, (was it for George 5th’s Jubilee?) with their control desks, facing each other, at right angles across the sender hall. Situated at the west end of each sender were twin Radio Frequency ‘Exciters’ and behind them twin Low Frequency (programme) ‘Exciters’. Senders 4 & 5 were designed to be semi automatic in operation for changing waveband, something which has to be done several times a day, in order to make use of optimum transmission conditions, when operating on the short wave bands. The Pen RF and Final RF stages being fitted with electrically operated turntables carrying the inductors required for four different wavebands. The turntables had an upper section for anode and output inductors and a lower one for grid components.

For the Final Radio Frequency stage there were also ‘shunt coils’, placed in parallel, to decrease the final anode inductance for the higher frequency bands. The ‘shunt coils’ looked like inverted toast racks on insulators. There were aluminium vertical screens between each section on the turntables. The original intention was for selector operating switches on the front panel to set motors in operation to select components for the next waveband. First raising anode and grid contacts then rotating the turntable to the required position then lowering the switches.

By the time I was let loose on Senders 4 & 5 (post war) the sequence was for the operator to switch on the motors to raise the anode and grid contacts, manually rotate the turntable to the correct quadrant then lower the contact switches. Senders 4 & 5 Final RF stages were operated in the ‘Grounded Grid’ configuration and, similar to Sender 7 later on and were not the most stable of senders. Originally the filaments of the RF stages were heated by alternating current this, I think, was one solution to the problem of insulating the filaments from an RF point of view. However the noise levels were not acceptable so a third filament Motor Generator was provided to supply the Pen RF and Final RF amplifier filaments. The problem of RF isolation of the Final RF filaments was then achieved using quarter wave length copper sections in the filament supply, these being housed in the crypt below. The Pen RF stage used Standard Telephone & Cables SS1971 valves (as in Senders 1 & 2), and the Final RF amplifier used ST&C 4030’s. The Audio Frequency stages used twin water cooled valves in the Pen AF with a third spare valve. The modulator used ST&C 4030’s with two spare valves which could be switched into service.

The MG rooms, behind each sender, housed filament and auxiliary MG sets and their motor starters, plus a power factor correction condenser and associated Oil Circuit Breakers. In the MG room and immediately behind each sender were interlocked enclosures containing the modulation transformer and smoothing chokes and condensers. There was one set of large oil filled components in a separate enclosure with cables running to both smoothing enclosures The sender control desks, in the sender hall, had meters for all the supplies and ‘raise’ & ‘lower’ controls which operated remote electric motors which controlled the 230 volt dc field current, and thus the output, of the generators. The EHT ‘raise’ & ‘lower’ control operated a motor which controlled the induction regulator on the selected MAR. (Mercury Arc Rectifier). The control desk had ‘ON/OFF’ switches for both the starters and outputs of three MG sets, plus controls for the MAR OCB and ACB (circuit breakers).

The crypt below the sender hall contained the valve cooling circulating pumps, distilled water storage tanks and the external valve cooling radiator fan motors starters. In the centre of the crypt was a common distilled water makeup tank with pumps automatically controlled maintaining the correct water level in the sender roof mounted header tanks.

Immediately under each sender was an interlocked enclosure which contained ceramic ‘pancakes’. These were insulating columns of water made into a flat circular shape and were in series with the water cooling flow to and from the valves. Adjacent to the large cylindrical sender steel water tanks were a number of ‘Electroflow’ units. Their function was to record valve cooling flow amounts and also to protect against cooling water failure. Also in the crypt were the motor starters for the external valve cooling radiators.

Senders 4 & 5 seemed very prone to Radio Flashovers, I recall one afternoon with many incidents and after each one the Senior Maintenance Engineer said, “Put 2 Decibels attenuation in the programme input and take 1kV off the Extra High Tension”. After some time I said to him, “I have only got 5 kV left, I might as well shut the sender down.” He sniffed the air and pronounced the problem was a bonfire outside ‘ionising’ the air, so he went outside and had the fire extinguished.

A common occurrence was the sender tripping as a result of the operation of a VPR (Valve Protection Relay) the sender was then off the air due to the VPR operating the Mercury Are Rectifier high speed relay (HSR) to suppress the arc. Another winter problem was ice on the feeders or the aerial array. This would manifest itself as adverse loading conditions in the Final RF amplifier. Power had to be reduced in order to avoid damaging equipment because of the high standing waves on the feeder. Unlike some other senders there was no way of reducing output coupling from outside the sender so one had to trip off and go inside to alter things with a spanner. I can clearly remember a 15 minute transmission one Christmas day and I never achieved full power.

In the late 1940’s two technical innovations came upon us. Firstly the carrier alarm. Before its introduction a sender could quietly drop off the air without being noticed, not so with the strident carrier alarm bell ringing. The second was the provision on the ‘Tram Driver’s Friend’, TDF in short. On the end of each sender control desk sprouted a metal box with a detachable handle, the was the ‘Auxiliary Isolator’ and its object was to make auxiliary circuits safe in the three sender enclosures. The state of the TDF was indicated by either a red or green lamp burning above each enclosure. These lamps were powered, as a safety measure, from the station 230v battery maintained supply.

Sitting on Senders 4 & 5 on a week day shift meant you were ‘on view’ as you were close to the office block. You were also expected to be industrious in some way, and monitoring the programme output with head phones. Also the Engineer In Charge at that time took a special interest in informing one, on his frequent strolls up and down the sender halls, that, “Your red light is out, Bowman!”

During the war the Senior Maintenance Engineer and his assistant had their office in the middle of the transmitter hall half way between Senders 4 & 5 and Senders 6 & 7. But when I was on shift post war the same area was occupied by ‘Supervisor 1’, later called GS1 (General Service Engineer) who was the equivalent of Assistant SME whose function was to oversee the station’s operations.

The aerial tally board was mounted on the front of the SUP desk, and as GS 1 had the job of verbally issuing array ‘Clear for Power’ to the senders. In the cupboard of SUP1’s desk were kept various forms which we needed to obtain and complete. Unfortunately there was a hole in the floor under the desk and one day it was discovered that the entire box of forms had dropped into the crypt and what is more they had fallen into the crypt water tank mounted underneath. A small friend of mine was persuaded to strip off and enter the tank and rescue the forms.

After a lengthy stint on Senders 4 & 5, I asked at an Annual Interview to be moved elsewhere and thus started a spell on Senders 8, 9, 10 & 11.

Aerials in the immediate post war years

Borough Hill, Daventry, is said to be the highest ground until you get to the Urals. Winter nights up there were not only bleak and cold. They were noisy! Strange whistles and howls came from the masts and wires, and led to stories of ghosts. After all, in prehistoric times it was an Iron Age fort, and that was long before the invading Romans built a villa up there. When on aerial switching duty in the dark, you could imagine all sorts of things as the wind bit through the thickest of overcoats. During the war days I somehow escaped aerial switching duties but not so after returning from Army service.

