Brown VL: Still chasing unpleasant tail note distortion

[bullkelp]

Back to scratching my head with my fender 1961 brown vibrolux 6g11a.

Great sounding clean amp but when pushed into even a little distortion it develops trailing crackling. Id love to be able to unleash the amps creamy over drive (which is there!) but cant abide the crackling!

I went through all the suggested process:
New preamp tubes, clean tube sockets, replaced grid stopper and screen grid resistors, addressed possible DC on board, seperated boards, checked bypass caps for leakage (blue moulded - all test ok), replaced EL caps.

Kinda stumped as to where to next?

Happens on both channels so I guess thats of some assistance.

Simple next steps

[pdf64]

How about if the nfb loop is opened, eg lift a leg of the 100k series feedback resistor, and volume turned down to compensate?
https://el34world.com/charts/Schematics/files/Fender/Fender_vibrolux_6g11_a.pdf

Edit - that OT can cause excessive phase shift, thereby reducing the margin of stability when negative feedback is applied.