reverb circuit causing distortion in entire amp

[pula58]

symptoms:
distortion (lots of it)

If I remove the cable to the reverb tank input the distortion improves.
Replacing the reverb tank with a known good one makes no difference.
The reverb mix is, in all cases, set to zero. So, it is not that the reverb signal is getting distorted and showing up in the mix of wet and dry, it is the dry signal itself that is getting distorted by the reverb circuit.

If I put a 10k 2W resistor in between the high-voltage side of the reverb driver transformer primary and the screen voltage suppy the effect becomes lessened, but it still there, but much better,

Other (possible) clue: When I received the amp (to repair), the the cathode of the reverb driver tube was at +50V (!), which cooked the cathode resistor. I repoaced the 12at7 wit ha new one and that brought the cathode down to +8V. I also replaced the cathode resistor since it was noticeably blackened.

Seems like maybe too much current is flowing i nthe reverb driver circuit???

bad reverb driver transformer?


Thoughts? ideas?

[jjman]

Make/Model? Schematic ?

[pula58]

Make/Model? Schematic ?

it is a Fuchs mod of a silverface deluxe reverb.
Not sure if I will be able to get a schem.

strange, if I place the o-scope probe at the grid of the reverb driver tube (12AT7, both grids connected together) the problem goes away, or at least seems to.

[ampgeek]

Maybe double check that the driver tube has a good ground reference (pot or fixed resistor)?

The O scope could have provided that during your test.

Good luck!
Dave O.

[pula58]

Maybe double check that the driver tube has a good ground reference (pot or fixed resistor)?

The O scope could have provided that during your test.

Good luck!
Dave O.

1meg fixed grid resistor to gnd. Checked gnd for that resistor, it's solid. I put in a new 1Meg grid res and 2.2k cathode res, just to be sure, and..... the problem still occurs. I also installed a new cathode bypass cap for the driver as well. It made no difference.

Things that are known to be fine:
* new reverb driver tube
* reverb tank input (two good tanks elecit the same behavior)
* supply cap for screen voltage (same supply that the reverb driver tranny uses)
* reverb tank cable (tried two different known-good cables)
* reverb driver tube grid resistor
* reverb driver tube cathode resistor/cathode bypass cap.

The DC resistance between the two primary leads of the reverb driver transformer is 1500 ohms. This seems within the ballpark of what it should be.

[jjman]

Sounds like a parasite that's captured by the scope lead. I would try a shielded lead to the grid and perhaps a grid stopper.

[Structo]

How is the lead dress on that amp?

Silverfaces are notoriously bad.

But maybe Andy cleaned that up when he modded it.

[pdf64]

Does the distortion reduce or disappear if the driver tube cathode bypass cap is lifted out of circuit?

[pula58]

Sounds like a parasite that's captured by the scope lead. I would try a shielded lead to the grid and perhaps a grid stopper.

10k grid stopper helped..I think it might be o.k. now, have to play with it some more to be sure. I also put a 10K resistor between the reverb driver transformer primary and the output tube screen supply voltage (the "downhill" end of the choke)

[xenrelic]

Just in case anyone comes across this with a similar issue, sometimes it's a damaged reverb transformer and cathode circuit that has gone bad if someone tried to use a 12AU7 driver in place of the 12AT7 or a 12AT7 failed at some point causing damage.

In some cases the voltage that hits the plates of the 12AT7 is too high and causes the tube to distort or even spark between the plate and grid if the transformer or components in the reverb circuit are damaged or the amp's B+ has risen since it was first purchased due to changes in the power grid.

A 15k to 22k 2W resistor in line with the high voltage side of the reverb transformer gets rid of it completely and seems to solve the issue if your transformer appears to be operational still, I've personally confirmed this. 360v to 400v seems to be the max this tube is comfy with without noise issues. If you're rocking higher gain, maybe see how hot the tube is running when your gain is at it highest setting and make sure it doesn't exceed 400v.

The data sheet for these tubes usually specs 300v max at the plates, so Fender is already pushing its limits and I'm not surprised that this tube tends to run hotter with a more limited lifespan unless that issue is addressed.