Tremolo Thump On Cathode Biased Amps

[Ted]

I've been trying to squeeze a little more tremolo depth out of a cathode-biased Ampeg Reverberocket.

Its the classic bias trem circuit, where the tremolo signal is injected at the base of the grid resistors of the 6V6 output stage.

As I wind up the trem voltage the thump gets more and more noticeable.

Anyone know any tricks to reduce thump?

The diode across the intensity pot does not work (unsurprisingly) for cathode biased amps.

[pdf64]

Are the 6V6 well matched?
If not, common mode signals, such as the trem, won't get properly rejected.
If matched tubes aren't seen to be the way forward, then consider a separate cathode resistor (and bypass cap) for each tube; that enables each tube to find its own static level, rather than being forced to operate at a common static level. Tubes then generally settle into closely similar static levels, so as long as their ac gains aren't too far dissimilar, common mode signals should then get amplified evenly both sides, and so rejected when summed.
Also ensure that the reservoir and screen node B+ caps are fit for purpose.
Pete