Resilient, Reliable but not eco-friendly – that’s the electronic marvel of the last and current century – the vacuum tube.
Through the last 100 years the biggest breakthroughs in thermionic tube or valve designs were the multi-electrode devices, from triode, to tetrode to pentode and of course the multi electrode hex-heptodes. All of this was before the second world war.
Our “modern day” audio equipment using this technology may be dated but just seems so resilient. With the innovation at Bell’s Labs, the invention of the point contact transistor and then later Kilby and Noyce’s integrated circuit it is no wonder that continued research into tube manufacture was deemed as fruitless. Yet tubes have survived, indeed very high powers today are still predominantly controlled by vacuum tube technology – even your domestic microwave oven uses an electron tube, the magnetron, which with ease radiates upwards of a kilowatt peak. With all its simplicity in operation the vacuum tube of the 1950s, 60s and even 70s carried a trademark of craftsmanship and resilience. It is also no wonder that the top lighting manufacturers world wide were also the biggest manufacturers of vacuum tubes. One such manufacturer is Philips, a Dutch company based in Eindhoven that progressed rapidly by hiring some of the brightest minds around at the time. Philips/Mullard tubes are still recognised today as being almost the ultimate in manufacture and are in huge demand amongst audiophiles. Much thanks here of course goes to the foresight of founder members (Gerard and Anton Philips) and scientists in their employ, one of which was the gifted Bernard Tellegen.
The ultimate electronic marvel – the pentode
With the pitfalls of the triode (Lee deForest) and thereafter the tetrode (Walter Schottky) becoming increasingly apparent in tube design and specification, especially in the R.F. field, Bernard Tellegen played a pivotal role in reducing secondary emission and the now well-known tetrode kink of tetrodes by including the suppression grid, to become founder father of the pentode and the pioneer of the EF50, accredited to the team lead by a Professor Jonker. The EF50 was originally designed for television use but found it’s way into radar receivers, an almost indispensable device due to it’s lower operating voltages, footprint and more importantly high gain and cut off frequency. Although it is widely known that Philips held the patent to the pentode tube, it is my opinion that Tellegen’s work is one of the biggest reasons why tube amplifiers still remain so popular today. (see Wiki EL34 / 6CA7)
Another electronic marvel – superhetrodyne receivers with triode-hexodes
With multiple electrodes in one package manufacturers were able to supply devices which were used as local oscillators/mixers, split triodes, split pentodes and triode/hexodes. In my research patents were granted to General Electric – triode/hexode, also Heintz and Kaufman.
In our line up one must not forget the cavity magnetron by John Randall and the Klystron by brothers Russel and Surgan Varian, both being extremely valuable devices in the development of microwave radar and of course, modern television and telecommunication microwave transceivers.
If one thinks of the array of semiconductors used to drive a 4 Ohm load to 1kW, the power outputs obtained through paralleling or push pull driven tubes to a few hundred kilowatts was absolutely unbelievable. Modulation at 50%, the power amplifiers of the modulator were equally awesome. All of these transmitters and modulators were water-cooled and had free standing power generation. To date, there are both old and new school engineers calling for tubes to be used in high power transmitters, In high lightning strike areas tubes are almost always preferred because of their resilience.
Some excellent further reading:
- The EF50 – an interesting look at the evolution of tubes and what made the EF50 significant.
- Paul Thurst – Blog on Solid State versus Tube Transmitters
- Station WLW in Ohio – overview of extreme powers