Super Reverb: Reverb channel distorting

[dude]

The 71 SR I fixed for a friends kid's tour has a problem. His band is the opening act for some weller known band. He's still on tour playing the amp, the problem is the reverb, vibrato channel is distorting but the main channel is fine so he's using that channel till he gets back and I can check it out. I know it could be a lot of things. The power supply is good (main channel is playing nicely). I replaced almost every component in the power supply.
So till I get it opened, could this distorting be caused by a tube? Some are very old. If so, what tube or tubes most likely could be the culprit. He can easily get a tube somewhere. Just a shot in the dark, I know. *Looks like the second 12ax7 feeds the input to reverb channel.
I suggested he let me replace all the coupling caps and R's, some there from 1971, it's an AA1069 (CBS 1971) circuit changed to AB763.


al

[Slimtim]

could be a tube,try switching 1 and 2

[pdf64]

Does the reverb control affect the nastiness?
If so I’m thinking a bad cap on v4 cathode. Were the board ecaps replaced?