Gibson GA-15RVT Repair

[rnevins]

I am trying to troubleshoot a hum issue with a '67 Gibson GA-15RVT amp.  The schematic is in the el34world schematic database as Gibson GA-15 RVT w_Choke.  I recently replaced the electrolytic caps and replaced the original power cord with a grounded cord.  Everything is working fine (including tremolo and reverb) however when I depress the reverb footpedal switch I get a fairly loud 120 hz hum.  The hum will go away if I unplug the reverb tank.  The hum is not affected by the setting of the reverb pot but does get louder when loudness control is increased.  I have checked all the plate and cathode voltages and all appear ok.  I've also checked the capacitance and resistance of all the caps and resistors in the reverb driver and recovery circuits.  I have another known working reverb tank from a GA-20RVT and it still makes the hum when reverb button is depressed.  Also if the reverb cable is attached at the amp end and disconnected at tank end it hums.  I'm thinking ground loop but I've tried jumpers to ground the tank to the chassis with no help.  Any ideas or troubleshooting steps I could take would be appreciated.  Thanks in advance.

[Dave]

Many times I have seen Fenders or Fender clones that have a ground loop at the reverb recovery cathode causing very similar symptoms. I haven't specifically seen it in an old Gibson, but I wouldn't doubt it, in fact, it would be my first suspicion.


Its easy to check, just remove the ground connection and alligator clip it someplace else to see if there is a change.


Dave

[sluckey]

when I depress the reverb footpedal switch I get a fairly loud 120 hz hum.

If you unplug the footswitch does the hum go away?

The cable to the reverb footswitch must be shielded. Is it?

[rnevins]

Sluckey - thanks for your suggestion.  When I unplugged the footswitch it made no difference.  It is the original footswitch cord.

Dave - when I did as you suggested and grounded the R15 resistor (see schematic) coming off of the cathode of V1B to a different spot, the hum went away.  I actually grounded it with the alligator clip to a lug on the footswitch pedal socket, which happens to be the same place that one side of the filament circuit is grounded.  The other side of the filament is ungrounded.  And V1 is the last tube in the chain so the filament of V1 is connected to the ground lug on the footpedal switch.  I have read/seen elsewhere that a virtual center tap using two 100 ohm resistors is recommended for the filament circuit, so I think I will do that and see what difference that makes also.  But at least for now I have a solution.  Would you recommend the virtual centertap of the filament circuit?  (there is no center tap on the power transformer).  Thank you.

[Dave]

Very happy you found a solution. Personally, I wouldn't like the heaters wired that way although I have seen it before and it seems to work fine. Yes, I would probably change it. Would it really make a difference? Who knows.


Dave

[dogburn]

I'm interested in this because I've got a reverb hum to deal with in my own project. But I have a question about which resistor was grounded differently - rnevins you said R15 coming off V1B, but on the schematic, R15 comes off V2A. Did you mean R20?

By the way, one of those amps came up for sale yesterday on Facebook marketplace for $250. It's probably sold already...

[Dave]

The one I'm talking about is the cathode resistor on the reverb recovery tube. Whatever it is being called on whatever schematic. I have seen it cause ground loops in Fenders and Ampegs and lots of homebrew amps.

To be clear, this is the stage where the grid is fed by the reverb tank.

Dave

[rnevins]

Sorry for the confusion - I mis-spoke.  It was resistor R20 coming off of V1 pin 9 that I grounded to a different spot that eliminated the hum.  It is the recovery tube connected to the output of the reverb transformer.

[dogburn]

rnevins - thanks for the clarification!

Dave, thanks - that's what I thought you were saying. I've recently built an AB763 single channel with reverb (based on Rob Robinette's layout), and the reverb recovery cathode is tied together with the third preamp cathode and then to the preamp ground bus through an 820 Ohm resistor and 25uF cap. Would you recommend separating the two so the reverb recovery cathode is grounded by itself?

[Dave]

I recommend you play around with it until you find where it likes to be grounded the most. That particular ground can be a little weird and sometimes it's only happy in a specific spot that you might not have guessed.


Dave