I want to make a micropower tube amp... calling tube gurus!

[petemoore]

  Use mushy feet for the cabinet.
  Thick dense wood for the cabinet.
  Consider a 'semi megaphone' type cabinet if you want
amplitude directionality, especially effective at higher than
bass frequencies, box-shape, something with a top or bottom board by which waves are
directed foreward, speaker in the back, offset, rear of cone enclosed and dampened.
  _____
  l  ___
  lV     l
  l------l
  "V" being the speaker, enclosed rear cone, channel for the soundwaves to be directionalized.

[Cliff Schecht]

I like the 6AK6 power tube in the microamp I posted in the submission thread. Running this tube with pentode operation gives a bright and punchy output that distorts nicely. Operating the tube as a triode would drop the power even more and would probably tame some of the brightness of the tube. I haven't tried a 12A*7 tube as a push-pull pair so I have no comment on doing this, aside from that it seems popular in designs like the Firefly.

Thanks for the suggestion.  Would you be willing to suggest some mods to your switchmode power supply for variable voltage operation?

Running any tube at a lower voltage will allow you to drop the power levels reasonably. But you should be careful about how small you really go, a 50mW amp won't sound much louder than an electric guitar with NO amp. You should probably be aiming for something more like 1/3rd to a 1/2 Watt of output power, anything else would be pretty useless IMO. The amount of power you get out also depends on the speaker size you're running into. An 8 Ohm 1W speaker isn't going to be anywhere near as loud as a 4x12 8 Ohm half stack.

Is there a certain sound you are going for with this amp? Are you looking for a clean amp or something that can get it's own distortion?

I guess the sound I'm going for is the "non-master volume tube amp cranked up to 11" but at about the volume level of a typical acoustic guitar.  The reason I was thinking 50mW is because when I crank my LM386 (at less than 0.5W) amp, it's WAY too loud to play at home.  Based on the "rules of thumb" for power vs. loudness, I figure a factor of 10 less power than that, with the same speakers, would be about half as loud, which might be usable.  I know simply lowering the voltage will drop the power, but will also change the way the tube distorts.  I don't want it to sound like a starved-plate "tube distortion" circuit.  I want it to be as close to a "real amp" sound as I can get, minus the volume.

On the topic of output iron, DC bias does have an effect on the overall frequency response and saturation level of the output transformer. More DC bias will cause the OT to saturate quicker and give you that nice sounding tube compression and distortion quicker. I believe that DC bias makes it easier to achieve asymmetrical distortion which is desirable in some amounts in a guitar amp.  Also under higher saturation levels, the transformer has a frequency limiting effect that depending on the application can also be desirable.

Another reason to keep the supply voltage higher... or add a third winding to the output transformer for adding DC offset.

I think that choosing a plate resistance as high as 40-80k is not going to give very pleasing results. In a single ended amplifier, you are going to get a LOT of odd harmonic distortion which is going to give you a very high gain, distorted sound. But this may not sound so nice, the amplifier may sound weak and, well, underbiased. The low end will be weak and you will always have a high end buzz that will sound more like a bad fuzz than a good tube amp. If you want that real tube sound, I recommend sticking to something more like 5k-15k.

Thanks!

The power supply is already designed to go from ~125V to ~250V without any component changes (rotate a trimpot). Unfortunately, the version that I have built in my microamp has a problem with "locking up" at higher voltages with the controller IC I used. I should have used the more modern version of the IC (old UC3842 vs. UCC3800) which has a lot more built in functional and safety features that usually must be implemented with external circuits. I need to build at least one more prototype before I release that design into the public though. The cool thing with that supply is that if you have a heatsink and a small piece of unetched double-sided PCB material laying around, you're only looking at about $20 to build the supply. If I didn't have 10 projects going at once I would have finished this by now but as always, I have more projects than time!

Also, with respect to the DC through the windings, I'm not sure a third winding just to saturate the transformer is such a good idea or really that feasible if you don't wind your own OT's. Just adding a DC bias winding to the transformer would give you a controllable saturation but at elevated power levels you are quite likely to blow something up if you aren't careful. This happens naturally in single-ended power amps because the windings have to carry both the DC bias and the AC wiggle whereas in a push-pull amplifier, the OT deals with an alternating DC bias that tends to cancel out if the system is designed properly. This cancelling action allows for much higher currents through smaller transformers because they don't have to deal with substantial DC biases AT ALL TIMES like a SE amp does.

Of course all of this has an effect on how the output stage will sound as well, at least by itself. Single ended power amplifiers with a reasonable plate load (say 3k-8k range) tend to have a lot of 2nd order harmonic distortion which rocks for achieving a musical sound, but not necessarily a great distorted sound. The best distortions do a nice job of emulating a well designed tube amp in that they both emphasize a certain mix of even and odd harmonic distortion. A push-pull amplifier is nice here because they introduce both even and odd distortion just by their operating principles. Even-order distortion in a push-pull amplifier is supposed to be cancelled out by the push-pull action with a set of matched tubes, but this is almost never the case because tubes and transformer windings are impossible to perfectly match. So use non-matched power tubes and you've already got your more pleasing even harmonics. Then there is crossover distortion, which is purely odd distortion. Negative feedback from the output speaker to the power stage driver (phase inverter) is meant to cancel crossover distortion but never does so perfectly and so voila, you have odd-order distortion. Mix these two together in the right amount and you'll have the next Trainwreck, albeit in your case it'll be a <.5W form .