The Bias King Pro modification that it desperately needs

[The Dude]

Without knowing the frequencies involved and understanding the positive feedback path, it's hardly possible to recommend specific measures without testing.......

I agree and it's why I prefaced by saying, "Hard to say EXACTLY what's causing the oscillation". I suggested trying the cap because it's a quick and easy thing to test.

[SoulFetish]

Without knowing the frequencies involved and understanding the positive feedback path, it's hardly possible to recommend specific measures without testing.
I tend to think that using shielded cable (withstand voltage!) is more promising than ferrite beads, as these mainly damp HF currents.

But it might not work with all amps. I generally recommend to pull the PI tube before taking plate measurements.

The good news is there is no high voltage in the cable runs. The 4M7/470R divider is built into the socket.
I like the idea of pulling the PI tube.

[Helmholtz]

I agree and it's why I prefaced by saying, "Hard to say EXACTLY what's causing the oscillation". I suggested trying the cap because it's a quick and easy thing to test.

Yes, I noticed and am curious if it works.