Blackface Pro Reverb LDR causing loud noise

[Randall]

I'm re-habbing and blackfacing a 74 Pro Reverb. I get all the intended work done and fire it up. Works well, looks good, except for this scritchy come and go noise, sounds like a cross between a condor and ripping a window screen, just harsh. While trying to locate the problem, which I admit is giving me fits, I notice the trem is not working, so I decide to get that out of the way first. Turns out the LDR is bad, the LED is flashing, but nothing happening on the resistor side, so I change it, and voila, the trem now works, but the awful noise is now gone as well.

I fail to see how the LDR can inject such a noise into the audio path, even with the resistive side lifted and the intensity turned full off. So I tack in the old LED side again, and the noise returns. I notice at this time that the noise is present with the old LED tacked in, but stops if I ground the trem pedal jack to turn on the trem. Noise returns every time I unground the jack. I installed the new roach and the noise is gone, whether the jack is grounded or not.

If it were causing havoc with the B node I would expect to scope it there and see it, but I do not. I saw it on pin 6 of the Vib channel tube, but not pin 1, nor anywhere on the Norm channel tube. Strange, I am at a loss. The amp is working well now and ready for pick up, but I just don't get what is going on.

pro_reverb_aa165.pdf

[Enzo]

It is a neon lamp, not an LED, but the questions all remain. But that MAY be involved, as the neon works at high voltage.

Does pulling the trem tube make it stop? That would eliminate current through the lamp. I would expect so, since turning the trem off and on has an effect.

You can disconnect half the bug. You can unsolder the photocell from the intensity pot, to see if that is the route of the noise. I suspect not, but we need to know. Then you can instead unsolder the neon wire to B+. any change? or leave that on and unsolder the plate end. or just unsolder both ends of the neon.

We may find the neon lamp is getting noise into the B+ line, for example, rather than through the photocell.

[Randall]

The amp is buttoned now, to be picked up soon.

Pulling the trem tube made it stop.

I disconnected the resistive half of the bug, issue stayed. I unsoldered the intensity pot, no change.

Once I had replaced the bug, I clipped in the old one in parallel with the new one on the neon side, and it made the noise. At some point, I unclipped one side of the old bug, and the amp went crazy loud with noises for about 2 seconds. And and it seems all was OK at that point.

[Mick Bailey]

I think you may have a fault causing the neon to oscillate. Neons can oscillate by design, or by accident. A fault condition could cause the neon to oscillate with a raspy sawtooth waveform that gets modulated with the trem signal. A neon's characteristic change with age and they can become unpredictable in how they operate.