Fender Quad Reverb clicks, pops and rumbles

[blackbird]

I have a '76 Fender Quad Reverb. It is basically a 100W Twin Reverb in a bigger box to house 4 x 12" speakers. It has a Master Volume control in addition to the two channel volume pots.
There may be 1,000 hours of use on it, and everything is original except for one 12ax7 tube.
It got noisy so I just used my other amp for the last year or so.

1. Yesterday I decided to check it out and fix it.
First I tested all the tubes. The 4 6L6GC's are a little leaky and read at 70% quality. The 2 12AT7's and 4 12AX7's are also a little leaky and read at 80% quality. I reinstalled them into the correct sockets and turned the amp on.
First noise to come out at warm up was the vibrato ticking quietly.
Then the reverb began echoing quietly. I plugged in and played my Stratocaster (a guitar of known quality - no noise with my other amp) through it. After 5 minutes a demonicly loud "buzzy- whisteling-screech" forced me to shut her down.

2. Today I fired it up again, doing two things different from yesterday. I noticed that the ground switch was a "off" so I immediately switched it to "A". I also took the reverb/vibrato footswitch out of the speaker cabinet.
Same startup sounds: quiet vibrato tick and reverb echo.
However, I got no loud buzzy whisteling screech. BUT I DID GET a popping sound when picking notes, and a "raspy rumble" on the bassy side of the scales. I tried to observe speaker motion from the rear, but didn't see any telltale signs.

That's it. I'm sure some of you can tell this is my first attempt at troubleshooting an amp, so I certainly would be grateful for any help. Thanks in advance!!!

Blackbird

[Nigel Goodwin]

Just a bit of general advice really!.

Try and isolate the section the fault is in, by trying what effect different controls and different inputs have. Valve/tube amplifiers are really very simple devices, and easily split into different sections.

Your static tests of the tubes doesn't mean anything, try swapping them over, and see if the fault changes.

A VERY common fault (which is really disgusting because it's been common for 50+ years!) is the anode loads of the triodes going high resistance - you would imagine the manufacturers would have started fitting decent quality components there by now!.