Tube Amp Hum Problem

Thread Starter

JackSpratt

Joined Jul 27, 2017
25
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 other than 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?
 

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nsaspook

Joined Aug 27, 2009
8,688
For old tube amps I would always check the B+ filters. C12 section A&B would be where I would first check for excess DC ripple.
 

AlbertHall

Joined Jun 4, 2014
11,506
Thanks, but it's not a ripple/filtering issue. When C15/R15 is grounded there is no hum.
That is not necessarily true. It may be that the reverb circuit is particulary sensitive to ripple on the supply.
Check C12A which filters the reveb supply.
Make sure that the heater supply is grounded.
Was this amp working correctly and then one day it wasn't or has it never worked for you?
 

dendad

Joined Feb 20, 2016
3,914
One thing sometimes done is to remove the heater gnd and add a 100R wire wound pot across the heater 6.3V with the wiper to gnd. Then null the hum with the pot.
But make sure that is the only gnd on the heater lines.
 

Thread Starter

JackSpratt

Joined Jul 27, 2017
25
That is not necessarily true. It may be that the reverb circuit is particulary sensitive to ripple on the supply.
Even when the grid is grounded? That would preclude any amplification of ripple, no?

Was this amp working correctly and then one day it wasn't or has it never worked for you?
I'm trying to fix it for someone else. He bought it about a year ago and told me that hum was there at least since he's owned it.
 

AlbertHall

Joined Jun 4, 2014
11,506
With the grid grounded, the anode is still connected to the supply, smoothed by C12A. When you ground the output from the anode the hum disappears.
You could try bridging another suitable capacitor across C12A.
 

KeithWalker

Joined Jul 10, 2017
1,919
Have you tried replacing V3? A heater to cathode partial short in that tube could cause the problem.
If that doesn't cure it, You could add a hum-dinger as dendad suggested in #5. Connect the 100 ohm pre-set pot across the heater winding. Disconnect the ground from the end of winding and connect it to the slider of the pot. Adjust the pot for minimum hum.This balances out any AC leakage between the tube heaters and cathodes.
 

Thread Starter

JackSpratt

Joined Jul 27, 2017
25
With the grid grounded, the anode is still connected to the supply, smoothed by C12A. When you ground the output from the anode the hum disappears.
You could try bridging another suitable capacitor across C12A.
Thanks, I just tried your recommendation, and a new 22uF by itself, no change in either case.
 

Thread Starter

JackSpratt

Joined Jul 27, 2017
25
Have you tried replacing V3? A heater to cathode partial short in that tube could cause the problem.
If that doesn't cure it, You could add a hum-dinger as dendad suggested in #5. Connect the 100 ohm pre-set pot across the heater winding. Disconnect the ground from the end of winding and connect it to the slider of the pot. Adjust the pot for minimum hum.This balances out any AC leakage between the tube heaters and cathodes.
Yes, have tried replacing V3. For the heater pot idea, is it critical what kind of pot it is? I've never heard of a wire wound pot.
 

KeithWalker

Joined Jul 10, 2017
1,919
Thanks, so does it need to be a wire wound pot specifically?
No but it will have 6.3V across it so it must be able to handle at least 0.4 Watts. The resistive track of most potentiometers is made from carbon particles suspended in plastic. That can not dissipate much power without becoming damaged.
 
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Thread Starter

JackSpratt

Joined Jul 27, 2017
25
No but it will have 6.3V across it so it must be able to handle at least 0.4 Watts. The resistive track of most potentiometers is made from carbon particles suspended in plastic. That can not dissipate much power without becoming damaged.
OK, well I went ahead and ordered a wirewound pot from digikey since I didn't have a 100R anyway. When it gets here in a few days I'll give it a try.
 

Jon Hoover

Joined Oct 10, 2019
33
I would remove C22 completely. Don't use that portion of the switch at all. Add a 3 wire grounded line cord and put 2-100 ohm, 1%, 1 watt resistors in series across the tube heater output of the transformer and ground the point where the 2 resistors connect to each other. C22 could be leaky and could be inducing line noise into the chassis. It's also called a "death cap". If it leaks too much or it shorts, it could make the chassis hot and could be very dangerous. That's why you should remove the cap and make sure you have a grounded, 3 wire line cord on the amp. 2nd thing is the humdinger resistor arrangement. You can use a 100 ohm wire wound LINEAR TAPER minimum 1/2 watt pot connected the same way but I have had excellent luck with just installing 2 fixed 100 ohm 1% 1 watt resistors in the same arrangement. The 1% rating will insure that the ground reference to the heater circuit remains balanced. This may not completely solve your issue but it is a very good idea to do this for ALL vintage amps to make them safe and to minimize any hum that may be caused by the tube filament being lifted from ground reference. Also, check C14 and C8 to make sure they are not leaky. Just lift 1 leg of each and see if your hum goes away. Those could really mess with your gain on the reverb recovery and the coupling transformer stages if they are leaking. Also check/replace C15 and see if that one is giving you any issues. That is what couples your post reverb signal to the coupling transformer driver tube. Good luck and let us know what you figure out!
 
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