I randomly found a 6V6GTA vacuum tube in the rubbish bin of a used book store. They wanted 3$ for it – how could I not? Visually, it did not have any indication of failure, or any mechanical rattling. So it might actually be functional! They did warn me that they had no idea if this was the case though :)

Anyway, I had heard that tubes can be configured to work at much lower that the typical voltages, if you design with them differently. I’ve seen as low as 3.3V reported! I figured it would be fun to make a portable tube amplifier powered by a rechargeable lithium cell.

I did not have a tube socket (and was not going to buy a $$$ one from some fancypants audio store), so jury-rigged one from protoboard using a drill press and soldering directly to the tube pins. So far I’ve tested the heater, it draws an appropriate amount of current (~400mA @ 4.2V).

I sort of expected it to explode (implode?), burn out immediately, or otherwise fail spectacularly at this point. So I was unprepared for the apparently normal operation – I did not have an audio source handy to test it with. That will have to be a story for another day.

It’s set up as an inverting amplifier, so it might be funny to give it a gain of 1 and subtract the input from the output (e.g. digitally or via a summing amplifier). People say things like “tubes sound warmer” – I have no idea what this means, so logically I must investigate. The difference between the two signals should give me the sound of pure warmth, right? :P