Standby Momentary Screech when switched on

[chunkitup]

JCM800 2204 clone

Instead of the DPDT standby switch on real marshall amps I have a SPST switch right before the B+ fuse. I get just an instant screech then it is dead quiet.
What is my remedy?

[Chuck H]

It could be a temporary oscillation caused by voltage spike as the main filters charge with current draw. Likely a ground issue. Though it's possible that adding bleeder resistors to the main filter could fix it by keeping the caps charged. It's a good idea to have bleeders anyway. This assumes your switch is between the main filter circuit and the fuse.

[chunkitup]

Yea I always add the bleeder resistor just a single 220k. The switch is between the first cap can and fuse.

[Chuck H]

If you have a bleeder in place the voltage spike isn't going to be as large. So you've done what you can there. Look over your ground scheme. Any daisy chained grounds? Any shared leads? These are places where decoupling is compromised. A spike in voltage (and therefor gain) between two equal phase circuits could cause a momentary oscillation.

You might also look at the occurrence with a scope if you have one. It might be tough to pick up on a DMM, but see about DC on the speaker when it happens. I'll assume your bias supply isn't affected by the switch because of it's location.?.

[52 Bill]

Bad switch? Or underrated.

[chunkitup]

I already ordered a new switch. It is rated 2A 250v which the new switch will be for 3A. This is a 1972 traynor
yba-1a so the switches are old.
What about the negative feedback? I'm still trying to get the grid voltage in a good range. Right now the el34's aren't pulling current when I flip the standby so what if the OT primaries are backwards and the neg feedback is really positive and there a brief moment when the feedback causes the screech. Of course I am at work as I thought of this and can't disconnect the feedback and see if that gets rid of the noise.

[pdf64]

Are you sure that the B+ caps are ok? Have you got any spare that you could sub in?
Does it have a choke or resistor to the 2nd / screen grid B+ node?
Pete

[chunkitup]

This is the first amp I used a resistor(390ohm) instead of a choke. The B+ caps are brand new. Do I need to swap out or check with a meter?

[Chuck H]

I'm still trying to get the grid voltage in a good range. Right now the el34's aren't pulling current when I flip the standby so what if the OT primaries are backwards and the neg feedback is really positive and there a brief moment when the feedback causes the screech.

If the tubes aren't pulling current it doesn't matter what the phase relationship is!?! I'm not sure how to interpret that. On that note though, I have seen amps with PFB loops where the NFB loop should be that behaved somewhat. No harm in checking.

[chunkitup]

Holy crap! I disconnected the feedback from the speaker jack and the screech stopped. Perhaps the inrush was enough to get a brief signal thru. I reversed the OT primaries with the feedback back in the circuit and it was quiet. I put my bias socket on and noted 1.68mA before falling to zero.

Just editing: I have the grid voltage down to -57vdc and still no current. What range am I looking for? For now there is 460 on the screens. Is there a formula or a chart to figure it out beforehand?