JCM800 - Oscillation

[Littlewyan]

I built a JCM800 out of a Pignose G40V. Its stock JCM800 except it has a 6v6 power section, NFB is on 8ohm tap and there is no bright cap, instead the mixer bright cap connects to the pot wiper.

For the most part it's awesome but if you crank the volume, gain, middle, bass and presence you can't turn the treble above 5 without it oscillating. You can't hear the oscillation except for some slight hum, but I can see the blue gas in the 6v6s changing and when you play it sounds horrible.

Am I right in saying that early JCM800s did this? Or could you quite happily crank them?

[Littlewyan]

Oh yeah the input grid stopper is 33K but I don't think this is the issue. I'm sure the oscillation is in the output stage. If I turn down the master then I can dime everything else.

[neikeel]

What have you done to counter PO in the amp?

Shielded grid wires in the preamp? Grid stoppers on the outputs?

Plate bypass caps?

Lead dress?

[Littlewyan]

The input wire is shielded and there are 2.2K grid stoppers on the output valves. Not tried anything else yet. Ill go through it with a scope some point next week (on a stag do atm). I was just wondering if these amps generally oscillate stock.

[danman]

Does your build include the small pf value cap across the cathode and plate pins (6 and of the v1 socket? If not, try adding a 100pf and see if this helps. I built a 2204 a short while ago and it does not show any signs of oscillation at any volume so you should be able to get yours straightened out.

[Littlewyan]

It doesn't no. I'm not sure if that will help as the oscillation is happening later in the circuit. Also don't forget I have shoe horned this into a Pignose G40V chassis. Which is tiny! Lol

I did change the NFB resistor to 56K last night. The amp only puts out 10W so I had to lower the NFB resistor value to bring the NFB back up. This actually helped with the oscillation a bit.

[Littlewyan]

Oscillation fixed. It was occurring at the V2A Anode/V2B Grid. I had to go up to a .001uF Cap across V2A's anode resistor before it stopped! Amp sounds much better now.

I also now may need that 100pF you mentioned Danman. When I dime every control I get pickup squeal unless I stand very far back or use 2 long guitar leads in series. So after sorting that oscillation I now have more high end? Go figure!