Modded AB763 Amp V2 Farting

[Matthias]

I built the stock ab763 circuit with success and started a new modded circuit.

Modded circuit is AB763 WITHOUT Vibrato circuit - result of a handful of gain mods.

The problem: V2 has distortion farts on the lower notes and increases with preamp volume. This is without the reverb on.

I installed a master volume which works nicely - I know its nothing with PI or power tubes. I disconnected v2 from the v3/reverb and went straight into the PI: Sounds perfect. Also, clean channel sounds great.

When is use a resister instead of a reverb pan, the reverb pot results in fair sounding distortion - so that part of the circuit may be ok

When I connect V2 back to the reverb circuit it goes all farty or non harmonic distortion like.


Will post VOLTAGE TABLE asap. I checked them briefly and they seemed ok but I hardly know what i'm doing!

Could the removal of the vibrato tube and circuit mess up the bias or supply voltages, or does my amp just hates me?

[sluckey]

Did you use a 47K resistor to simulate the removal of the Intensity control? See this schematic, just prior to the .001µF PI input cap.