Deluxe Reverb Oscillation

[ChrisM]

I finished up a Deluxe Reverb build a couple of months ago. Everything was fine.

Recently though it has developed a bit of an issue. I am getting a high pitched oscillation. The sound is faint and when you play you cant her it at all. It's only when you stop playing and you listen you hear it.

None of the controls affect the oscillation. The volume control does not make the oscillation louder.

At first, well I hoped it was a microphonic tube. Turns out it wasn't. I started pulling tubes to narrow down the problem. When I pull V4 the noise stops. V4 is the reverb recovery tube.

So seems the Normal channel is fine and the beginning and end of the Vibrato channel is. Just that damn recovery stage me thinks. Disengaging the reverb with the footswitch doesn't stop the noise.

Schem: http://www.jmiamps.com/schematics/fen/d ... _schem.gif

Any ideas what to look for? I am going to open the amp up tomorrow. Check lead dress, joints, microphonic components(?) and so on.

Makes sense the vol control doesn't control the noise. The vol control is prior to the reverb recovery.

Thanks...

[VacuumVoodoo]

Check cathode cap on V4, or better yet install separate resistors and caps for each cathode : 1.5k and 22uF. That said not having seen your wiring, could be something there too.

[ChrisM]

Check cathode cap on V4, or better yet install separate resistors and caps for each cathode : 1.5k and 22uF. That said not having seen your wiring, could be something there too.

What's the rational behind this? Voltage?

Just wondering not questioning you.

[ChrisM]

Was a ground loop between output jacks and RCA jacks. Solved it, noise gone.

[Structo]

Cool!
Always a great feeling when you solve one of those problems that drive you nuts!

[jjman]

Was a ground loop between output jacks and RCA jacks. Solved it, noise gone.

Can you expand with the details of how it was setup before and after the fix?

I have a small "buzz" in the reverb recovery of my '71 DR. I actually noticed that there seems to be microphony on the components soldered on the recovery RCA from the tank. But no microphony on the grid wire to the recovery stage. I have the A1270 so I have the "added" cap on the return rca jack. Maybe it's the culprit.

[ChrisM]

Tom, it really is rewarding. I've gotten to the stage where I can usually diagnose and fix a problem fairly quickly now. Makes all the reading and learning worth the effort. Kind of noticed the problem happening about a week ago and just decided to fix it today, haha.


The problem was the output jacks and RCA jacks (as well as other things) are insulated from the chassis. Now the RCA jacks are really nice and insulated quite well. They have a ground wire running back to the preamp ground. I decided to also isolate the output jacks as part of my grounding scheme. I checked and they weren't insulated. They were ground on the chassis and also with the RCA jacks (preamp grounds). Anyways I just made the output jacks have a chassis ground and that solved it, it seems.

Try isolating your RCA jacks. A ton of noise is injected from the recovery stage I found when building the amp.