Night Shift

There were two lads on each shift on ‘Aerials’ sometimes assisted by a GS (General Service) engineer at very busy times. One lad on the ‘Gantry’ the other lad on the ‘Field’. The first function of the aerial gantry was to provide manual selection of one of six possible array (aerial) feeders to each sender. The switching comprised (for each sender except Sender 7B) the six array feeders, each terminated, with two ‘eye connectors’, mounted on short pylons, arranged in a semi circle.

The feeders from the individual senders were also terminated on short pylons, each sender feeder fitted with two flexible lengths of ‘sausage’ feeder, long enough to reach any of the six ‘array’ feeder pylons, and terminated in ‘hook connectors. There was also a small winch and cable in order to provide the necessary tension, the arrangement being similar to a six position rotary switch.

One of the two ‘Gantry’ tasks therefore involved operating the sender selectors, lifting the flexible feeder, on the end of a long pole, from one array feeder position to another. However, due to the total weight of feeder and wooden pole, this could easily place the unwary operator on his back when ‘unhooking’ the feeder hooks unless the pole was maintained at the correct angle.

Biff and Triff

The Gantry lad also did the ‘Biff (bifurcate) which meant crawling through a tangle of feeders to operate a Bifurcating switch. Quite exciting as some feeders were alive and even more exciting at night! Because many of the arrays had to be made available during a current schedule to as many as three senders (but not at the same time!), the incoming 600 ohm array feeder could be manually switched to one of three sender selector positions.

This was achieved by a simple arrangement of terminating the array feeder in twin hooks which could be connected to one of three ‘eye’ connectors on feeders to the sender selector positions. This then got the name ‘biff’ (Bifurcate). So the word ‘Biff’ was accurate when only two sender feeders were involved and ‘triff (trifurcate) for three senders.

A further complication, which was of no concern to the aerial switching TA, was that between the ‘biff’ position and the sender selector, matching and impedance transformation from 600 to 320 ohm had to be carried out (the array feeders being 600 ohm and the sender feeder 320 ohm Characteristic Impedance). (I wonder what the present day Health and Safety people would have to say about the dangerous operating conditions of those days?)

Biking About

At very busy times even the GS Engineer (General Service) was pressed into service to assist with the gantry switching. The ‘Field’ lad had the job of either slewing or reversing the direction of aerial arrays. Charging along on a ‘sit up and beg’ bicycle on narrow concrete paths, in all weathers, hoping in the darkness the sheep got out of the way. I doubt whether there was a light on the bike and the path lighting was usually rather feeble.

Quite often an array would require either slewing within a fifteen minute period. As some of the arrays were as much as a mile from the main building there was not time for the TA (Technical Assistant) to accept the array was off power, cycle to the array, and carry the necessary switching within 15 minutes. So a current practice was for the ‘field’ aerial TA to go beforehand to a field telephone adjacent to the array and accept the ‘array off power’ instruction from the GS1 (General Service) engineer by telephone. Frequently he would go and switch the array, ‘phone in to say he had done it, possibly change another array prior to returning to the main building to write up the details in the aerial log. When an array was taken off power and was required later on another bearing it was also the principle to carry out the switch as soon as possible, and of course log the fact that this had been done.

The site extended to many acres (700?) (before we knew about Hectares) and of course involved going out in all weathers, sun, gales, fog, snow, frost. The standard issue included Duffel coats, oilskins, rubber boots, leggings, gloves, a torch and ‘sit up and beg’ bicycle often with no brakes and ‘square wheels!’. The switching on older arrays involved operating up to 16 ‘quarter wave switches’ each contained in a small fenced enclosure. Later on the array switching involved the use of a ‘C’ frame and ‘on line’ switches.

Windy in the Willows?

There was a story of someone confessing that one foggy night he only located 15 of the 16 switches for an aerial! Quite a common event was an aerial TA being scared when in the field. At night the aerial field was an eerie place, wind whistling in the rigging and stories of a ghost on site. Often one could be startled in the field by hearing a cough, convinced it was the ghost, only to realise it was a sheep. During severe gales the aerial TA had to monitor the array balance weights which could lift in severe wind, ice or snow due to extra loading on the arrays. Sometimes this would require tension on the array to be decreased using a winch to avoid undue forces on the mast. It wasn’t all bad as on summer mornings one might ‘accidentally’ stumble across mushrooms somewhere down the field.

Following the extensive modernisation in the 1960’s manual aerial switching was a thing of the past. Modern technology converted Borough Hill, Daventry to a matrix switching station with array switching, all remotely controlled and pneumatically operated.

The ‘DAVENTRY Pattern’ - a study in RF Interference

About 1948/1949 Sutton Coldfield started transmitting 405 line TV programmes for the Midlands with the sound frequency of 58.5 MHz and a vision frequency of 61.75MHz.

It was very quickly discovered that harmonics of the Daventry Transmitters Short Wave carrier frequencies, particularly 19 metres, (15x4=60MHz), interfered with television reception in the Daventry area. TV screens were often covered with something like a Scottish tartan (in black and white). It got the name ‘Daventry Pattern’ and caused so much annoyance that soon questions were asked in Parliament by the local MP. All types of filters were tried out on individual’s TV sets but they were not satisfactory.

The BBC were stung into doing something about it so the Engineer in Charge from the BBC Woofferton Transmitting Station, Mr. L. F. Ivin was sent to Daventry to conduct research with a view to finding a cure.

New Problem - No Kit

As at that time there was no commercial equipment capable of being used to measure TV interference thought to be generated by the short wave transmitters. So L. F. Ivin constructed his own. This was using a (valved) Hallicrafters Short Wave receiver mounted in a Ford ‘shooting brake’. The high tension supply for the receiver valves was derived from a number of cycle headlight type dry batteries connected in series, tied up with string, wrapped in brown paper parcels and mounted in a wooden box. Valve filaments were supplied from a lead acid accumulator.

A rod aerial to cover the range of frequencies involved was provided by a local firm and this was fastened to the door hinge on the side of the vehicle. Measurements of harmonic radiation was carried out at a series of locations more or less in a circle in villages around Daventry. L. F. Ivin held the view that harmonies were carried to the transmitter aerial along the aerial feeder. So he experimented with different types of filters. Originally some of the filters were placed close to the aerial.

Making Smoke

Another measure was fitting a filter mounted in a large copper box, and placed in series with the transmitter aerial output. This filter had been designed for use on an RCA transmitter, of lower power output, at another transmitting station. I think I was partly responsible for the destruction of this filter by applying too much power from the transmitter we were using! I still remember the lid of the filter being removed to make some adjustments and a cube of smoke came out of the box! I tried not to laugh.

In time L. F. Ivin discovered that a filter, mounted on top of the transmitter close to the output could be adjusted to give minimum interfering harmonic signal and this was proved at several of the measuring spots.

