Deluxe Reverb Distortion

[lowell]

This is model AB763 and schem is below. This amp begins breaking up in the vibrato channel at around 2.5-3 on the volume control. The amp has been recapped all electros but not coupling caps. I have resoldered the entire fiberboard and all grounds and they measure good. I have gone through and all voltages are fine... aside from higher plate voltages than specified on the schem. I'm guessing this is due to higher mains voltages now. The plates of vibrato channel 1st tubes are 185v and then 202v. I have scoped the amp with a 1k 1.5v p-p sine wave and the signal seems fine until post PI on the first side of the coupling caps. The clips quite hard when the vibrato channel volume is set at the above stated 2.5-3. All tubes are new or at least nos. I will try another PI to make sure it is indeed a 12at7. Can someone suggest what might be the problem here? Maybe this amp just sounds this way? It could be.

[slidincharlie (Carlo P)]

This is model AB763 and schem is below. This amp begins breaking up in the vibrato channel at around 2.5-3 on the volume control. (....) Maybe this amp just sounds this way? It could be.

I would say YES for a blackface DR, especially with humbuckers.

[lowell]

I checked all coupling caps and they're fine. I swapped a 12ay7 for the vibrato channel first tube and the amp sounds cleaner and just all around better in my opinion. we'll see if customer likes it.

I have a questions regarding all this too. Shouldn't a 12ax7 plate be biased to C+/2 to get maximum undistorted output? This amps voltages certainly aren't abiding by this rule.