2204 JCM800 build bias question

[M Fowler]

2204 JCM800 build bias question

My meter is set to 35mA with 396vDC on the plates.

If I turn the master pot it goes from 35mA to 24mA mid turn and to 35mA at full turn with oscillation at mid turn.

Never had that happen before any suggestions?

Mark

[sluckey]

That's odd. Is this a new build? Do you hear the oscillation? How does it react with the NFB disconnected?

[sluckey]

That's odd. Is this a new build? Do you hear the oscillation? How does it react with the NFB disconnected?

[M Fowler]

I just check in Steve and yes new build and I hear oscillation while turning that master pot plus decrease in bias mA's.

I will check the NFB and get back to post, thanks.

Mark

[M Fowler]

Yes if I disconnect the NFB wire off the tone board the bias max is 29mA with no oscillation. I can lower the bias resistor since I had to keep increasing the value to get my 35mA I wanted.

If I reconnect the bias wire back onto the tone board it goes ups to 35mA but has oscillation and the before noted change in mA's on my meter.

Will swap the OT wires and see what happens.

Mark

[M Fowler]

Swapped the OT primaries and decreased the bias resistor down to 68k and now at 35mA.

Put the chassis in a cab and ready for guitar/amp show in my town March 10th.
I put my logo on the cab front but photo is right before.

Mark

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