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

  • Saigonauticon@voltage.vnOPMEnglish
    1·
    4 months ago

    I hooked up a random MP3 player module, and… no output.

    Then I fixed a dumb wiring error in the bias network, and got signal! Gain is OK, but the output impedance is terribly high. I guess this is why people use tubes at their recommended voltages.

    Anyway, I can hear it on an 8 ohm speaker, and it might be able to drive headphones! If not I can always add a little buffer. The important thing is that the audio is actually passing through the tube at some gain, and you can see the heater glow slightly.

    I don’t have any music, so it’s just playing the horn sound from the tripod walkers from ‘War of the Worlds’.

    • Saigonauticon@voltage.vnOPM
      1·
      4 months ago

      I added a 21700 lithium cell and charging circuit, it can power the tube for 8-10 hours. It is now a ridiculous pocket tube headphone amplifier.

      You know, for those times when you don’t mind your pocket being uncomfortably warm.

      Actually, that seems to be this device’s main feature. I have renamed it the “thermionic pocket warmer”. It also sort of works as a headphone amp. I fully realize that this device is both ridiculous and bizarre.

      • Saigonauticon@voltage.vnOPMEnglish
        1·
        4 months ago

        OK, so while the tube does produce (inverted) output, the output impedance is waaay too awful to drive anything much more than my oscilloscope. So I increased the voltage to 5V, and I could hear it better on headphones, but it still sort of sucked. So I added some salvaged Soviet capacitors I found in some other junk pile to stabilize the power supply a bit, and then added a mystery Chinese audio IC (not much is known about it, I also found it in a pile of junk) as a buffer.

        This thing… is an atrocity that has no business existing – in other words, I think it’s pretty cool. It plays sound out of an 8 ohm speaker quite loud. I should probably add some volume control at some point.