AB763 Deluxe Reverb runaway Oscillation

[watchman]

The amp sounds is great until I turn the reverb knob past three. Then it starts whistling so loud I need to shut it down. And it whistles even with the pan and footswitch plugs removed.

I had it under control with a 5751 as the reverb driver, but the recovery 7025 failed, so I pulled the chassis and started checking values. Everything appears normal, the 220K is there on the grid of the recovery 7025. Seems like the gain is too high and cooked the recovery tube?

any help is greatly appreciated!

... watchman

[Swarty]

I don't think a 5751 has the current capacity required for the reverb driver. However, I think your problem may be a failed cathode cap on the recovery tube (I've had this symptom/solution on several amps).

[Blue Strat]

Yep, only a 12AT7 (or possibly 12AU7) is appropriate for the driver tube.

If the reverb still misbehaves, try padding the tank more heavilly. If there's no padding at all, that's probably your problem.

[John Phillips]

It's nothing to do with the pan or driver tube if it does it with the cables disconnected :).

It must be from the recovery tube, and if that's been changed already, the two possibilities are either that the new one is bad as well (not that likely, if the sound is exactly the same), or what Swarty said. If the cathode cap has shorted it will drastically increase the gain in this stage. Not only that, if it's shorted it will blow the tube in fairly short order too, which is almost certainly why the previous one failed.

[Blue Strat]

It's nothing to do with the pan or driver tube if it does it with the cables disconnected :).



Excellent point! I missed that part. Move along, nothing to see here....:rotflmao