Help tracking down amp noise in Super Reverb...

[reswot]

I've been tinkering with this '69 dripedge Super Reverb (AB568 circuit) I picked up a few weeks ago. So far, I've replaced the torn grill cloth (a PITA with the drip edge!), replaced the filter caps, replaced all the bypass cap, replaced all of the plate resistors, and Blackfaced the circuit. I also disconnected the small cap to ground on the reverb return.

Tubes are an Amperex 7025 in V1, a a grayplate RCA 12AX7 in V2, an Amperex 12AT7 in the reverb driver, a GE 6681 in the recovery, a blackplate RCA 12AX7 in V5 (sounds glorious in V2 until you hit a D note and then it's microphonic as hell, but works in this spot) and a Phillips 12AT7 as the PI. I got a pair of NOS GE 6L6GCs from Tubetramp last week and am using a TAD GZ34 for the rectifier.

The amp sounds pretty good now, much stronger since the cap changes.

I'm getting a weird sound in the Vibrato channel, though, when the amp is turned above 5 and I'm using a Les Paul. When I hit a chord, it initially sounds really good, but then a crackly, fizzy, static-y sound fades in and then abruptly cuts off and the volume decreases quite a bit.

I only get this sound in the Vibrato channel; I can crank the Normal channel up all the way and it doesn't happen. That seems to rule out speakers, trannies, the PI circuit, or the output section.

I initially thought it was a bad tube, but still get the sound after swapping all of the tubes. I thought it could be a cold solder joint, but I still get the sound after reflowing all of them. I also went through the amp with chopsticks and couldn't duplicate the noise.

Has anyone experienced this? Any ideas? I can record a clip in the next few days, but not today, as I have to get the amp cranked a bit to get the sound.

It doesn't happen, at all, if the volume is around 4, or below....

[AustinTx]

I fixed a combo amp last week with the same problem..was a poor speaker cable and wire.
Fine until you reached a certain volume and the vibration of a loud amp caused the
connection to fail. Hope it is something easy to track!

[briango]

Almost sounds like some blocking distortion symptoms.

[Roe]

could be blocking distortion in the reverb driver. install a 68k grid resistor on v3

[noworrybeefcurry]

Ive had this problem alot with my scratch build, it really does sound like blocking distortion as briango said. try those bigger grid stoppers.

[reswot]

Thanks... I've got a few ideas to try.

I'm intrigued by the 68k on the grids of the reverb driver. They go to a 1M resistor to ground. Would putting a 68k between the 1M and the grids really help?

[toner]

I'm intrigued by the 68k on the grids of the reverb driver. They go to a 1M resistor to ground. Would putting a 68k between the 1M and the grids really help?

If it is blocking distortion then a grid stopper will help. Put it after the 1M, ideally right on the tube socket.

[reswot]

That sounds like a good start. I'll give that a whirl tomorrow...

I definitely think the problem is in the reverb circuit, as it goes away if I unplug the leads to and from the tank...

[noworrybeefcurry]

Im not very versed with reverb circuits, but if the noise stops when you unplug the tank then could the tank be the culprit?

[Roe]

this is a common problem, just try the 68k at the socket and you'll be happy

[reswot]

Thanks a bunch, Roe.

That did the trick.

[Roe]

I'm glad it worked! it will remove blocking distortion and potential oscillation since it removes highend in the 15k+ range