EL34 Grid Stopper resistor

[ca7922303]

Yes take it out and see if power tubes still redplating. I don't have the whole picture of you amp but still may try to isolate the problem. Did you checked the bias of power tubes ?

Removed V6 and there is no red plating and no output.

[ca7922303]

Seem to be wandering about all over the place.
The original question was ... ? ... no idea, it has gone on too long and is hitting deaf ears. From memory it was adding a grid stopper but there already is one and now it seems it is oscillating ..... why are you disconnecting the heater supplies?
I'm gone.

Removed V6 preamp tube only. Thanks for your help.

[Chuck H]

Yes, I believe C37 which is near R43. I'll send you a link for schematic I have.

Interesting. There seems to be two C37's and one isn't a power supply cap. But I'll assume you did replace the C37 that is a power supply cap. Did you replace this cap because it was bad? Did replacing it solve a problem? I ask because it could be relative to the condition of the other caps in the power supply.

Since the amp is a crowded PCB, dual sided with SMD on one side and board mounted sockets there's not much to be done about grounding and lead dress. So I suspect the power supply caps for the preamp may not be decoupling signal well enough. It looks like the power supply nodes pair up out of phase circuits, but there's still some switching I can't quite follow in their crappy schematic. So it's still possible that some circuit is being cascaded with an op amp or channels are stacked to form gain circuits and this could cause like phase circuits to share a power supply/signal decoupling node. Since the amp may be over twenty years old it might be time to replace more of the power supply filters.

Short of that, instability at extreme settings isn't all that weird for amps from that era. Most leaving the factory are fine. Some may not have been. And a little age on components can sometimes change things just enough to make an amp that only borders on being stable become unstable. I've seen a couple of amps from that era that did just that. Bone stock and worked fine, then became unstable at extreme settings. In your case I still suspect the preamp power supply filters for the moment. But when I was dealing with these problems the amps weren't as old as yours is now. My fix was to find a place in the signal chain where adding a small value capacitor to ground or a capacitor and resistor in series to ground would stop the oscillation. I had to experiment with values to make as little audible change as possible. in the two cases I can remember there was no audible change to one and no significant audible change to the other.

Others may key in with better ideas.

[ca7922303]

Interesting. There seems to be two C37's and one isn't a power supply cap. But I'll assume you did replace the C37 that is a power supply cap. Did you replace this cap because it was bad? Did replacing it solve a problem? I ask because it could be relative to the condition of the other caps in the power supply.

Since the amp is a crowded PCB, dual sided with SMD on one side and board mounted sockets there's not much to be done about grounding and lead dress. So I suspect the power supply caps for the preamp may not be decoupling signal well enough. It looks like the power supply nodes pair up out of phase circuits, but there's still some switching I can't quite follow in their crappy schematic. So it's still possible that some circuit is being cascaded with an op amp or channels are stacked to form gain circuits and this could cause like phase circuits to share a power supply/signal decoupling node. Since the amp may be over twenty years old it might be time to replace more of the power supply filters.

Short of that, instability at extreme settings isn't all that weird for amps from that era. Most leaving the factory are fine. Some may not have been. And a little age on components can sometimes change things just enough to make an amp that only borders on being stable become unstable. I've seen a couple of amps from that era that did just that. Bone stock and worked fine, then became unstable at extreme settings. In your case I still suspect the preamp power supply filters for the moment. But when I was dealing with these problems the amps weren't as old as yours is now. My fix was to find a place in the signal chain where adding a small value capacitor to ground or a capacitor and resistor in series to ground would stop the oscillation. I had to experiment with values to make as little audible change as possible. in the two cases I can remember there was no audible change to one and no significant audible change to the other.

Others may key in with better ideas.

If I recall correctly, C37 was bad, possibly due to R43 burning a hole in board, and of course it was replaced also. Board hole was cleaned and epoxy was used to patch hole. Four(4) diodes were replaced near HT lugs, and several things were done to midi board. A hum balance pot was replaced twice and V5 burnt a hole in board under socket and a wire was run under V5 socket to repair that damage.

[catalin gramada]

Removed V6 and there is no red plating and no output.

Now put the v6 back and shunt its input to the ground. See if osscilate.

[ca7922303]

Now put the v6 back and shunt its input to the ground. See if osscilate.

Not sure what you mean by shunt it's input to the ground? Can you give more detail. Thanks.

[catalin gramada]

Not sure what you mean by shunt it's input to the ground? Can you give more detail. Thanks.

Yes. Instal v6 back and just do a strap wire from pin2 cap input of V6 to ground. Power on and see if osscilate. But put it before the input capacitor

[catalin gramada]

You have a cap over there c36 22n/ 400v you may shunt the input before those cap not directly from pin 2 c36 have one end tied to pin 2 and one end to r47 220k . You may tie a wire between r47 / c36 to the ground to be more precisely. Do not tie pin 2 directly to the ground. You may tie through c36 capacitor. Or you may tie a wire from pin 2 to any cap you have on hand then to ground. You may shunt the ac input not the dc bias voltage of v6 which is elevated build it.

[catalin gramada]

Do you have a capacitor at hand, say 22-100n or so?

[catalin gramada]

Check if is a track on board between red mark point to r47. measure capacitance between red mark and yellow mark. Not sure where c36 is placed on the board

[ca7922303]

Check if is a track on board between red mark point to r47. measure capacitance between red mark and yellow mark. Not sure where c36 is placed on the board

O.L measuring capacitance between red and yellow marks. I see r47, going to try to locate c36 without removing board. Wires connecting to bottom of board have no slack.

[ca7922303]

Check if is a track on board between red mark point to r47. measure capacitance between red mark and yellow mark. Not sure where c36 is placed on the board

Best I can tell from schematic and board, that is c36 between red and yellow marks on photo.

[ca7922303]

Check if is a track on board between red mark point to r47. measure capacitance between red mark and yellow mark. Not sure where c36 is placed on the board

C36(22vdc/1.1mvdc).

[catalin gramada]

Best I can tell from schematic and board, that is c36 between red and yellow marks on photo.

Ok put a wire between red mark point and ground. Power on and see if osscilate

[ca7922303]

Ok put a wire between red mark point and ground. Power on and see if osscilate

What would be best point for ground?