A Show of Embarrassment

L. F. Ivin was so convinced that he had the solution that various head office people descended on Daventry with measuring equipment in order to satisfy themselves regarding Mr Ivin’s theory. Unfortunately when simultaneous measurements were carried at several locations results showed all that was happening was the interfering signal was being slewed to be a minimum in one place but strong in other. There were red faces all round!

Class C Spikes

So it wasn’t long before it was realised that a two pronged approach was necessary, a filter on each transmitter aerial output terminal and the whole of the Final RF amplifier screened with copper sheeting. It soon became apparent that the majority of the harmonies were produced from the grid circuits of the Final RF Amplifier, presumably due to the ‘angle of flow’ of the grid current being low and thus producing copious harmonies, and radiating from the transmitter enclosure.

It was also established that even with the 11 kV dc EHT on the Final Amplifier removed, and the feeder disconnected at the sender, the harmonic radiation was very little attenuated! Resulting from this knowledge it was clear that the Final RF amplifier, required screening to contain the radiation from the grid circuit. Not only this but the sheath of every cable entering the Final RF Amplifier required bonding to the case and a harmonic filter was required for both feeders. Following all this knowledge the BBC Planning and Installation Department arranged for a London firm, Maryatt & Place, to carry out the work to screen the Final RF Amplifier casing with sheets of copper.

Sometime later L. F. Ivin left the project and the investigation was solely directed by BBC Planning Department. Meanwhile harmonic filters, based on the ‘dust bin’ prototype, the case of which was hand made by a local metal work firm, were manufactured by Mullard Ltd. One further complication was the addition of the Final RF Amplifier screening reduced the ‘natural’ ventilation of the unit and therefore two extractor fans were installed on the roof of the unit.


During the investigation into harmonic radiation affecting Television another problem occurred: complaints were received from the Civil Aviation Authority that interference from Daventry senders was affecting aircraft communications. This was in the region of 120 MHZ and was attenuated by a small modification to the centre ‘PI’ section of the harmonic filters.

Harmonic Interference Testing!

Measurements of harmonic strengths was continued on a regular basis, particularly on receipt of a complaint or on the use of a new frequency. Later, commercially designed battery operated portable measuring equipment was used for harmonic measurements. Although test measurements of harmonic radiation were regularly carried out, someone was sent occasionally to view a TV receiver in Daventry town. An engineer would visit the TV room in the BBC Club in Sheaf Street to see if any interference with the picture was evident. One certainly got some queer looks from mothers in the room with their children viewing ‘Watch with Mother’.

The screening of the Final RF Amplifiers, and installation of harmonic filters, was then completed on the three remaining SWB 18 senders, plus Sender 6. Sender 7, which had units of a different mechanical design, was also screened in this case with a ‘top hat’ of aluminium over each RF channel. The knowledge gained was then conveyed to other Short Wave transmitting stations. The requirement for harmonic suppression was an integral part of the specification of the new generation of senders, appearing in the 1960’s, and was accepted as a matter of routine without much thought about the research into the problem a decade before.

Senders 8, 9, 10 & 11 - ‘Boy’s Town’

In the east sender hall, which was a replica of the west hall without the office block, Senders 8,9,10 & 11 were installed just before the 1939-1945 war. These were Marconi 100 kW SWB 18 senders, with slight differences between Senders 8 & 9 and Senders 10 & 11. The east sender hall received the title ‘Boys Town’ as the senders were usually staffed by young lads. Up to 1951 there was a Technical Assistant allocated to each sender.

Senders 8, 9, 10 and 11

Below the sender hall was a large crypt containing the valve cooling equipment, and unlike the west hall crypt, as it had two staircases at either end. Behind Senders 8 & 10 there were two choke enclosures in the long MG room, also the west end of it gave access to the LT Switch room as it was known. Similarly there was a long MG room, plus two choke enclosures, behind Senders 9 & 11.

Each sender had wavechange trucks for the Intermediate Amplifier and Final RF amplifier stages, the Final Radio Frequency Amplifier stage trucks running on rails with points and sidings. Auxiliary power supplies were derived from the ‘B’ Unit located at the end of the senders.

The RF chain started with a ‘Channel’, in other words one of four tuneable multi stage RF amplifiers to which the RF drive was connected to its input. The channel output was connected via a small concentric feeder to the air cooled ACT9 input stage of the IA. Then on to two CAT9 water cooled valves forming the Pen RF stage. The output being fed into the Final RF amplifier, and like Sender 6, originally used CAT14, then CAT 17 valves and latterly CAT 27 thoriated tungsten filament valves.

Tuning controls for the Final Amplifier were hydraulic operated by a hand wheel on the front of the FA, ‘Anode Tune’, ‘Balance’ and ‘Output Coupling’ being selected via three selector levers. Wavechanging could easily be carried out within the maximum time of fifteen minutes, however newcomers initially found this difficult to achieve in the time.

The AF line up commenced with the first LF stage with two PX25 valves in Push-Pull, the output going to two air cooled MT14 valves which in turn feed the Pen LF stage of two water cooled CAM2 valves with one spare valve. The Final modulator stage used two CAT 20 valves with one spare valve. When all the front doors were opened the sender looked more like a train in a railway station!

Sender 9

The Surge

Something which happened quite often was a surge in the mains power supplies. This could result in the sender tripping off the air, one common cause being a contactor releasing in a valve cooling pump starter box or in the air blower starter, which in turn would trip the filament machine and so on. The solution was not to run - anywhere but to study the meters on the control desk and to take stock of what (if anything at all) had been lost.

I can clearly recall one TA after a mains surge dashing into the crypt assuming his pump and sender had tripped, whereas in fact his sender was still on the air. All he did was to shut down his air blower and restart it unnecessarily thus shutting down his sender.

Another very common incident was an inter electrode short (e.g. Grid/Filament contact ) on a CAM2 valve. The solution was for someone to trip the sender, rush into the machine room, wind down the filament MG field control by hand, whilst someone else switched the spare valve into service, then wind up the filaments by hand and re-power the sender. This could be easily done in a break in programme and therefore wouldn’t count as a shut down. I recall this happening on one occasion and the sender TA’s realising the situation said to GS2, “Shall we put the spare into circuit?”. He (nameless) said, “I will go and have a word with the SME”, and as he walked down the MAR room on his way to see the SME the lads decided the quickest and correct thing to was to insert the spare CAM2 which they did!

Schedule Changes

A fact of life on an HF transmitting station was the need several times a year, due to Ionosphere changes, to change the wavebands on which senders operated. The new operating Schedule would be published a week or so beforehand thus enabling preparations to be made. Sometimes this involved a sender ceasing to operate on a given band thus releasing the IA and FA trucks to be rebuilt for use on another waveband. Then, out would come the drawings showing how the IA and FA trucks should be rebuilt. I recall a lot of metal polish also being involved. Trucks were built; then came the task, sometimes lengthy, of lining up the sender on the required frequencies on the new waveband.

