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Having resolved the locked-up toggle switch dress-nut ordeal last week, I was able to remove the control panel PCB assy, pulled and cleaned all the pots. Exercised them and put them all back into place, finally re-installing the PCB’s to get back to checking the results. No more noisy pots. Now just left with the rest of the problems. The chassis is very microphonic
I began by swapping the input preamp tube, no change. Same with the second tube, no change The most recent AC30-CC2X I worked on had issues on the main PCB that the three 12AX7’s plug into. I got burned by this next move, which was removing the main PCB assy, and not finding any solder fractures or component issues. Put it back together, and I still had the same microphonic problems. I removed all three 12AX7’s, and still had the issues with the Master Volume pot up. All the power tube hold-down wires behaved like harp stings, being able to pluck any one of them to get the system to sing. Same with the hold-down clamp of the rectifier tube.
Turning the Master Volume pot fully CCW stops it. The pot is across the two input grids of the push-pull cathode-biased set of tubes. I turned the amp off, moved the cooling fan box into place and cooled off the tubes, then released all of the hold-down clamps. Powered back up. Still had the microphonic problems.
I had already been to the Guitar Dept to ask for a fresh matched quad set of EL-84 tubes, as they had two sets left just weeks before. Now, not any EL-84 tubes on hand! Great! A sizable rental inventory of Orange and Vox amps, and others that use EL-84 power tubes, and NONE on hand!! They didn’t even have 12AX7’s on hand. I won’t get into my thoughts of how the department is run.
Chanced are, not all four power tubs are bad (microphonic). Trouble is, pulling one pair out doubles the plate current in the pair that’s left installed. Vox already runs them too hot, which shortens their useful life. So, I had to act fast in trouble-shooting.
I pulled out the outside pair V4 & V7 and also had the three 12AX7’s pulled. None of the clamps were installed, rectifier tube included. Still had microphonic behavior. Swapped the outside pair V4/V7 with the inside pair V5 & V6. No more Microphonic noise. Moved those two tubes to the outside pair, just to test the tube sockets. Still quiet. Moved them back to the middle pair of sockets, then, one by one, swapped one of the tubes with the suspect pair. I got noise in both conditions. So, I had isolated the microphonic problem to V5 & V6.
I then put the quiet tubes back in place, checking all four tube sockets again, still quiet. Next, after cooling down the tubes, I re-installed the hold-down clamp for the rectifier tube, still quiet. Did the same with the two power tubes, still quiet. Now, I restored the 12AX7’s, starting with the driver tube. After getting all the tubes back in, I found the input preamp tube was noisy, and swapped it out with a known working ‘pull’ on hand, and got the amp back to acceptable behavior, but missing a full matched pair of power tubes.
That set of power tubes was reasonably matched in plate current, being 37mA, 44mA, 41mA, 38mA for V4 thru V7, just no longer usable as a quad set. Now, how long will I have to wait for the new Matched Set of EL-84's?