A Madcap look at Radio Valves (Thermionic tubes) – part one

I think everyone over the age of 40 should be culled! Youngsters at work have never heard of the Holocaust, the Berlin Wall and now, thermionic tubes. Remember the book Logan’s Run? Sometimes I think I am in the book. Well for starters when I was about 13 I took up electronics as a hobby – finances at 13 don’t normally allow one to build snazzy circuits but I do recall getting a lot of parts from my dad and grandfather both whom were avid tinkerers. Remember the OC71, the OC72, the AD161 and the AD162? The OC71 lead to disasterous results when used in amplifier  circuits once the layer of black paint becamse scratched. The OC series were old MULLARD devices but due to cost ‘tinkerers’ kept to thermionic tubes/radio valves. Can you imagine that today?

Power Tetrode
        Power Tetrode http://www.indiamart.com/impex-international/vacuum-electron-tube.html

My first audio amplifier used two EL84s in push pull and at 15W was pretty powerful in those days. Actually 5W per channel in those days was pretty powerful, 15W was positively ‘disco’. My first semiconductor amplifier used an AD161/162 which my old man helped me build – actually he built it but it never worked. I actually built my own one subsequently and it did – testament that fathers should never get involved in their kid’s projects.  But no matter how enthralled I was with semiconductors radio valves captured my imagination – most of the time the circuits worked and secondly, possibly more importantly was the fact that I worked on 350V DC equipment without supervision, strictly VERBOTEN of course, but who cared. After school I used to work on these projects and then slip them under the bed so the folks wouldn’t know. Audio and wireless were always my first passion. I found ‘tubes’ were easier to work with because one only needed tag-strip and I had hundreds of tubes, chokes, capacitors etc – semiconductors would self-destruct and were costly.  Most semiconductor circuits I fiddled with were of the germanium variety and they were not at all very robust, especially at the hands of a young teen. I learnt two things through my own tinkering – experience comes from doing it yourself, not like Googling, secondly, I could help others with their own projects without Googling.

Once I had gone passed the ‘learning curve’ which at 13 or so is only a matter of hours I became an ‘expert’ – had some pocket money stashed and embarked on my first ‘real’ project – a 100W amplifier using 2N3055s. They were pretty new devices in those days so I couldn’t wait to get the project underway. Like all enthusiasts the end result always looked a bit shoddy, especially since I wasn’t a sheet metal worker – in those days sheet aluminium was pretty cheap and like all enthusiasts ‘patience’ is something which is sadly lacking. At time of switch on I also realised that knowledge was also sadly lacking – there was a loud cracking sound which blew the drivers off the veroboard and the 2N3055s were no more. I never found out what caused this but in hindsight it was a combination of impatience and over-confidence. Funny enough years later I could design and build my own audio amplifiers and most of them worked well – this was after a bout at college where they taught you things like electronic theory and practical design. One thing which college taught me well (this was in the mid to late 70’s) was how popular thermionic tubes still were when it came to reliability. The first Raytheon radars used a SCR/snubber arrangement instead of the tried and trusted Thyratron – how quickly they went back to the old tried and trusted arrangement.  (possibly the lesson learnt was never to use display engineers to design high powered applications used in radar).

Radio valves or ‘tubes’ are actually simple devices and I found that understanding the theory behind their use in audio and radio applications made it easier to understand how an NPN transistor would operate in the same circuit – especially oscillators and amplifiers (using the triode as the active component).  Our digital youth would no doubt smirk at this but remember that we were made to think ‘tube’ – semiconductors only started becoming popular in the 70’s and most marine equipment still used ‘tubes’ in their circuitry – almost certainly all SSB transmitter power stages. A very big lesson I learnt in my younger days was never to probe around on valve equipment while live – anything which uses high voltages and low current which in turn means the active component (the ‘tube’/valve) is very sensitive to tampering around the input – a lesson which bodes well when working with MOS devices. Just touch the gate of any FET in a high energy circuit while live and you’ll see what I mean. Valves are just way more robust than semiconductors – transistor transmitters fail because of a mismatch to the load so the protection circuitry is often more sophisticated than the drive and frequency generating circuitry. Not so with valves, one could hear the arcing to know that there was a problem – I never had a valve fail on me. Blast air cooled valves were small devices and at 1500W PEP there were normally two devices in parallel that would out perform a stack of transistors any day – this is still seen nowadays. But don’t switch off the blower – these valves would overheat within 20 seconds and trip the PSU. Another thing, all SSB transmitters used conventional power supplies, none of the switcher type which in those days was VERY unreliable. Radar sets are a case in point.

I am not a keen follower of switched mode power supplies in circuits where the current draw is not constant or where there is rapid switching between low to high current draw states, audio amplifiers are a case in point. Simplicity for reliability is the way I look at it – likewise most over-engineered machines just seemed to lack reliability in the 70’s and 80’s.

Many years ago I wokred on a fishing vessel where the owners had moved one 400W transitter over to another vessel because they were selling off the older vessel. They complained of a loud cracking sound coming from the transmitter. On inspection I found that the older vessel was running at 12V, the newer at 24V DC. They had not configured the transmitter to run on the higher voltages, the inverter strangely enough wasn’t damaged – the 900V plate voltage had stepped to 1800V as my poor Fluke tried to tell me – but after configuring the input supply the transmitter ran perfectly. I doubt any transistor would have managed to dissipate that amount of energy without shorting out – yet the only protection circuitry in this transmitter was heat detection and an LED which came on when there was a serious mismatch at the antenna.

Going one step further, a friend of mine worked on a fishing vessel and he asked me to check the unit because they felt the range was a bit limited and the skipper was thinking about getting another Tx – what had actually transpired was the transmitter had been tuned for maximum cathode current (into the red zone) which is where they thought it was maximum transmitting power and NOT a dip in cathode current – they had been using the transmitter like this for going on two years with not a grunt of disapproval from the output devices. In fact the current they were drawing from the PSU was over the specification of the tube. Transistors have a max current rating, a max voltage rating but don’t try the two simultaneously.

End of Part One