One would start with the RF channel putting in the new coils and hoping the drive room was able to supply the test frequencies required. Tuning the 5TH. RF and Pen RF were more involved as neutralising the stages had to be carried out. Then, providing an array was available (properly matched by the Aerial SME and staff ) one could tackle lining up the Final RF amplifier stage. But was the sender stable?? Unplug the RF drive input to the sender, stand back and see what happened! Then it was necessary to compile a new settings chart for use on the new Schedule.

Rota Valve Changes & Pirani Tests

For many years it was deemed necessary to change the Final Amplifier valves on a rota basis. This involved swapping a service valve with a spare, this was a job that more often than not was left to the night shift to carry out. The spare ‘rota’ valve would be collected on its trolley from the valve store and preparations made to remove the service valve. The whole process was straight forward providing you knew what you were doing, particularly on the plumbing side! It wouldn’t be the first time someone got a wetting!

The correct routine was to turn off the water inlet, then the outlet, afterwards the drain cocks could be opened and the petcock opened to admit air and so let the water drain from the valve jacket. After putting the replacement valve into position came the tricky part in opening valves in the correct sequence in order to remove air from the valve jacket!

Changing a valve in the "East Hall"

The Pirani Tests

Another job, again often left to the night shift, was checking the vacuum state of a new valve. It was necessary to find the Pirani trolley and hope the batteries were fully charged! The test entailed passing a constant current through a valve filament and accurately measuring the voltage developed across the valve filament pins. During the time the filament was rising in temperature so would its resistance and thence the voltage across it. Eventually the filament would reach a certain stable temperature, resulting in the filament resistance and voltage levelling off. This final voltage was known for that type of valve filament, based on the idea that with a good vacuum very little heat is carried away from the valve filament.

All very well, but it was a time consuming procedure. Sometimes time could be saved by carrying out Pirani test of two valves connected in series.

Distortion Tests

These were tests carried out regularly, by the Senior Maintenance Engineer, to establish that each sender was achieving set limits of noise, frequency response and Harmonic Distortion. I have included a note on this subject as I still have a clear memory of a couple of the dodges the SME could get up to in producing good figures, if only at the time of the test! One was to increase the filament voltages with the idea of increasing the emission of the valves. But one incident I always recall is one particular SME, who on an occasion after being told the figure on the meter was too high, responded, “Not from where I am standing”.

Pranks & Incidents

Dropping the truck rails: A frequent prank was for someone to go round the rear of a sender then trigger the carrier alarm bell and the same time dropping the brass extension rails on to the steel covered floor making a terrible clatter.

Cleaner sleeping on spare MG set: It wasn’t unknown for one particular cleaner to rest himself by leaning on the motor casing of a spare filament motor generator set. The joke, having seen this, was to go back to the control desk and switch on the motor!

AEiC trying to open Sender 8 doors: One day, Sender 8 was off the air and the AEiC for some reason wanted to enter the Final RF amplifier unit. At the time the ‘X’ bolt was released so the doors were free to open. Mistakenly the AEiC saw the two door handles were horizontal so he pulled on the handles, but as the handles had locked the two doors they didn’t open. So he thinking all the sender doors were locked by the ‘X’ bolt moved the handles to the vertical (unlocked) position. He then approached the ‘X’ bolt to unlock the sender doors, naturally nothing happened as in this position the ‘X’ bolt would not operate. By this time the sender TA was in hysterics and in no position to render assistance.


During the war we had the ‘double talking’ Stan Unwin on shift, creating chaos. Perhaps he developed it from the technical slang that this and every other job seems to manufacture. You must have heard him on TV or films, and realise what this brilliant distortion of everyday speech could do to a serious discussion of technical procedures.

For several years I was on duty as GS2 giving me responsibility of overseeing the lads on Senders 8 to 11. One night shift I had gone into the crypt and was having a rest on a table close to the crypt stairs. Every few minutes I heard a bell ring, so after a while I went upstairs to investigate this strange occurrence. I found the lad on Sender 10 checking his transmitter was still radiating then resetting his alarm clock, after it aroused him, then going off to sleep again. It’s amazing what we can train ourselves to do if we try!

Senders 6 and 7

When I had been back at Daventry several years after war service, someone must have had the idea that I could cope with the technicalities of Senders 6 & 7. For many years the Senders 6 & 7 shift rota was associated with GS duties and was always considered a senior position and not for the likes of us juniors! One complication was that 6 and 7 were totally different from one another. Sender 6 was a forerunner of the Marconi SWB 18 senders, the main difference being that it used a ‘floating carrier’. The idea was presumably to conserve power. It required a Very High Tension of 22 Kilovolts, as the Final RF and Final modulator stages were connected in series, the EHT being derived from a pumped BTH Mercury Arc rectifier. (This meant that MAR’s 1 to 6 were all capable of 22kV output). When not on ‘float’ the Final RF anode current was 9.5 amps, shared of course by the modulator stage.

Visual Pips

When the carrier was on ‘float’ the final anode current would swing from about 5 Amps to 9.5 Amps depending on the percentage modulation. When on ‘float’ the effect on the ‘mains’, when the six pips were transmitted. could even be seen with the streetlamps of Daventry town, dipping six times in sympathy!

Sender 6 required special arrangements for the modulator stage filaments which were fed by alternating current. Other bias supplies were provided from a motor generator set in the crypt enclosure insulated earth.

Changing Frequencies

Wavechanging on Sender 6 was carried out using trucks to carry the inductive components for the Intermediate and Final RF amplifiers. Originally, the Final RF amplifier had the commonly used Marconi CAT 14 valves, which used water cooled filament seals, the water flow protected by water operated tundishes and mercury tilt switches housed in copper boxes. Wavechanging also involved selecting another low power RF channel and changing both the Intermediate and Final Amplifiers wave change trucks. The valve cooling equipment, as for Sender 4 & 5, was located in the crypt below, and the remaining MG sets in the machine room alongside. Sender 6 was finally taken out of service about 1961 and the BD253 Senders 14 and 16 were installed in its place.

Four Channels

Sender 7, opposite, was a Standard Telephones & Cables four channel CS 8 transmitter, but different from those at the former Skelton B station, as it had four Radio Frequency channels, each usually set up on a different wave band, of which two adjacent channels were known as S7A, and the other two as S7B. One channel of S7A and one channel of S7B could be used simultaneously and both modulated with the same programme enabling signals to be sent to two different parts of the world using separate frequencies and aerials. Again similar to Senders 4 & 5, which were also ST&C, the Final RF amplifier of Sender 7 used a grounded grid configuration. At one end was a row of four RF exciters, plus the LF exciter. The Final RF amplifier used CAT14 and later on, CAT17 water cooled valves, the modulator using four ST&C 4030 valves. The usual valve cooling equipment was located in the crypt and in the interlocked crypt enclosure were lengths of long white ceramic tubes to insulate the valves.

