Need help locating cause of "congestion" (blocking distortion?) Tremolux AB763

[acorkos]

I appreciate any help you can give me regarding the cause of what sounds like blocking distortion in a 1965 Fender Tremolux AB763. It occurs on both channels...signal sputters as the sound decays, comes back with a sharp whack on the guitar strings. Everything appears to be original with the exception of one 16uF/450V power supply electrolytic capacitor. I swapped out all the tubes with no change. I metered all the resistors, all are within 10-15% of spec. Will tracing the signal with a scope help locate the problem? What should I be looking for? Thanks for your help

[Mick Bailey]

Your description doesn't sound like blocking distortion to me due to the sound coming back with a whack on the strings. It sounds like a high resistance or mechanical joint problem that's being momentarily overcome by the sudden higher voltage of a string being plucked hard.

I've had similar symptoms caused by;

Poor solder joint or fractured component leg
Tube base pins that are dirty or slack and need cleaning and re-tensioning
Coupling capacitor that's got an internal connection issue.

I would start by making sure the socket contacts are good. Discharge the amp and clean the with Deoxit or similar and use a jeweller's screwdriver or dental pick to close up the contacts. If the contacts are poor you'll be chasing your tail trying to find the fault. As the problem is with both channels I would take a look at where the channels combine into the PI and see what's happening there. If the signal is good on the PI input, take a look at the signals on the power tube grids to narrow down the problematic section.

[Helmholtz]

Please post schematic and measure all PI and power tube voltages (plate, cathode, screen).