65 super reverb Clipping/break-up

[Normybaby]

OK, A Friend brings me his 1965 Super Reverb...Complains:
Clipping/distortion/break-up when played/pushed hard.
Story: He brought the amp to a (fender expert) tech a year ago. The tech did something to it but, the problem is still there.
I opened it up and this is what I found:
Power Supply and Cathode bypass caps all replaced with good ones (Sprague Atom) caps; a few resisters where also replaced. I found that the coupling cap to the PI was replaced with a non-stock (.018 uf) cap. Also the Neg feedback wire had been disconnected. I installed the correct coupling cap (.001uf) and reconnected the feedback wire (he wanted everything back to stock values). I checked all the voltages and also ran a scope test with full power ( started to clip at 36 watts at 400 hz); the speakers where also check, everthing looked OK. To me it sounds like that amp is just pushed a little too hard. He is a strong player and it happens when he hits a power cord or low string/5ths. He claims that it didn’t do this before. Any suggestions where to start?
Normybaby

[Enzo]

Yeah, start by isolating the problem. WHERE is it clipping?

The caps may be new, but how well are they working? That is how much ripple is there under load, and is the signal getting onto the B+ at any node?

Is it actually clipping or is it a parasitic?

[Normybaby]

Thanks, I only had the amp for a short time. The only thing that I checked was the voltages and the SineW signal at the output, full power. The amp sounds good, i think when played at a moderate level. He described the sound as being like snapping a deck of cards on table when shuffling with your thumb; again this happens only on the attack when playing low string hard
I'm going to have the amp back and I will follow your suggestions
Thanks again
Normybaby

[Sweetfinger]

Try a different speaker cab. Maybe the spkr(s) are bottoming out.

[Normybaby]

We tried different speakers with the same results. I just received an email from my friend with amp, he got this info from the Fender forum. I would have never suspected that the reverb driver tube would interfere with the signal path:

I pulled the 12AT7 (reverb driver tube - V3) and no clipping. Also no clipping when I disconnected the reverb tank, even with the tube in.

Response from an amp builder:

Ah, good news. You've narrowed it down.

Make sure the cable and connections on the 'send' side of the reverb driver are all good. Be sure to get inside the jacks on the amp and the tank and snug up the contacts for the center conductor as well as slightly crimping in the shield connections on the cable. Check the cable for good continuity. Re-tension the tube socket contacts.

If everything is tight and solid, try a different 12AT7. (I tried a different 12AT7 Norm to no avail) If it's not that, try hanging a 500pF cap between the plate and the cathode like the SF amps.

If all that still doesn't cure it, it's probably an arcing reverb transformer. They can die from being run into an open load just like a regular OT. Fortunately, they're cheap.

Let your tech do the work if you're not 100% confident around tube amps and high voltages.

[Normybaby]

Update: The problem was in the tube socket on the rectifier tube. I cleaned and reset the pin holders on all the sockets and the problem went away.
Normybaby