1960s Ampeg GS-12 Rocket 2. Crazy 120hz noise issue

[BrianS]

Hello folks,
For some reason, this amp is giving me fits.  Have never had this problem.
Link to schematic: https://ampeg.com/support/files/Schematics/G%20Series/GS-12/GS12%20Rocket%202%20Schematic.pdf
Without going into every detail of the entire repair, I'm hoping that describing this one thing will give you some ideas:
I have some nasty 120hz buzz that "originates" on pin 7 of V3.  Of course that follows through and shows up on the output...it is pretty dang loud.
When everything is wired up properly, the amp is otherwise fully functional.  It amplifies a signal & sounds good...other than the 120hz buzz.
In the course of troubleshooting, I have discovered that if I disconnect the shielded wire (shield wire terminates at the volume control ground) that goes from the wiper of the volume control to pin 7, V3, the 120hz buzz goes away, and there is just a faint 60cycle hum on the output, which could be attributed to my "noisy" workbench and/or the layout/design of the output stage.
HOWEVER, if I take my needle nose pliers and touch them from pin seven to the chassis of the amp (ground)...which would be the same condition that you'd find with the amp wired correctly and the volume control set to zero...the 120hz buzz jumps right back in. 

How is it that I can have this signal present only when pin 7 is grounded?  I searched for the source of this buzz for a long time, and was surprised in my findings.  I did replace all the power supply capacitors and everything looks to be wired correctly and all voltages are correct.  Again, the amp is "fully functional" at this point other than the buzzing, which is definitely only present when pin 7 of V3 is grounded.
This condition exists even with V4 (preamp & tremolo circuit tube) pulled as well as the other components of the trem circuit are disconnected...I basically disconnected everything before V3 while testing.

This condition also exists with 2 different tubes used in the V3 position:  the original 7199 OR a 6GH8A used with an adapter provided by the fine folks over at CE Distribution.
There is no DC voltage present on pin 7.

Any ideas?  Thanks.
--Brian

[HotBluePlates]

Without going into every detail of the entire repair ... I have some nasty 120hz buzz that "originates" on pin 7 of V3. ... When everything is wired up properly, the amp is otherwise fully functional.  ...other than the 120hz buzz.
... if I disconnect the shielded wire ... that goes from the wiper of the volume control to pin 7, V3, the 120hz buzz goes away ...

Vintage Ampeg amps have a very specific method of grounding; if you go changing this to "ground stuff right" the result is often added hum/buzz.  IIRC, the only place the ground buss touches the chassis is at the input jacks.

I think you'll have to tell us what else was done in the amp to have a sense of whether you accidentally created a ground loop.

[AmberB]

I would suggest that you don't need a shielded wire going from the volume control to pin 7 of V3.  It's pretty unusual to see a shielded wire in that part of a preamp circuit. 
Have you tried grounding the wire shield to the back of the volume control instead of the ground terminal of the control?

[PRR]

Have the main power filter caps' "grounding" been changed since it left the factory?

[BrianS]

Yes, I did change the power supply grounding scheme just a bit, as I could not use the original can cap.  I've been thinking about this a lot since I discovered this buzz.  The first 70uF cap is mounted to terminal strips I added in over the can cap area.  The other 3 electrolytics are mounted to the terminal board.  There was a lot of "extra" space to mount them there and so it just seemed logical. 

I will look at some gut shots of original amps to see about how the grounding was done...specifically where the terminal board buss meets the chassis...and try to determine how much change there is in this amp.

As to the shielded wire, I think I will keep it, as it is part of the original design.  I don't really want to "re-engineer" anything...other than the grounding scheme, which it seems that I've already screwed up!!
Thanks folks.
EDIT:  Looking a some gut shots of a GS-12-R (reverb version of this amp), it looks like the terminal board ground buss terminates at the can cap negative connections.  The chassis mounted pots have their own buss wire that connects to the terminal board at one point.
I have:
- terminated the 70uf cap at a separate chassis grounded lug on an added terminal strip.
-added a footswitch for the tremolo.  This did add a new ground from the terminal board to the chassis.
-added a jack for the speaker connection.  I did not isolate this jack from the chassis; just mounted it like a Fender jack.  Originally, the secondary of the OT is connected directly to the speaker terminals, with no chassis connection at all.
-The OT secondary has two parallel connected hook up wires for both the sides.  So on the "positive" side (green wires), one wire goes to the speaker, and one wire is used for the negative feedback circuit.  On the "negative" side (black wires), one wire goes to the negative speaker connection, and the other wire is connected to the ground buss...i think...I connected this wire to the ground buss on the terminal board.  I think at the same point as the original.
-The OT is a new Mercury Magnetics OT built to the "exact" specs as the original.  I am confident that it is exactly as they describe.

So I have essentially created 3 new grounding points.  Guess I have some work to do...

[BrianS]

PROBLEM SOLVED.
As HBP mentioned, the only chassis ground point is indeed at the potentiometers.  The can cap ground lugs are isolated from the chassis by a fiber washer, so all filter caps actually ground on the terminal board ground buss.

Once I moved the 70uf ground off the chassis and back onto the terminal board, the buzz was absolutely GONE.  There is also no other hum at all...the amp is dead quiet.
I tested every "new" ground I made before I changed the filter cap ground...none of those made a difference.  I did add in and leave isolation washers on the speaker jack, so that does not have a chassis ground.  I disconnected and reconnected the footswitch jack, and that did not affect the signal in any way, so I left that new jack in place and grounded at the chassis.
Thanks for your help!
-Brian

[HotBluePlates]

PROBLEM SOLVED.

Thanks for reporting back!  (not everyone comes back to say what resolved their problem)

[BrianS]

Yeah, I wish more people would report back with their solutions.  It really can be useful for those searching the forum before asking a question.
I've been doing electronics repair for a living for over 10 years now, but I do work alone and so sometimes I get a problem that for whatever reason just stumps me, and I don't have anyone else "in house" to bounce ideas off of. Sometimes just having someone else with more experience and knowledge suggest a couple things can open your eyes enough to get the problem solved.
So thanks to HBP and PRR for setting me on the right path to the solution.  And thanks to Doug for hosting this website & forum.  I've used it many times over the years and is always my first stop for schematics that I don't have on hand.
Brian @ Tree Strings, LLC

[tdvt]

Yeah, I wish more people would report back with their solutions.  It really can be useful for those searching the forum before asking a question...

..And thanks to Doug for hosting this website & forum.

Agreed that follow-ups are always helpful, & many thanks to Doug as well.