Not so easy

The entire operation of the sender was fraught with snags, there were two power supply units (located in the MG room) one of which was also used to feed the screen grids of the Pen RF valves, plus the LF exciter. You could easily get into an unholy mess on a channel change, deciding which PSU was the Slave and which was the master particularly when a channel was removed from service and preparations were being made to select another. Then it was very important to remember the correct routine in terms of which way to walk round the sender so that operations were done in the proper order. It was only too easy to trip the remaining channel off the air. Once a Technical Assistant was informed that there were a number of classical operation errors on Sender 7 and that he had done all of them but one and he would be shown how to do the last one!

I was lucky when I started, as a colleague, Dick, drilled into me how to avoid making operational errors. Setting up a channel on a new waveband ready for a schedule change presented many difficulties as Sender 7 needed very little encouragement to become unstable. It was not unusual to see a grid-grid vacuum condenser glowing red hot!

Going Modern

During the 1960’s modernisation, Sender 7 had two of the four channels removed and the remaining two channels were ‘rehoused’ to be similar (in looks) to the BD253 Senders 12, 13, 14 and 16. Later on Single Side Band senders were installed. The control circuits were redesigned and an AR64 Excitron high voltage rectifier unit installed so as to make the sender semi automatic like the BD253’s.

Hot Lettuce

I will end with one story. We used to have a night watchman, during the war, (perhaps after the war too?) whose job among other things was to make tea at midnight, 3.00 and 6.00 a.m.. At 3.00 a.m. he would also reheat and bring to a Duty position a meal prepared by the evening shift cook. One night he delivered the food to a TA who just then was preoccupied elsewhere. When he came to eat his meal he found it was stone cold. Then he played havoc with the Watchman, telling him that in future he expected to receive his meal piping hot whatever it was!

The next night, the TA sat down to cat his meal and removing the metal cover from his plate discovered a very shrivelled salad! When he tackled the Watchman, the latter replied, “Well, you said you wanted me to keep it hot whatever it was, and that is what I have done!”

Memories of GS Engineers

About 1950-1951 there was a shortage of General Service Engineers so I was one of several Technical Assistants (Class 2) who were promoted to TA1, following an oral examination, and was able to carry out GS duties on shift. For some reason there were no vacant Posts on paper! The result was that we became experienced in operating a wider range of switchgear, rectifiers, and diesel equipment without actually being paid for the work. By this time shift staffing had settled down, so there were three GS engineers on shift.

Number 1

GS1, mentioned elsewhere, who was really the Assistant Senior Maintenance Engineer, was responsible for overseeing the operation of the station. He sat at a table in the Sender’s 4 & 5 hall, where in former times the SME’s office had been located. In Post War Years the drive room was moved upstairs and the SME had his office in the former drive room, off Sender 5’s machine room. GS1’s job was supervising all operations also he was responsible for issuing ‘Clear to Power’ senders. On his desk was a board containing all the numbered tallies for the aerials.

One GS1 engineer I seemed to be on shift with more often than not had a habit of not listening to what you told him. He was often referred to as ‘Cloth Ears’ - behind his back, of course. One lad had never heard him called by his own name and one day innocently addressed him as “Mr Clothier”.

Number 2

GS2 was located in the east Senders 8 to 11 sender hall and possibly his first duty was the supervision of the TAs operating Senders 8 to 11. He had a roving commission over the MAR (Mercury Are Rectifier) room, Diesel Engines, ‘Low Tension’ Switchroom, Boiler, sender Motor Generator rooms, crypt valve cooling equipment and external sender cooling radiators. There were also sundry odd jobs like switching on and off the red masthead lights and the gantry lighting. On Saturdays the GS2 had to remove the 230v station battery ‘off float’ and re-charge it, using the spare DC generator. Then came the task of measuring, and recording the specific gravity of over a hundred main battery cells.

Mercury Arc Rectifier (probably part of Senders 8, 9, 10 or 11)

‘Load shedding’

For several years after the war the station was often called upon to reduce the electricity loading for the then Northampton Electric Light & Power Company (before Nationalisation.) This was called load shedding and was often about 7.40 a.m. at the end of night shift. Fortunately the diesel mechanic had recently come to work and started the two engines. The GS2 engineer would then have the task of switching the mains power input to several MAR’s from the mains to the alternator bus bars. Sometimes this situation continued until mid morning, and possibly we would be requested to load shed again about noon. Sometimes we were also asked to trim the station mains power loading by reducing EHT on some of the senders. GS2 also had to carry out the weekly test run of the two 750 HP diesel alternators involved starting the engines, putting one alternator on the diesel bus bars, find some load, usually the MAR of one sender, then synchronising the second engine and applying more load.

Number 3

GS3 had a roving commission and maintained a watch on all rotating machinery associated with Senders 4 to 7, the valve cooling water distillers, plus MARs 1 to 6, the external valve coolers, checked drive frequencies, the main water tower and, in the 1950’s, the Third Programme Transmitter (T3 in the old 5XX building). Both GS2 and GS3 were responsible for controlling the temperature of the sender cooling water, by switching on, or off, the external valve cooling radiator fan switches in the crypts. (There were thermostats but they were considered unreliable). In winter it was often necessary to venture outside, particularly when the transmitter was off the air, and close the louvres on the coolers to avoid frost damage. Another common task was unlocking the gates at the rear of the sender supplies switch board in the MAR rooms to check that the HRC fuses protecting the sender’s 4I5 volt supplies were not overheating.

Starting up

One can still hear in one’s mind the ‘click’ of the lubricating oil priming pump on a diesel engine, whilst attempting to raise a pressure of I5 pounds per square inch, before starting the engine. Then checking that the cylinder water jackets were all full of water before opening the valves from the two air receivers and admitting compressed air to start an engine. Slowly the giant three ton flywheel would revolve as the engine fired, slowly increasing to the operating speed of 375 rpm. Then after loading the engines, starting the compressors to replenish the air receivers, watching engine water temperatures and adjusting water cooling flow. In those days the diesel engine cooling water flowed into an external ‘hot’ pond from whence it flowed into a ‘cold’ pond from which it was pumped to the top of a wooden slatted construction called a cooling tower. Alas those engines are no more - more water under the bridge.

Diesel Generators

Finally, a few ‘Now it can be told’ tales

(1) Prior to the modernisation work in the 1960’s there was a large duct conveying exhaust air from an extract fan in Senders 4 & 5 crypt. This passed through the west end of Sender 4 & 5 Transmitter hall. One day the Assistant EiC complained to the GS3 Engineer that there were marks of oil on the outside of the duct. The GS Engineer (whom I shall not name) told the AEiC that this was due to the bearings of the fan leaking oil. In fact the cause was generally accepted as being somewhat different. The real cause of the oil marks was the habit of several GS3 engineers of throwing the contents of the circulating pump drip buckets into the fan!

