Buzz on 8080 valvestate

[spebblechunky]

Hi all - I'm getting this buzz that sounds like a ground loop on this amp. It is audible with everything turned to zero, goes up a bit with the master up, and then goes up a lot with increase of volume.



I replaced the filter caps and tried re-flowing the solder on the output resistors and a lot of other places, looking for cracked leads. Cleaned the hell out of the pots. Tried another tube. It's still there when I disconnect the reverb.

I don't have a scope but I have a signal tracer so I can listen to the signal path to try to figure out where it starts. Could anyone recommend where to look for that ground loop, given what I have mentioned so far?

 

[Jon Snell]

Check the input socket. Probably a broken ground link or a faulty socket.

 

[Tatzmann]

All the 8100 have this as far as i know. 8080 is the same amp.

Cant remember if i ever read about a solution of this and it never bothered me too much that i had to investigate it myself.

It may had something to do with a cap in the powersupply, but since they are changed already, idk.

Its a fairly known quirk of those amps, some extensive googling could bring up an answer.

 

[spebblechunky]

Check the input socket. Probably a broken ground link or a faulty socket.

Thanks Jon. I checked the jack and it seems okay. Actually the buzz is there at a low level even if I plug a cord into the effects return with everything zero'ed out. Any other leads?

 

[Jon Snell]

What happens with VR13 at zero. If there is still hum, what frequency is it? Mains or twice mains?
If R13, master vol, is at zero, that removes the preamp/reverb recovery from the equasion ... assuming the ground is good on VR13 pot!
Have you checked the reverb?

 

[spebblechunky]

What happens with VR13 at zero. If there is still hum, what frequency is it? Mains or twice mains?
If R13, master vol, is at zero, that removes the preamp/reverb recovery from the equasion ... assuming the ground is good on VR13 pot!
Have you checked the reverb?

Okay with vr13 (master) at zero, the same hum is there at a low volume. Turning up gain and volume brings it up much higher. I guess it's a mains frequency 60hz, but clipped like in this screenshot, which is maybe why I thought it was 120hz ground loop. Seems like ground is good on vr13.

Unplugging the reverb tank doesn't change the buzz.

 

[spebblechunky]

What happens with VR13 at zero. If there is still hum, what frequency is it? Mains or twice mains?
If R13, master vol, is at zero, that removes the preamp/reverb recovery from the equasion ... assuming the ground is good on VR13 pot!
Have you checked the reverb?

Actually, with a signal tracer, I cannot hear the buzz on IC1, IC2, IC3, but then I can hear it on the input of IC4, so it seems it is somewhere around there. When I wave the signal tracer through there not touching anything, I can hear it picked up from the auto transformer pictured here. So I guess that is the source.. I don't know if something is grounded wrong, or that xformer would need to be replaced.. Still poking around.

 

[Jon Snell]

Tat means the hum is coming from the valve pre amp stage.
I would firstly check the valve, then look for ripple on C53 and C59.

Think back a few suggestions ... the grid of V1 connectes to ground at the input socket! Have you replaced the socket and checked the soldering?

 

[spebblechunky]

Tat means the hum is coming from the valve pre amp stage.
I would firstly check the valve, then look for ripple on C53 and C59.

Think back a few suggestions ... the grid of V1 connectes to ground at the input socket! Have you replaced the socket and checked the soldering?

Thank you - I'll check that out. I do get continuity from tube pin 7 grid to ground. I can't find fault with the tube(s) or the socket so far..

Do those capacitors 'filter' the voltage from the autotransformer?

 

[Jon Snell]

You may get continuity but if the ground is not zero Ohms, it is likely to cause issues.
Connect a piece of wire between the grid and the ground end of R70 to prove it with a blank jack lead plugged into the input socket. That will prove its innocence.
Remove the valve ... does it still hum?

Yes those two aforementioned capacitors are the smoothing capacitors for the valves HT supply.

 

[spebblechunky]

You may get continuity but if the ground is not zero Ohms, it is likely to cause issues.
Connect a piece of wire between the grid and the ground end of R70 to prove it with a blank jack lead plugged into the input socket. That will prove its innocence.
Remove the valve ... does it still hum?

Yes those two aforementioned capacitors are the smoothing capacitors for the valves HT supply.

Okay, thanks for your patient replies. Sorry if I sound completely thick here; the Grid of the valve leads back to the input jack switch, and with a plug in there, it opens the switch and breaks the connection of the grid to ground.... is it not supposed to?

The grid-ground settles down to 0.3ohm with no plug in there, and with a plug in the input, there is no connection grid-ground. Is this wrong?

I think this input jack is furshlugginer and I might have to rewire it point to point if I replace it, which is a drag, but I'll do it for science.

 

[Jon Snell]

Okay, thanks for your patient replies. Sorry if I sound completely thick here; the Grid of the valve leads back to the input jack switch, and with a plug in there, it opens the switch and breaks the connection of the grid to ground.... is it not supposed to?

The grid-ground settles down to 0.3ohm with no plug in there, and with a plug in the input, there is no connection grid-ground. Is this wrong?

I think this input jack is furshlugginer and I might have to rewire it point to point if I replace it, which is a drag, but I'll do it for science.

Did the wire link make the hum go away?
The grid should connect to the ground switch in the socket.
If the link wire makes no sifference, the hum is probably not from the socket, so no need to replace it.
What happens if you remove the valve?

 

[spebblechunky]

Did the wire link make the hum go away?
The grid should connect to the ground switch in the socket.
If the link wire makes no sifference, the hum is probably not from the socket, so no need to replace it.
What happens if you remove the valve?

Just tried the jumper from grid to ground side of R70 - made no difference. With the tube pulled, the channel still functions, but it makes no difference to the buzz (with or without jumper). I'm confused about the grid to ground, if that is only when the plug is pulled.

The buzz definitely enters the system after the tube - it's not present prior to that.

What do you think?

 

[Jon Snell]

If the valve is removed and the hum is still there, it is not from the valve.
Remove the valve and place a link across R71. Does the hum stop?
If it doesn't, move to R13 and link that to ground. If it is still there, move to VR10 and link wiper to ground. Keep on going until it stops. That will take you to your fault or show you where it is.
I think it is a design fault but what do I know.
Good luck.