Fix for ground loop hum in Gibson Falcon GA 19 RVT

[SoulFetish]

So we got one of these amps in yesterday with the customer complaint about noise in the reverb. It wasn't in the reverb, and after an audio test to demonstrate the issue, it sounded like your run of the mill failing power supply cap.
Here's where it gets a little interesting; After clipping in helpers on each of the nodes to see if we could identify a failing cap, the noise persisted. We checked all the tubes, but I was suspicious the noise was due to a grounding problem. The caps had actually been replaced at some point, but here is the schematic:
Gibson Falcon Reverb
So, I have to run, but I'll holler back at you guys later on today and finish the story. Sorry for the tease

[loudthud]

I see the amp has no ground switch on the schematic. Has the line cord been replaced with a three wire type ? If not, try spinning the line cord plug around 180 degrees.

[SoulFetish]

I see the amp has no ground switch on the schematic. Has the line cord been replaced with a three wire type ? If not, try spinning the line cord plug around 180 degrees.

It has been updated with a grounded power cord.

So, here's what was going on.....
The amp came in like this:



Firstly, the 5Y3 has a design max input capacitance of 32µF, so the 47µF replacements for the 20µF listed in the schematic was probably installed as a punt to try and solve the hum issue.
The strange part was, if I pulled the reservoir capacitor out of circuit altogether, the hum almost disappeared completely. If I reconnected it back in circuit, the hum returned. If I pulled the second 47µF cap after the 470Ω dropper, the hum died down, but not as quiet as the first. (lifting last two 10µF caps made the noise worse).
Clearly, the amp was never intended to be operated with no reservoir cap, so I was pretty certain we were dealing with a ground loop in the power supply. It turns out, that's exactly what the issue was, and here is the fix:



The amp is now extremely quiet at idle, and the customer should be very happy with the results. The amp is in fantastic condition as well.

[Tom Phillips]

Good job and excellent clear writ-up.

[Leo_Gnardo]

Good job and excellent clear writ-up.

Agreed! I've done these and similar amps like SoulFetish but ... hey dude you wrote it up, with photos, schematics & everything! Good work!

[BirdSG]

Im working on my Gibson Falcon for the better part of a year now. But I am also trying to get rid of unwanted noise. Was the noise you describe more of a "White noise." Mine is coming across as a white noise/frying pan noise. It gets louder with the volume knob and the tone knob changes the white noise pitch. Ive replaced all of the preamp tubes, all of the electrolytic caps, replaced the carbon comp resistors around V1 with metal film and retouched solder joints but the noise is still there. If we are talking about the same type of noise, I may have to try this tomorrow.

[SoulFetish]

Bird, welcome to the forum!
it’s been a while, but I remember it being more of a hum type of noise. I think that what I wrote up in this thread improves the original grounding scheme, so its not a bad thing to do when your in there.
but, i don’t think that’s your problem, by the sound of what your describing:
heres what I would do:
turn on your amp with the output going into a speaker.
pull your tubes, one at a time starting with you first preamp tube closest to your input jack (usually V1).
Pull them in order, stopping after each one to see if the noise stops.
Try this and report back to us which one you pulled stopped most of the noise.

[BirdSG]

Bird, welcome to the forum!
it’s been a while, but I remember it being more of a hum type of noise. I think that what I wrote up in this thread improves the original grounding scheme, so its not a bad thing to do when your in there.
but, i don’t think that’s your problem, by the sound of what your describing:
heres what I would do:
turn on your amp with the output going into a speaker.
pull your tubes, one at a time starting with you first preamp tube closest to your input jack (usually V1).
Pull them in order, stopping after each one to see if the noise stops.
Try this and report back to us which one you pulled stopped most of the noise.

