jcm800 mk2 lead pitch noise in V1

[binito]

Hi,
i´ve a JCM800 master Model 50w Mk2 Lead '81
http://ewilkins.com/amps/jmp_rebuild..._2204_4010.jpg
When I set up to 9 the potentiometer of pre-amp. High frequency beep begins.
The circuit was not changed and the valves and I changed to new
The inputs jacks are fine.
I have placed a 220pF 2kV ceramic cap between the plate and the cathode of High channel and the High Frecuency noise disappeared.
I think that in this way lose tone. Can you think what may be the reason.
Thanks

[Chuck H]

Many new preamp tubes today are very microphonic (mechanically loose). It's very possible to have a tube become microphonic and replace it with one that is just as bad or worse. You need a known not microphonic tube to try in the first preamp socket.

If a known good tube doesn't stop the beeping you may have a bad ground connection or possible a bad power supply capacitor.

[binito]

Many new preamp tubes today are very microphonic (mechanically loose). It's very possible to have a tube become microphonic and replace it with one that is just as bad or worse. You need a known not microphonic tube to try in the first preamp socket.

If a known good tube doesn't stop the beeping you may have a bad ground connection or possible a bad power supply capacitor.

Is true that the current valves can be more microphonics.
However, the problem seems input impedance.
With the 220pF 2KV Cap between the plate and cathode of V1 beep is fixed.
But when I plug the guitar in the bridge pickup no beep, but the neck pickup has beep. (Preamp pot above 8)

[Chuck H]

The input impedance of the tube is very high. So high that it can be ignored, so the input impedance is controlled by the input circuits 1M resistor. This effectively makes the input impedance roughly 1M. Since the guitars output impedance will be somewhere between 5k and 20k (with the guitar volume up) this cannot be an impedance issue. The 220pf capacitor from grid to cathode does reduce input and output impedance for the tube at higher frequencies. It is a local negative feedback loop. So it also reduces top end and I think the effect on amount of frequency is more important than specific impedance at frequency for this problem. If you get no beeping with the bridge pickup, only with the neck pickup, and only at higher gain settings, the problem is probably a microphonic neck pickup.

Even if it never did it before. Even if that pickup doesn't do it with another amp. Everything is related and there are resonant peaks specific to guitar/pickup/amp/setting combinations. You should "pot" the pickup or have it done. There are many threads here about how to do it.

[binito]

I believe that solved the problem.
I changed the wire of grid in v1 for shielded wires and the high pitch noise dissapear.

[Chuck H]

Because that fixed it, it seems unusual that you only had the problem with a specific pickup. But as I mentioned, there are many things happening with the guitar pickup/ amp combination. Glad you got it fixed.