sizzling when switching standby??faulty vol pot

[grumpy]

Hi all
My D style amp (hybrid A based) has developed a strange fault. When I switch the standby on, I get a sizzling/fryingpan sound for a few seconds. I am assuming that it is something to do with the B+ reaching it's operating point, when it gets to it's quiescent state the sizzling stops and the amp is acceptably quiet. I currently have a 390ohm resistor in place of the choke and when I play a loud passage and then stop I can hear the sizzling for a short period. I am guessing that because the voltage sags when I play loudly,I get the sizzle when I stop and the B+ goes up again.
Has anyone had this problem? I have replaced some components around the V1 stage as the sizzle is not present with V1 pulled. The fault seems to be centred around CL2 as with the CL1 plate resistor disconnected I still get sizzle when turning standby on or off. I suspect that the CL2 feedback network may have something to do with it but I have replaced all components in the loop and there has been no improvement.
Anyone have any thoughts or ideas???? I don't seem to be getting anywhere and I've been trying to pinpoint this issue for a few days.

Cheers, John

[Darkbluemurder]

I doubt that the fault is with the feedback network. To be sure remove it and see if the sizzle is still there. If it is you can be sure that the loop has nothing to do with it.

I would suspect the plate load resistor of CL2 and/or a bad tube in V1.

Good luck
Stephan

[ayan]

Hi all
My D style amp (hybrid A based) has developed a strange fault. When I switch the standby on, I get a sizzling/fryingpan sound for a few seconds. I am assuming that it is something to do with the B+ reaching it's operating point, when it gets to it's quiescent state the sizzling stops and the amp is acceptably quiet. I currently have a 390ohm resistor in place of the choke and when I play a loud passage and then stop I can hear the sizzling for a short period. I am guessing that because the voltage sags when I play loudly,I get the sizzle when I stop and the B+ goes up again.
Has anyone had this problem? I have replaced some components around the V1 stage as the sizzle is not present with V1 pulled. The fault seems to be centred around CL2 as with the CL1 plate resistor disconnected I still get sizzle when turning standby on or off. I suspect that the CL2 feedback network may have something to do with it but I have replaced all components in the loop and there has been no improvement.
Anyone have any thoughts or ideas???? I don't seem to be getting anywhere and I've been trying to pinpoint this issue for a few days.

Cheers, John

I would suggest you try a new set of screen grid resistors....

Gil

[skyboltone]

I would suggest you try a new set of screen grid resistors....

Gil

Hey, are you guys using wirewounds here or metal films. Seems like a scary place to be adding inductance, but I'll sure go with the consensus.

Dan

[ayan]

I would suggest you try a new set of screen grid resistors....

Gil

Hey, are you guys using wirewounds here or metal films. Seems like a scary place to be adding inductance, but I'll sure go with the consensus.

Dan

I use "cement blocks," which are wirewounds, as did Marshall. In one of Kevin O'Connor's books, he suggests that this is a good idea because it adds some desireable inductance. I can't remember exactly what the rationale offered was.

Gil

[Bob-I]

I would suggest you try a new set of screen grid resistors....

Gil

Hey, are you guys using wirewounds here or metal films. Seems like a scary place to be adding inductance, but I'll sure go with the consensus.

Dan

I use "cement blocks," which are wirewounds, as did Marshall. In one of Kevin O'Connor's books, he suggests that this is a good idea because it adds some desireable inductance. I can't remember exactly what the rationale offered was.

Gil

I've been using 2 watt metal films to avoid inductance. I'd like to see his reason.

[thejaf]

I had the exact same problem on a combo amp. It was an old Guild from the 50's that was gutted and modified. Everything was new except for the power and standby switches. It turned out to tbe the 50-year old standby switch, which wasn't making a very good connection.

[mlp-mx6]

I've been using 2 watt metal films to avoid inductance. I'd like to see his reason.

IIRC, the reason is the "ever so slight" smoothing of the ripple that the inductance offers. Like a very small choke for the screen supply, right there on each screen!

Not saying it works, or that it does not work. But I'm pretty sure that was the rationale.

Michael

[grumpy]

Well thanks for the suggestions guys! I appreciate it. After trying everything suggested with no luck, I went through the whole front end of the amp, reflowing just about all the solders, replaced the whole dropping string along with a bucketload of other things. The last thing I would have suspected was the Pots, but, It turned out to be a faulty Vol Pot? It was a brand new Alpha. I don't know in what way it's faulty but there you go!
The odd thing is I can't quite make sense of why it caused the symptoms that were there. The sizzling was exactly as described in my first post, and was much reduced with the CL2 local feedback switched out, I think I replaced that network three times!!
Anyway it's fixed!! Ya F..ing Hoo. The amp is also generally quieter than it was before too, so it's all good.

Cheers, John

[skyboltone]

Well thanks for the suggestions guys! I appreciate it. After trying everything suggested with no luck, I went through the whole front end of the amp, reflowing just about all the solders, replaced the whole dropping string along with a bucketload of other things. The last thing I would have suspected was the Pots, but, It turned out to be a faulty Vol Pot? It was a brand new Alpha. I don't know in what way it's faulty but there you go!
The odd thing is I can't quite make sense of why it caused the symptoms that were there. The sizzling was exactly as described in my first post, and was much reduced with the CL2 local feedback switched out, I think I replaced that network three times!!
Anyway it's fixed!! Ya F..ing Hoo. The amp is also generally quieter than it was before too, so it's all good.

Cheers, John

Hi John:
This here is a WAG, and remember I'm frequently full of it but, I've had pots taken out by faulty/leaky blocking caps in old radios I've worked on.

Dan

[Bob-I]

Hi John:
This here is a WAG, and remember I'm frequently full of it but, I've had pots taken out by faulty/leaky blocking caps in old radios I've worked on.

Dan

That's exactly what I was thinking. I had that happen recently on my Super Reverb clone. I'd check if there's any DC voltage on that pot just to be safe.

[grumpy]

Thanks blokes. Yes that's possibly what happened. I have replaced pretty much all the coupling/tone caps in the front end as I thought the sizzling was from a leaky cap originally and the Vol pot was making that heavy breathing kind of sound when you turned it. There doesn't seem to be any DC there now but it's quite possible that there was previously although I did check for it as one of the first things I did and I cant remember there being more than a few millivolts. I think I'll check it all out again just to make sure.

Cheers John