Gibson Medalist 4/10 - Bleed thru of signal w/volumes all the way down

[Mars Amp Repair]

OK...this driving me NUTZ,
I'll attach the schemo for your reference.
Initially, this amp had a blown EL34. Replaced the pair. This amp only has a tube balance pot that takes the place of the cathode resistors w/fixed negative bias to the grids. Idle wattage was extremely high so paralleled the neg bias series resistor to achieve idle wattage that was within acceptable range.
Main cap-cans in preamp & main amp all not leaking & functioning.
I only replaced 2 1uf bypass caps in the preamp section and one1uf on cath of V6.
At this point I really hadn't turned the channel volumes down all the way so unfortunately, I cannot say whether or not it was this way before I worked on it. Also it is new to the customer but he doesn't recall if he ever turned the volumes down all the way. Once I did that I realized that there as a fair amount of signal bleed-thru on both channels with volumes all that way down.

The signal goes from V1a p7 thru the volume pot and tone stack to V1b p5 grid. Grounding out the signal at the volume pot has no effect on the bleed-thru signal.
After a bunch of signal tracing & disconnecting this and that and filter cap jumping (to be certain signal not a decoupling issue), I finally just disconnected the V1b grid p5 and even that did not change the bleed-thru!
The only thing that cancels the bleed-thru is grounding that grid. BTW, I have removed all the tubes except V-1 and V-6.
It seems that the signal is either bleeding thru internally in the tube itself (I have xnged the tube) or the socket is somehow weirdly conductive but only between pins 5-V1B & p7-V1a the plate of the input amp section of the tube. P 6 is the plate of
I've even signal traced the grounds to see if somehow weirdly the signal is sneaking thru them....weird I know but a dying man grabs at straws .
I've added a shielded cable to V1b p5 just to eliminate the antenna effect while it's disconnected to be certain it's not picking up the amplified signal from V1a plate to no avail.
I tried to measure the actual tube socket to see if it's somehow become conductive...not.
I've thoroughly cleaned the tube socket, too.
Much of this seems very implausible as channel 2 suffers from the exact same malady.
Any ideas...Thanx, glen

[Helmholtz]

Conductive board?

[Mars Amp Repair]

Thanx Hemholtz for the quick response.
This amp is from 1969 and is point to point wired. Good idea, tho. glen

[Helmholtz]

Can't exclude the possibility of a little crosstalk between the triode sections.
Of course then all these amps would have suffered from the issue.

Otherwise I'd suspect some bad ground contact(s).

[The Dude]

I know you said you checked/changed the node filters, but I'd insert some audio, hang a scope on node F, and make sure there's no audio riding on that supply just to be sure. It could be a connection problem somewhere on that node. That's about the only thing common to both channels.

[Helmholtz]

... but I'd insert some audio and hang a scope on node F and make sure there's no audio riding on that supply just to be sure. It could be a connection problem somewhere on that node. That's about the only thing common to both channels.

So would I.

But observe probe and scope voltage limits.

[Helmholtz]

If the voltage limit is too low, a say 10n/600V cap in series with the probe would block the high DCV.

[Mars Amp Repair]

Here are a couple of pix of the underside of the chassis. You can see in the close up of V1 socket the p-5 green wire unsoldered from the terminal strip where it goes. If I signal trace the terminal strip where the signal comes from the tone stack, there is no signal there. If I signal trace pin 5, I hear the low signal there as well as on the corresponding plate p6. Ground pin5 and the signal disappears on p5 & as well on the plate p6.btw
BTW, the signal originates from p7 of this 6EU7.
I'll update this when (IF) I find this issue or some kind of solution.
Thanx all...g

[Mars Amp Repair]

Regarding the post by The Dude (I can't figure out how to respond to specifically one post) where he suggests signal tracing).
I have audio traced the power supply nodes with no signal passing there as well as for the first stage B+ actually removed the connection to the filter cap and subbed the cap to no avail.
I have signal traced the terminal strip grounds as well as bridged them to ground given they're merely rivetted there to no avail.
A real conundrum!
Thanx all...I'm still plodding on.

[Helmholtz]

Here are a couple of pix of the underside of the chassis. ...

Leaving the grid free-floating creates an unrealistic situation, likely making the grid highly susceptible to nearby fields.
I would at least add a grid leak.

[Mars Amp Repair]

Agreed,
I added the 470K grind to ground resistor as it would have been hooked up to try & emulate the original situation & put a shielded cable on the grid to try and prevent it from acting like an antenna. Still the identical situation of a signal traceable signal on the end of the shielded cable. Difficult to emulate the exact situation.

[Mars Amp Repair]

Next I tried completely isolating the control panel from the V-1 Circuit by disconnecting the grid from the input circuit & feeding a signal into p-9 of V-1b. Then disconnected the signal path from the plate of V1b on pin 7 connected thru a shielded cable to air to prevent it from acting like an antenna.
I left the grid of V1a p5 disconnected with a shielded cable and 470K grid leak resistor to ground from P-5.

Experienced the exact same scenario. Signal imposed on the grid p5 of V1a with absolutely nothing connected to it and of course signal on the plate on V1a P6. No signal found on either the grounds (which are all grounded at one point like their own star ground) or the 'F' node of the power supply.

The customer never turned the volumes down all the way to notice if this was an issue for him and as I mentioned he has only had this amp for a very short time before the power tubes shorted.
I am really beginning to think the signal is bleeding thru the tube somehow internally (crazy I know) or somehow thru the tube socket as there really isn't anything left.
Yes, all freakin' crazy ideas.
I may just at this point explain to cust that it's something he's just going to live with unless he wants to spend a shit-load more labor for me to do something crazy like try another tube socket.
What else can ya do.
Thanx for listening-glen

[g1]

(I can't figure out how to respond to specifically one post)

All you can do is use the 'quote' button at the bottom of each post. From there you can delete parts but you must leave the tags at beginning and end of quote.

I would tend toward internal crosstalk of the tube, you said both channels act same?

[Mars Amp Repair]

[QUOTE=g1;n968817]
All you can do is use the 'quote' button at the bottom of each post. From there you can delete parts but you must leave the tags at beginning and end of quote.

I would tend toward internal crosstalk of the tube, you said both channels act same?


Thanx for the info on replying to a specific post.
Well I was even beginning to tend towards that one until I made this make-shift 6EU7 to 12AX7 adaptor and came up with the same results! I'm missing something obvious here or testing for the crosstalk in the circuit a way that is masking the effect like having my scope/audio tracer ground in a position that is masking the issue...I'm really baffled at this point.
Thanx again....glen

[g1]

[QUOTE=Mars Amp Repair;n969123]

All you can do is use the 'quote' button at the bottom of each post. From there you can delete parts but you must leave the tags at beginning and end of quote.

I would tend toward internal crosstalk of the tube, you said both channels act same?


Thanx for the info on replying to a specific post.
Well I was even beginning to tend towards that one until I made this make-shift 6EU7 to 12AX7 adaptor and came up with the same results! I'm missing something obvious here or testing for the crosstalk in the circuit a way that is masking the effect like having my scope/audio tracer ground in a position that is masking the issue...I'm really baffled at this point.
Thanx again....glen

You left off the end quote tag, that's why the quote didn't show properly.

Agree with you that the 12AX7 showing the same issue implies the crosstalk is not internal to the tube.