As part of my delvings into things vacuum tubes, I of course found that many others have been down this road, including this rather interesting receiver built by Alan Yates. Being a novice at this, I was intrigued by the fact that his circuit used the 12DZ6 tube, which could apparently be powered by ordinary 12 volt supplies. Its application was in automobile radio circuits, where high voltages were inconvenient (when needed, they were often generated by “vibrators”, which created an AC voltage from a DC battery voltage, which could then be stepped-up by a transformer). When I mentioned this to Tom, he thought (as I did) that it was odd, and that he didn’t understand how tubes could operate with such low plate voltages. But of course, if you search, you shall find, so I uncovered this terrific page that describes some theory and circuits that use these low voltage (and presumably safer) tubes in radio circuits. Nifty.
Ah, that old hack – I still use it some times. It isn’t a very good design, mainly because I simply wasn’t raised with tubes and have little experience designing for their high impedance properties. The plate resistance should be larger and I cheated to generate significant AF power and used solid state electronics in the AF amplifier (largely because I lacked a suitable AF output transformer).
Space charge tubes are a fun and safe way to play with empty-state electronics. But you can’t get much power out of them without higher voltages on the plate, their impedance is just too high. It is a limit of the physics of vacuum tubes.
I’ve been thinking about this lately. I have a collection of vacuum tube technology in the junkbox and was working on switchmode inverters to build a lab supply for tinkering with it… However I have come to the conclusion that while an interesting exercise in history there is really no point to use vacuum tubes in any modern circuit not involving enormous powers. Solid state is just superior in almost every way except for a few niche areas.
That said I still have a huge ambition to build a neon-lamp based clock or computing device. I am going to use solid state rectifiers though and probably a solid state PSU. Neon-logic is fun, simple and different.