(2) Instead of hand pumping the drip can contents of the crypt pumps into the outside drain, some GS engineers would throw the contents under the cylindrical valve cooling water tanks with a phrase such as, “It will soon dry”. On one occasion someone did this only to surprise a cleaner who had crawled under a tank for a crafty snooze!

(3) After the war, the BBC got rather excited about Civil Defence and we had to visit a burning building, enter it, and using a wartime ‘Stirrup pump’, extinguish the fire. As a preparation for this, a number of us were assembled in a small office in the T3 building to be lectured by the AEiC. Well, at what seemed to be about the middle of his lecture, he abruptly departed without any explanation. Sometime later, the door at the back of the room burst open and we turned and looked about 5 feet from the floor where one would expect the face of someone entering to be. Then to our amusement, which was difficult to suppress, we saw him on his hands and knees waving his arms about. Later we realised he was demonstrating how one should enter a smoke filled room. It’s the unexpected event which often makes a good unforgettable lesson.

DAVENTRY: post Evesham: promotion to Engineer

Returning to Daventry after the engineer training course at Evesham in early 1954 meant going back onto shift work again. For the next eighteen months or so I carried out GS duties, usually with the same people on shift. As my experience progressed I was asked to deputise for the GS1 and sometimes even to be in charge of the shift. (They must have been short handed to use me!). Meanwhile the Third Programme Transmitter(s) had been installed in the former 5XX building and one ‘C’ engineer had a six month attachment to look after it on day shift, so my turn arrived in 1955.

T3, the Third Programme transmitters

After the departure of the 5XX transmitter the same 1925 building was used to house T3, which comprised two medium wave transmitters working in parallel on 464 metres. So having ‘qualified’ as a BBC engineer I was given the day shift job of overlooking the Third Programme transmitters. From the early days T3 had always been in the care of one grade C Engineer on day shift. In those days, in the mid 1950’s, this meant Monday to Fridays plus Saturday mornings. It usually took the first couple of months of duty, by the T3 Engineer, to comprehend the operation of the complex relay control circuits! The remainder of the time (evenings, nights and weekends) T3 was the responsibility of the shift staff. This involved the shift GS 3 Engineer in routine and fault visits.

The Third Programme transmission commenced, in those early days, at 6.00 p.m. and the ‘resident’ C Engineer had to switch control to ‘remote’ and then was also required to stay with the transmitters until both were radiating satisfactorily. Originally it was intended to operate T3 A & B remotely from London, but this idea did not materialise. Instead, they were remotely operated from the control room, in the main building, only a 100 yards or so distant!

The keen student would observe that, by this time the Station transport had departed, at 5.30 p.m., with the Day Shift, to Daventry town! This meant leaving the T3 Engineer to make his own transport arrangements at his own expense. My solution was to purchase a second-hand 98cc Bown motorcycle.

More Power

The Third programme, which had been carried by the ‘Ampliphase’ Sender 3, from March 1950, was transferred to the much more powerful T3 in April 1951. These two air cooled transmitters were the first modern transmitters Daventry had seen since before the outbreak of war! Each transmitter was air cooled, used no rotating machinery (except the 20 HP cooling fan), had its own control & monitoring system. The valve and unit hot exhaust cooling air could either be discharged to the atmosphere or into the transmitter hall, to act as building heating, as each transmitter had manually air flow operated louvres. It was intended eventually to heat the building using waste heat from the transmitters. Similarly in warmer weather the cooling air intake could be taken from outside the building or from the inside.

The Medium Wave output of the two transmitters was fed into a Bridge ‘T’ combining network with a water cooled ‘dummy load’. The dummy load could be used as a test load for one transmitter whilst the other one was connected to the feeder.

Big Mast

The aerial took the form of a 725 foot high Mast Radiator situated close to the village of Dodford some 7000 feet away. The mast stood on insulators and the top 300 feet, which was crowned with a ‘sweeps broom’ (as a capacity top), stood on insulators at the Mast Break. The design was intended to reduce the typical medium wave interference between the ground and sky wave signals. The ATH was adjacent to the mast & connected to the transmitter by a lengthy open wire RF feeder and a catenary underneath for the control and interlock cables.

Trunk feeder and mast for T3

The a.c. heated valves filaments were of interest as they were made using the modern concept of thoriated tungsten and operated at low voltage (9 volts) and high currents, the filaments being ‘fat’ increased their thermal inertia. This and other techniques minimised hum which can be a problem when a.c. is used for valve filament heating. Quite a lot was made of the relatively high overall efficiency, of about 40% for an air cooled transmitter.

Starting Up

Starting the transmitters used a sequence control system. whereby the main cooling blowers started first, then when sensors detected adequate air pressure, the filament supply was switched on. Next followed, grid bias, auxiliary HT and main EHT, using six individual mercury discharge AR64 Excitron rectifiers in a three phase bridge configuration. A further control system looked after the paralleling of the two transmitters and the combining unit. It was also the first appearance of the Automatic Monitor Minor, which compared the input audio signal with the demodulated AF transmitter outputs.

On detecting a fault, the monitors were capable of taking executive action and closing down a transmitter. The monitors had also a self test routine and by means of a combination of coloured lamps were always able to display the current state. This meant if the transmitter were unattended and was closed down by the monitor the reason for the close down would be indicated.

A few tales!

(1) Many years ago, one warm evening a fault occurred in the ATH at Dodford. Two Engineers (known, but names withheld) and the AEiC (JES) went to the ATH. It was hot inside the ATH, so jackets and slipovers were removed. After the fault was dealt with there was some oil spilled on the floor which required cleaning up. One of the Engineers grabbed something nearby with which to mop up the oil, saying, “My, this is a good piece of cloth!”. To which the AEiC (not a man known for his sense of humour) retorted, “That is my pullover!”.

Aerial Tuning Hut at Dodford for T3

(2) Another T3 Engineer was missing from T3 after lunch one day. He was eventually found dozing in the Rest room adjacent to the Main Building. On being aroused he asked what time it was. When told it was 3.30 p.m. he replied, “Oh dear! I have missed my tea!”

(3) The third story concerns a well known SME in the late 1950’s. Joe, an engineer, was in T3 with this SME on some investigation during a night shift, which involved the use of an oscilloscope. Suddenly the SME said, with some urgency, “Fetch me some toilet paper”. Thinking all manner of things, Joe dashed off and on his return with the toilet paper was relieved to see it was only required to make a tracing of the waveform on the ‘scope! Known afterwards as a ‘Bronco Graph!’.

