Deluxe reverb AB763 blocking distortion

[Leevi]

I have built several AB763 but never met this before.


http://tubeamplifierparts.com/schematics/fender_deluxe_reverb_ab763_schematic.gif


I this circuit there is only vibrato channel and PPIMV master isolated with caps.


The problem exists on the second triode of 7025 (12AX7) just before PI.


If I turn preamp volume more than 6 the overdriven signal starts to break and sounds like a blocking distortion.


It helps if I drop the signal level with resistors before 3.3M resistor + cap.


/Leevi

[Leevi]

I found the cause.
It was a wiring problem, two wires were too close to each other which caused the problem.
/Leevi