Gibson GA-15 RVT Reverb Hum

[bobloblaws]

Hello everybody, a number of weeks ago I had a thread going in the troubleshooting section discussing my problem but that discussion seems to have run it's course so I thought I'd try this section, since I still haven't been able to get to the bottom of the issue.

So, I have a Gibson GA-15RVT tube amp here, circa mid 1960's, with a hum problem that seems like a no-brainer, but I'm completely stuck on it. The reverb recovery stage is adding a significant amount of noise, more than is acceptable. It is there even with the reverb tank disconnected and/or the grid of V1B grounded. One of the footswitch buttons grounds the junction of C15 and R15 when toggled. When that junction is grounded (reverb effect OFF state) the hum in question is not present. When the footswitch is toggled (or disconnected) the junction is no longer grounded and the hum is present in the output. I've done a number of things in the way of troubleshooting, e.g. the grounding of the tube grid that I mentioned, also removing the wire that goes from C15/R15 to the footswitch, replacing the V1 tube, etc. but have not been able to eliminate the hum apart from grounding C15/R15. So my question is. looking at the schematic, is it a given that this design will result in noise being introduced? In other words, is it a bad design and there's nothing that can be done to eliminate the noise?

GA-15RVT.pdf

[bobloblaws]

Here's the schematic again, I tried editing the post above but I can't get the link to the schematic and the link to the older thread to play nice together.

GA-15RVT.pdf

[Jazz P Bass]

Try relocating the return ground somewhere else.

[bobloblaws]

Try relocating the return ground somewhere else.

Thanks for the reply. Your response implies the answer to my question is that no, it's not the design and there should not be any significant hum introduced.

What do you mean by "return ground".

[Helmholtz]

Is C12A (20µ) good?

Tack a good 22µ cap across C12A and see if it helps.

[bobloblaws]

Is C12A (20µ) good?

Tack a good 22µ cap across C12A and see if it helps.

Yes it seems to be good, I just tried swapping it for a new one and also paralleling the new one for more filtering and no change.

[g1]

Can you try disconnecting V1B pin7 at the socket. Tape the wire off and leave it in same basic position.

[bobloblaws]

Can you try disconnecting V1B pin7 at the socket. Tape the wire off and leave it in same basic position.

That gets rid of the hum. In that case there is no change in amount of noise whether the C15/R15 junction is grounded or not.

[g1]

Just wanted to be sure it wasn't induced into the wiring after the plate of that tube. That seems to indicate the hum is definitely being introduced by the tube. Grounding the grid didn't help. The hum could be introduced through the cathode or may have to do with it's ground cathode resistor grounding.
Haven't checked back, but I'd think you have tried other tubes in that position so it should not be internal bleed from heater.

[bobloblaws]

Just wanted to be sure it wasn't induced into the wiring after the plate of that tube. That seems to indicate the hum is definitely being introduced by the tube. Grounding the grid didn't help. The hum could be introduced through the cathode or may have to do with it's ground cathode resistor grounding.
Haven't checked back, but I'd think you have tried other tubes in that position so it should not be internal bleed from heater.

Yes, swapping V1 and V2 did not change anything and I even bought a new 6EU7 to try. I posted my problem also on All About Circuits and one of the things those guys are suggesting is that it could be heater related and to try a wirewound 100R pot on the heater circuit to null the hum.

https://forum.allaboutcircuits.com/t.../#post-1623198

[Helmholtz]

The hum could be introduced through the cathode or may have to do with it's ground cathode resistor grounding.
Haven't checked back, but I'd think you have tried other tubes in that position so it should not be internal bleed from heater.

Still makes sense to check the cathode cap.

A scope would be a big help.

I faintly remember a similar Gibson amp hum problem lately but don't seem to recall how it was resolved. Anybody?

A humdinger pot as advised in the other forum is always a good addition. It is not necessary to go down as low as 100R and with a 500R (470R) pot it doesn't have to be wirewound as dissipation is only around 80mW. But this will not adress the root cause of the hum.

Anyway, make sure the negative end of C12A is well grounded.

[vintagekiki]

Remove reverb from V1B.
Withn ohmmeter measure whether there is a resistance of 220k between pin8 V1B and chassis (gnd).
Check (clean) the reverb jacks for any oxide.
Check metal Reverb unit is connect to the reverb chassis (gnd).
(check red color continuity)

[Helmholtz]

What happens to hum when pulling V1?

[bobloblaws]

Still makes sense to check the cathode cap.

You mean C14? It actually is missing from this particular amp.

A scope would be a big help.

I have a digital scope, but I don't know how to utilize for this application. I'm keen to learn though!

I faintly remember a similar Gibson amp hum problem lately but don't seem to recall how it was resolved. Anybody?

You might be referring to the other thread for this same issue that I linked to in post #1 above. Or maybe something else?

A humdinger pot as advised in the other forum is always a good addition. It is not necessary to go down as low as 100R and with a 500R (470R) pot it doesn't have to be wirewound as dissipation is only around 80mW. But this will not adress the root cause of the hum.

In any case I have a 100R wirewound ordered from Digikey, should get here in a couple of days. I don't have any pots around less than 50K.

[bobloblaws]

What happens to hum when pulling V1?

It eliminates that hum.