(4) I was doing maintenance in T3 one day and the old Engineer in Charge, Frankie Calver, appeared with some visitors. He tried to open the door interlock to enter an enclosure. He couldn’t open the door as I had removed electricity from a ‘fail safe’ lock on the bolt. He moaned at me, “Bowman, you can’t get in with the power on and you can’t get in with the power off.”

So on completion of my stint at T3 it was back on shift again carrying out GS duties until 1960 when I was roped in for the Daventry Re-Engineering Project.

Re-engineering in the 1960s

By 1960 the transmitters in the cast hall were about 25 years old. Pressure had mounted steadily for greater transmitter output powers and improved efficiency through automation and staff reduction. It was planned for programme switching to be automated and for transmitters to be switched on and off from a central point, rather than from control desks adjacent to the transmitters, a system used ever since the station opened. So the future implied a shift comprising all Engineers, but fewer of them (no Technical Assistants) with a roving commission, rather than staff being tied to a specific transmitter.

Initial Steps

In August 1960, work started on the construction of the new control centre. Its function was initially to house the automatic programme and later, aerial switching unit, the drive room and the programme input equipment. This was located on the first floor and building work would take in the stores area. In mid 1960 I was taken off shift work and assigned to assist the Planning Engineer, David Driver, who was involved in repositioning the drive equipment. This meant moving a single piece of equipment, putting it in service, then moving another and so on. We had masses of temporary cabling, so much that we had to place wooden boards above the cables so that we could move about in the room.

Short Interval

During this time I was sent to the works of Siemens in Charlton, London, for a course on the maintenance of the relays and ‘uni-selectors’ being installed in the programme automatic switching unit (ASU). This was probably the last time mechanical switching would be used as solid state switching units were on the horizon.

The ‘Informer’

The control centre work took several months and on completion of the drive part of the work I got my next job, which was to last until Spring 1963, of being a liaison engineer working primarily with Laurie O’Neil (L’ON). I was intended to be the focal point of information to give the station staff so they knew what modernisation work was going on. However I seemed to become a sort of Clerk of Works. The initial work involved the removal of Senders 4 & 5, in the west transmitter hall, and the commencement of building work to accommodate modern Marconi Senders 12 & 13 in their place. I remember the EiC being amused at the way I always attached a label before removing cables.

Energy Saving

The Motor Generator sets were removed to make space for a stores area, power distribution unit, modulation enclosure and constant level distilled/deionised water tanks for the transmitter steam cooled valves. The crypt under the transmitter hall was modified to house the brick built air cooling enclosures for Senders 12 & 13 and it was the intention to use the sender exhaust air, to provide ventilation and some heating, by directing it from louvres built into the facade above the senders into the sender hall. The original crypt cylindrical valve cooling water tanks were modified and lagged to store the heal given off from the valves in the form of hot ‘tap’ water used in cooling the sender heat exchangers. Later on facilities were introduced so that the heat in these tanks could be used for building central heating, and in the summer time cooled by external blower units. The building work created a fair amount of dust and the Assistant Engineer in Charge, (AEiC) whom L’ON called ‘Jack the Wireless’ used to complain to me, “Look at all the dust fellah”. With a man with a pneumatic drill working a few yards away, with only a plastic screen between us, it wasn’t surprising. The EiC, Harry Masters was christened ‘The Station Master’ by L’ON.

New Power

As the replacement senders required an 11kV supply for the Excitron HT rectifier units and also replacement 415V 3 phase supplies, a new 11kV sub station, and Medium Voltage Switch room, were installed in the former valve store area. After Senders 12 & 13 were operating, Sender 6 was replaced by Senders 14 & 16. The sender on/off control circuits were extended to the centre console in the control centre and from now on, stopping and starting a transmitter involved pressing a few buttons.

Total Control

The new aerial system was now fitted with an array switching matrix, in place of the old Gantry, and the slewing/reversing of arrays was converted to remote operation. Aerial changing on a push bike would be a thing of the past. Compressed air was used to operate the aerial switches. At first, the compressed air contained moisture and the switches froze in the winter! Modifications followed. to dry all air to pneumatic switches. Control of aerial switching was interlocked with the sender control circuits and partially automated from equipment in the Control Centre. Sender 7, the last original East hall transmitter, was modified, two of the four RF channels being removed and control being made semi automatic. This eliminated the control desk and so cleared the floor area in front to the newly installed Senders 14 & 16. The five British Thomson Houston pumped Mercury Arc High Tension rectifiers, provided in 1937 for the use of Senders 4,5,6 & 7 were now redundant. They were replaced by the four Marconi 250 KW output transmitters (Senders 18, 19, 20 & 21).

New aerial system, showing trunk feeders in the foreground going to a switching matrix

This meant that half of Daventry Transmitting station had been modified with the installation of eight modem transmitters and the modified Sender 7. Meanwhile, Senders 1 & 2, the first short wave transmitters to go into service at Daventry in 1932, were taken out. The building was then used for workshops and as a main store. The main workshop, in an outbuilding, formerly the home of Sender 3, was demolished to allow room to route sender aerial feeders.

These three years proved to be a great experience as I was involved with builders, plumbers, pipe fitters, electricians, jointers, welders and many more trades. Quite a change from being a TA, cycling round Borough Hill, changing aerials in the dark!

Closing Down 1992

Many more developments took place at Daventry during the last 20 years of its life as an HF transmitting station, but this story now jumps to the day of its closure.

Sunday 29th March 1992 meant an early start for many as, in addition to travelling long distances to Daventry, it was also the morning after clock altering! Although a few people stayed the night in local hotels, many had travelled from home that morning, including one man from Bridport who was due to return home to work a night shift.

New Sights for ‘Old Hands’

Approaching Daventry seemed strange and somewhat confusing in view of the many new roads, roundabouts and large factory estates. I even got lost looking for our previous home, ‘The Malverns’ on Ashby Road. Some of the town streets, Shed Street for instance which once had the Northampton double decker bus running down it, were closed to traffic.

High Street seemed narrow with pedestrian crossings marked with ‘traffic calming humps’. The top of North Street had a ‘No Through Road’ sign. The former Methodist Chapel building on New Street, we attended, was still there even though a new Chapel had replaced it on Lodge Road. Leaving the town centre and going to Borough Hill meant turning left off London Road and finding a wide road, passing industrial units in what were once green fields, replaced the former narrow BBC driveway.

Somewhere near the location of the former bridge over the Blisworth, Weedon, Daventry to Leamington Railway line, of earlier times, was a large roundabout, one of many around the town. Climbing up the hill, passing on the left a housing estate, the remnants of the BBC concrete drive appeared on the right. Rounding the final bend on the BBC drive, then came into view on the left the former 1925 5XX building, now with the sign, ‘Tape Reclamation Unit’.

On the final stretch of the hill the familiar West Front of the Main Building and the 1932 former Senders 1 & 2 building could be seen. Checking names and addresses of all the arrivals at the main gate on the top of the drive, and manning the cloak room, were uniformed youngsters (presumably of the Air Training Corps.)

