Distorting reverb tube

[Tomislaw]

Even more fun with reverbs in Fender clones. I'm builing a second AA1164 Princeton Reverb now with some design modifications like additional decoupling, elevated heaters, proper ground wiring etc. The reverb driver tube is doing some weird things in both builds and I have no idea what to think about it. Reverb sounds OK, but scoping the V2 plate I see how it starts to distort and most likely oscillate when the preamp volume pot is turned up. Actually the voltage swing on the 12AT7 grid is quite high at the maximum preamp output, around 66V peak to peak. The signal is fed to the grid through a 500pF capacitor and there's a 1M grid leak and a 47K grid stopper (not included in the original schematic). As expected, increasing the grid stopper value does not prevent the oscillation and attenuating the signal at the V2 input makes the output too low to drive the reverb tank. No improvement with different Rk values either. Design mistake perhaps?

I'm going to examine the wiring again to make sure there's no ground loop or interference with the tremolo circuit which is on the same power node as the reverb driver, running from the Vs filter cap. I think I will temporarily disconnect the oscillator triode, leaving only the V2 which is already properly decoupled. The ground wiring follows the supply node arrangement, meeting the main ground bus at the Vs filter cap. It looks like this:

Galaxy 1:
Star 1 (preamp): input jacks, V1A and V1B cathode resistors, 6K8 eq resistor, Vol pot
Star 2 (reverb recovery): V3A grid leak, reverb off switch, V3A Rk, reverb pot
Star 3 (V3B gain stage): NFB tail resistor and speaker jack
Star 4 (phase inverter): 56K tail resistor
Star 5 (power amp): V5, V6 cathodes
Galaxy 2:
Star 1 (reverb driver): grid leak, Rk and reverb transformer secondary
Star 2 (tremolo): grid leak, V4A Rk, 22nF cap, 100K resistor and tremolo off switch