Princetonreverb negative feedback instability

[Diverted]

Hi all, just built a straight clone of a Princeton Reverb for my wife, using a Fender eyelet board and layout. Voltages right on spec. Only change to the circuit is a lar-mar master volume.
It plays great in all respects but one:
With master volume dimed, I’m getting a hellacious him, loud, and no or very little guitar signal. I back off the MV just a tiny bit and all is well.
The issue also vanishes with the 2.7k feedback resistor out of circuit.
Any ideas? Thanks!

https://schematicheaven.net/fenderamps/princeton_rev_gz34_aa1164.pdf

[sluckey]

I’m getting a hellacious him, loud, and no or very little guitar signal.

To get rid of that hellacious him you should see a Catholic priest. But first, swap the OT plate leads.

[Diverted]

Thanks. I'll go see the Diocese. The devil comes out when I crank that master

Till then, I'll try switching plate leads. However it's not the normal squeal you get right on startup that I've heard before that was cured by swapping leads. I'll give it a shot though, thank you!

[pdf64]

If that doesn’t fix it, try grid stoppers, resistor body close to socket terminals #5.
The extra wiring you got in the grid circuit for the master vol invites unexpected coupling, possibly causing oscillation (which may manifest as hum, due to high current draw).

Is there an actual benefit from putting a master vol after (rather than immediately prior to) the cathodyne?
An LTP adds a heap of gain and, arguably, pleasing overdrive; a cathodyne has no gain and a questionable overdrive character.