What’s Left?

Entering the West Sender hall, in which Senders 4, 5, 6 & 7 had been replaced by the more modem semi-automatic Senders 12 to 16, some 30 years previously, one was not prepared for the rather derelict scene of blank floors and walls. All transmitter equipment had been removed from the West Hall. However on the positive side, the remaining BBC Daventry staff had gone to great lengths to furnish the hall with most interesting and comprehensive display of the history of BBC Daventry. The displays included posters of the Borough Hill Iron Age fort, photographs of equipment and staff, also newspaper cuttings of BBC Borough Hill taken during its 67 years of service.

Attractive Show

A display of photographs, entitled ‘Sender 7 1940 - 1964’ showed both the inside and outside of the sender. Other photographs of the commissioning of a BD6126 300 kW transmitter and showing staff in March 1987. There was an aerial colour photograph of the Third Programme Transmitter Mast at Dodford, now long since demolished. There was also a tribute to Daventry’s contribution to early research into Radar in 1940.

Under the headlines:- ‘DAVENTRY CALLING... DAVENTRY CALLING ......’ was a view of Daventry East Sender Hall in 1951, a general view of the modern Daventry East Sender Hall, a picture full of sparks showing Sender 5 being cut up for scrap, and some of modern automatic switching equipment. There were also photographs of the 250 kW BD272 transmitters installed in the mid 1960’s but now no longer in service.

List of important dates

July 1925: Start of the Longwave 5XX National Service.

1927: The 5GB Midlands Service.

1932 Dec. Start of the Short Wave ‘Empire Service’ with Senders 1 & 2.

1934 Construction of the ‘A’ & ‘B’ Mast Towers.

1936 Main Building construction commenced.

1937 (May) Senders 4, 5 & 6 in service for George V’s Coronation.

1938 Main Building and aerial system extended.

1939 Senders 8 & 9 in service.

1940 (June) Senders 10 & 11 in service.

Dec 1940 Senders 7 in service

1962 Sender 12 & 13 in service.

Important anniversaries were featured both in print and photographs as well as the presentation of decorative medals to members of staff. In another display were three of the original 5XX transmitter valves, possibly a CAR2, CAM1 and CAT1. Someone had also rescued part of the front operating panel of Marconi Sender 12 installed in the early 1960’s. This was complete with the metering and tuning panel, three valves and a solitary Excitron rectifier.

One reminder of broadcasting in earlier days, on display, was part of the original 2LO transmitter, with even some valve filaments alight. In the East Hall, the only visible reminder of the former home of Marconi SWB18 Senders 8 to 11 was a Final RF amplifier wave change truck and, mounted on a wooden block on the wall behind, the final amplifier tuning wheel and selectors.

The Guests

There was a list of the 500 or so names of ex-Daventry staff and wives, who had been invited to attend the closing down ceremony. Outside caterers provided morning coffee in both East and West Halls. As 12.30 approached both Sender halls became packed with an ever increasing crowd of former Daventry staff and their wives, not forgetting several widows of former staff.

The Last Switch

One remaining modern transmitter, in the East Hall, Sender 204 of recent years, was still broadcasting on 19 metres to South West Europe and North Africa and this, at 12.30, was the focus of attention as for the last time it was switched off by Bill Bird, a member of staff over 49 years. The closing down ceremony was also accompanied by speeches by Stanley Unwin (who had been a wartime engineer at Daventry) and the Mayor.

A buffet lunch was set out on a long table in the East Hall, on a table covered with a white cloth, decorated with flowers and matching blue candles in silver candelabra. The centre piece was an iced cake with the inscription ‘DAVENTRY CALLING, 1925-1992’ topped with the logo ‘Nation Shall Speak Peace unto Nation’.

End of the day

On leaving later in the afternoon guests were presented with a commemorative booklet which, in the words of the foreword, “provides a short pictorial history of Daventry Transmitting Station, and is a tribute to the technical innovation and achievement of those 67 years at Daventry and to the commitment and dedication of the staff.” The opening page showing a colour print entitled “The Prospect Daventre and Borough Hill July 1719”. The booklet which indeed contained interesting archival pictures and stories over the 67 years and concludes with the following: -

“Although transmissions from Daventry have now ceased, the BBC link with the Borough Hill site will continue. A single mast will remain for communications purposes, and Daventry will continue to be the base for the maintenance of BBC Domestic Services Radio and Television Transmitting Stations within a 50/60 mile radius of the site. Other specialist maintenance and stores and supply services will also remain along with the Radio Tape Reclamation Service, and it is hoped that some form of permanent exhibition may be established locally illustrating the near-67 years life of the Daventry Transmitting Station.”

Questions and answers related to the above article

Q1: Why did the Daventry site close?
A1: Mainly because of the end of the cold war as VOA did not need Woofferton anymore.  By this time the Daventry antenna infrastructure needed major re-engineering work and so it was more cost effective to close Daventry and move services to Woofferton.

Q2: Didn’t the transmitter site in Droitwich replace 5XX?
A2: Droitwich took over the National Programme transmission from 5XX on 7 October 1934.  According to Pawley: "In January 1945 the former 5XX transmitter at Daventry, which had been used in the Overseas Service long-wave group until December 1944, was transferred to an Air Ministry service on 530 kHz, using keyed Morse transmission, until the end of hostilities in Europe".

Q3:  Why did they build the site in Dodford?
A3:  The third programme mast and ATH (Aerial Tuning Hut) were at Dodford to avoid interference with HF transmissions.

Q4: Why did they have the Third Programme transmitter in the old 5XX building when they had the site in Dodford?
A4:  a) A substantial power supply was needed, b) To have the transmitter close to the staff and administration.

Q5:  How long did 5XX exist in the old building until they replaced it with the Third Programme transmitter?
A5:  The Third Programme transmitter started its running-in period on 8 April 1951 so 5XX was probably removed in about 1949.

Q6:  Why did they have diesel generators at the Daventry site?
A6:  Diesels were installed in about 1936/7 when mains electricity was not sufficiently reliable.

Q7:  Why was the site re-engineered in the ‘60’s?
A7:  To increase power output radiated, provide more senders, replace worn out equipment, automate and reduce staffing.

Q8:  Why were the Rampisham, Skelton, and Wooferton sites built?
A8:  To contribute to the war effort and combat propaganda on short wave by Hitler and Mussolini.  The Rampisham site was acquired in 1939 then, according to Pawley, "In early 1941 consideration had been given to a much more ambitious development.  It had become clear that the demands of the Overseas Service could be met only by a major expansion of short-wave facilities.  It was decided to build eighteen more high-power transmitters at three separate sites.  One station [Woofferton] was to be used mainly for services beyond Europe, one for the European Service, and a third to serve Europe by day and more remote target areas by night [Skelton A and Skelton B]."

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