M15mk1 - mod for more gain on crunch channel

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    • M15mk1 - mod for more gain on crunch channel

      I've managed to finished my m15mk1 build. It's been a long time completing this build.
      I encountered problems with the output transformer. I was sent another one, which also didn't work!
      I sourced a different 18w transformer and now the amp works and sounds great.
      It would appear these ATRA 0427 transformers are prone to faults as many seem to have the same problems as me.

      Anyway. Build is done. Voltages see a little high but my wall voltage is 245v so to be expected I guess. Sound is beautiful. Loud and clean. No hum or excessive noise.

      I would like a little more Crunch channel gain though. Not sure if this is because I'm using a different output transformer.

      Any mods for creating more preamp distortion. I've read that lowering the b+ to the preamp tubes (crunch) and altering the cathode bypass cap can also have an effect. My knowledge isnt good enough to figure out the values though.
      Can anyone help? Anyone tried this?

      Ryan uk
    • Hi Ryan - and welcome to the forum.
      The sound might be different if the ratio of the new OT is completely different to the original one but it's very unlikely that this is the reason for missing crunch - I also assume that the new one is not too much different to the original one. Are you using a Strat, Tele or other guitar with single coils? With these guitars the amp will stay clean for higher volumes but even with single coils the power stage will distort at higher volumes. So if you don't hear any crunch sound there must be something wrong with the circuit you've built. If it's OK and you want more crunch from the power stage (and the phase inverter stage) the easiest way is to replace R13 and R14 with the other value respectivley (R13 to 33k and R14 to 68k, this reduces the attenuation). If you still want more crunch you can change the connection from C11/R15 to R13/R1, connect it directly to the cathode of V2b (pin 3), this completely removes the attenuation before the tone stack.
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