Wartime Radio Transmitting Valves

The following valves were used during WWII in transmitters and being in plentiful supply afterwards were snapped up by growing numbers of radio amateurs

The 4X150 RF Power Tube

This is a pair of 4X150D high power VHF valves using what's commonly known as "disc seal" construction.

Cooling air is forced through the fins at the top of the valve to get rid of waste heat.

The pair may be run in push-pull to deliver 600 Watts of power with a couple of thousand volts applied to their anodes.

These examples have 26.5 volt filaments. Why that voltage? Maybe its related to 12 volt accumulators, a pair of which would have a combined terminal voltage of 26.4 volts would suit their rating? Many military vehicles used 24 volt accumulators.

Valves not unlike these examples are still used by amateur radio enthusiasts to generate power at 2 metres. Although transistors may now be a better source of power at the sort of level provided by the 4X150, bigger brothers of this type of valve, the 4CX1000, are still used to develop kilowatt powers at both 2 metres and 70 cm and if you want a REALLY BIG valve try the 4CX35000C. Check the junk box for a suitable transformer first though because the filament needs 10volts at 300amps!

The overall diameter of these 4X150Ds is a little over an inch and a half

By the way did you know that some of these types of valve can be lethal. They use beryllium oxide in their construction. This is an excellent insulator as well as being an excellent conductor of heat. Its dust will kill you if inhaled. The same goes for a lot of HF, VHF and UHF power transistors. They use a disk of the stuff to provide a good heat path from the base of their chips to their mounting stud. When purchased new, information is supplied to warn you of the danger but when bought second-hand or "government surplus" they rarely do!

The 813 Beam Tetrode

A pair of 813 transmitting valves used to represent every radio ham's ambition. They could provide 650 watts of RMS RF output with 2,500 volts HT. Although this really represented the maximum power output allowed by licensed amateurs in the USA, many in the UK would happily operate this sort of power level until caught out when interference to local TV sets, caused the GPO to descend on them and demand a demonstration of their rig. Sometimes a wily amateur would be prepared and have something up his sleeve to thwart the measuring equipment! The reading on my plate current meter had to multiplied by two for a true reading and the voltmeter read half!

In the UK we were allowed 150 watts of DC input power together with an efficiency of not more than 75%. Whatever the sums one did, the permitted maximum output power allowed was no more than 100 watts RMS. When SSB, or single sideband techniques, arrived on the scene it became quite difficult to determine the equivalent output power from this mode as RF power was only generated whilst one spoke into the microphone. To reduce the power one spoke in hushed tones. To increase power one could SHOUT

LOUDER

and LOUDER

and LOUDER

but a GPO test was conducted at only

a whisper.....

The mathematics said that the power output was 2 times root 2 times the average DC power. A pair of 813s could develop nearly 2 kilowatts of what was called PEP output.

The terms of the license unfortunately only permitted one to use 440 watts PEP INPUT so a pair of 813s was not a bad bet especially when their carbon anodes were glowing red!

The 811 High Power Triode

A few people used an 811. The valve could put out 200 watts of power in CW, that's using morse code. At that power the anode which was just shiny metal wouldn't get the chance to glow red unless the key was held down. For speech the input power could safely be allowed to run at 150 watts. As this was the legal limit a single 811 was adequate in the UK.

These valves could also be used in Hi-Fi applications to run lots of audio power. A pair could develop 155 watts. This is interesting looked at in terms of modern power ratings. The 155 watts represents what is called "Root Mean Square" power....

When transistor amplifiers started to take over from valves, manufacturers needed to think up a way of making them look attractive. To this end they came up with a number of new terms intended to hoodwink the user. "Peak" power was one and "Music" power was another. When stereo arrived on the scene they added together the two channels and allowed users to think the power was really associated with one loudspeaker! The other day I bought a pair of loudspeakers for a computer. They were sold as 160 watt speakers. The plug-in power unit had 12 volts 9VA marked on it. The power supply therefore delivers a maximum of 9 watts RMS. There must be a new conversion scheme for computer speakers! Allowing for a realistic amplifier efficiency of 70%, the waste heat is 2.7 watts and the power available for the audio is 6.3 watts. Multiply this by about 25 which is presumably the new fiddle factor, to arrive at the "rating". In other words to find the actual power from your computer speakers, divide what it says on the box by 25. For a single speaker of the pair make that 50. Therefore the real output, given a fair wind from one speaker is about 3 watts. That's about right as half a watt RMS can fill a room with sound.

Back to the 811..... if you used a pair of 811s with your computer you could expect to hear 3,870 watts. Now that's what I call POWER! The only problem is finding a big enough loudspeaker!

The 807

Most law abiding radio hams using government surplus parts for constructing their transmitters used a pair of 807s. Shown here on the right is a CV124, a military coded "807" with a ceramic base. On the left is an ordinary type. As these valves cost around five shillings (that's 25 pence in modern monopoly money) not many hams couldn't afford a pair of 807s.

The valve represented the UK legal limit in terms of CW power. That is....one's morse transmitter provided 100 watts of power. Using AM the power output had to be dropped slightly as the anodes tended to run a bit hot at this level. Some surplus types were not very understanding and residual gas was liberated from the white hot electrodes causing the valve go soft and making it glow in all sorts of pretty colours until finally expiring. Usually the chap at "Super Radio" or whatever the local store was called would accept it back and hand over another!

The ubiquitous 807 also made a good audio amplifier and was often pressed into service for providing the modulator for the AM transmitter. A pair in Class B could provide 120 watts of RMS audio power.. quite adequate for a legal AM transmitter.

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