Help Troubleshooting Reverb Buzz

[chinrest]

Hi folks,

I'm working on a scrap-parts project: blackface/reverb/EL34 power.

I'm getting a nasty buzz when the reverb control is turned up--even with no tank. I'm assuming that it's a ground loop, caused by my layout, but I'm not sure what to do. I've attached a pic that shows the ground scheme. The reverb send/return/footswitch jacks are all chassis ground.

Any tips are appreciated.

[chinrest]

Sorry -- I said buzz, but I meant HUM.

Well, it seems to be in the recovery stage (like other threads I've read). Grounding the return signal at the grid does not kill the hum. I shielded the wire from the reverb pot back to the board--no dice. I played with the orientation of the reverb transformer--nope.

I guess try lifting the reverb jacks off the chassis and grounding them elsewhere? Grounding the pot elsewhere?

[Enzo]

The reverb transformer is on the drive side of the pan, and especially with the pan removed, it is not going to introduce hum.

How did you ground the signal at the grid? Clip wire to chassis? Nah. Tack solder a short wire from the grid pin of the recovery tube to the ground end of the cathode resistor - right at the resistor. Disconnect the input jack cable from the tube even. That will tell you the trouble is coming in that grid or not.

There is no wiring on your layout.

You wanna lift wires and stuff? Does the reverb control turn the hum all the way down? Ground the top of that control. does that kill the hum?

Have you put a different tube in the reverb recovery socket? Does just pulling the tube kill the hum?

[chinrest]

Thanks for the reply Enzo.

How did you ground the signal at the grid? Clip wire to chassis?

Actually, I alligator clipped it to the cathode resistor ground. It did not effect the hum at all. I'll remove the grid wire and hard ground it just to be sure though.

Does the reverb control turn the hum all the way down? Ground the top of that control. does that kill the hum?

No hum with the reverb control all the way down/grounded. The amp behaves normally.

Have you put a different tube in the reverb recovery socket? Does just pulling the tube kill the hum?

I tried a few different tubes with the same result. Pulling the recovery tube kills the hum (and also interupts the signal to the PI).

[Paul P]

How close is your reverb transformer to your power transformer ? I ask
because I had a similar sounding problem, no hum with the reverb at zero,
hum with it turned up, independent of the volume control, and it was because
my reverb tranformer was right next to my PT. Moving it further away solved
the problem.

Paul P

[chinrest]

Paul,

I have about 13 inches between the PT and reverb transformer.

[Paul P]

That should be enough .

Paul P

[chinrest]

I had a thought (uh oh).

I originally had the reverb transformer B+ tap connected to the screen supply, but with the extra beefy PT, I was getting about 520V on the plates of the 12AT7 driver. I have since moved the tap one node further downstream, resulting in about 325V (not ideal).

I noticed that the transformers B+ lead is now very sensitive/microphonic (duh). I know this probably has nothing to do with the hum--just some first-time-reverb general troubleshooting. Would it be better to tap off the screen supply with it's own dropping resistor?