Sorry for the delay! Ive been busy the past couple of days, but greatly appreciate the response.
So I've been watching a lot of videos with the guy from D-Lab that works on the amps. One of his common tactics is banging on things with the ol chop stick. lol. So inside the chassis I haven't really found anything to note..but tonight, with everything reassembled and the amp on and at 10 I may have found something. When I tap on my tunes, V1 gives me a pinging sound out of the speaker, and a little bit of crackle if a little bit of pressure it applied to wiggle it. All of the other tubes are silent with the exception of one crackle I got out of V3 when pressure was applied. I swapped out V1 with another tube I had and the same result. So its not tube itself related. I wondef if there is something wrong with my socket.

I also took your advice and yanked my first two tubes one at a time.
I pulled V1 and fired up the amp, the noise was there. Quieter, but there.
I pulled V2 and it was dead silent.

Having both tubes in place and firing it up, returns it to its loud normal white noise level.

Also, in my recent attempts, i installed a shielded cable to go from my input jack to V1 with no success on noise elimination

Any thoughts?

[Jazz P Bass]

If pulling V2 kills the noise then the issue is at V2 or V1.

[SoulFetish]

I also took your advice and yanked my first two tubes one at a time.
I pulled V1 and fired up the amp, the noise was there. Quieter, but there.
I pulled V2 and it was dead silent.

Having both tubes in place and firing it up, returns it to its loud normal white noise level.

Also, in my recent attempts, i installed a shielded cable to go from my input jack to V1 with no success on noise elimination

Any thoughts?

Okay, now we can narrow it down even further. So, we know the noise is in the circuit(s) driving V2, or directly proceeding the stages.
First, what happens if you disconnect the return from reverb tank? If the noise goes away, try and swap it out for another reverb tank if you have one handy.
Next (if the previous suggestions had no effect), we can make a probe to narrow our search down. Take a .47uF/630V cap and solder/connect one lead to an alligator clip, which you connect to ground. You are going to use the other lead to probe the grids and plates of V2.
++YOU ARE GOING TO BE PROBING LIVE HIGH VOLTAGE NODES, SO PLEASE USE PROPER SAFETY MEASURES.++ ie-good practice is to keep you free hand in your back pocket during this procedure
This acts as a shunt and should tell you where your problem is. Start at the grid of V2 where the volume control feeds to. If that doesn't kill the noise, move to the plate of that side of the tube, and then the grid of the other half of the triode and so on.
Report back.

[g1]

A little ping and crackle when wiggled from V1 with amp turned up to 10 seems pretty normal.

[BirdSG]

Okay, now we can narrow it down even further. So, we know the noise is in the circuit(s) driving V2, or directly proceeding the stages.
First, what happens if you disconnect the return from reverb tank? If the noise goes away, try and swap it out for another reverb tank if you have one handy.
Next (if the previous suggestions had no effect), we can make a probe to narrow our search down. Take a .47uF/630V cap and solder/connect one lead to an alligator clip, which you connect to ground. You are going to use the other lead to probe the grids and plates of V2.
++YOU ARE GOING TO BE PROBING LIVE HIGH VOLTAGE NODES, SO PLEASE USE PROPER SAFETY MEASURES.++ ie-good practice is to keep you free hand in your back pocket during this procedure
This acts as a shunt and should tell you where your problem is. Start at the grid of V2 where the volume control feeds to. If that doesn't kill the noise, move to the plate of that side of the tube, and then the grid of the other half of the triode and so on.
Report back.

So using your above method, Probing the grids and plates of V1 does nothing.
Probing grid 1 of V2, get rids of the white noise but instead turns it into a straight low, loud hum.
probing Plate 1 of V2 kills the signal altogether. No sound.

Probing grid 2 and plate 2 of V2 does absolutely nothing to the white noise and does not produce any extra hum or noise

Not sure what this means. But maybe it means something to someone that can help me out.
Side Note - i did not have a .47 capacitor. I only had a .047/630volt cap. Not sure how much difference that would make. I would think just its filter effect on the white noise.

Thanks in advance!

[SoulFetish]

Then I would suspect either the 680k grid leak resistor or the 470k plate resistor in V2a.
do you have metal films you can replace those with?