Distorting reverb tube

[Tomislaw]

Is this interaction between the reverb driver and the preamp tube before or after changing the grounding? Because the reverb driver is a small power amp unto itself, it is sometimes beneficial to place it out of sequence as far as the signal path goes, and/or to give it a huge filter cap of its own.

It was still happening after the reconstruction but only with the 12AT7 in place. The amp had a different tube there when I got it, but I assumed they had been changed and possibly mixed up at some point and since it's basically a slightly modified Princeton Reverb circuit, I thought the 12AT7 should be reinstated as in the original Fender amp. There were so many other potential causes of oscillation/intermodulation that I thought the problem was not actually caused by this particular type of tube. But I think I also tried the 12AX7 in V2 before the reconstruction and it was even worse. 

Reverb driver is on the screen node along with the first half of V4 which is the tremolo oscillator, but I added 8K2 and 12K resistors between them and installed two 22uF caps. V4B (concertina phase inverter) had its own stock filter cap. I also separated V1 and V3 with a 10K resistor as they were hanging off the same node, and added a 22uF decoupling cap for each tube. 

V1 was running smooth without V2 in the socket, regardless of the preamp output volume. Probably the 12AT7 should have been biased a bit cooler, but I left the stock Rk/Ck values when it turned out the 12AX7 does the job. I actually ended up putting the ECC832 from the original tube complement in there. But it was kind of surprising to see a lower mu tube cause some trouble, but it wasn't the case of a reverb driver actually clipping the plate signal.

Another lesson learned - don't waste your time trying to troubleshoot an amp with really bad grounding and shielding. I thought I'd be able to get away with just a few quick fixes, but not this time.