Signal leaking through with volumes off

[Chuck H]

I've been following along and I just can't help but think that there may be something less nefarious than induced signal from radiant fields or a conductive circuit board. There are a lot of grounding or other wiring errors that could cause this problem. I'm sure the ground scheme and circuit wiring has been inspected multitudinously (my two dollar word for the day ) but I've fussed over such things for days myself until I eventually found my own mistake looking things over for the hundredth time.

[Greg_L]

I've been following along and I just can't help but think that there may be something less nefarious than induced signal from radiant fields or a conductive circuit board. There are a lot of grounding or other wiring errors that could cause this problem. I'm sure the ground scheme and circuit wiring has been inspected multitudinously (my two dollar word for the day ) but I've fussed over such things for days myself until I eventually found my own mistake looking things over for the hundredth time.

I agree with the notion that it's always probably something simple and stupid. Totally. Whenever I fix anything I always focus on the simplest most likely situations first. But damn...I just can't see it on this thing. I've tapped and wiggled and moved wires, I've redone and relocated grounds, I've traced and traced and traced to the best of my admittedly meager abilities. I measured, tested, and verified every cap and resistor before it went in. The amp works flawlessly and sounds amazing when you're actually using it for what it's supposed to be. It's just got this dumb slight signal leakage!

Having said all that, I hope you're right and my own mistake just pops right out at me soon.

[galaxiex]

Didn't read the whole thread...
...but could it be something as simple as V1A and/or V2A modulating the power supply?
Those triodes are before any volume controls so they are at "full" output even with the volumes at "0".

Sorry if this has been covered already.

[Chuck H]

I'm with Enzo. Divide and concur. You can use basic trouble shooting techniques to isolate the matter. Enzo's brass tacks practices have worked for me and many others here on more than one occasion. Just look at his rep! One step at a time... Starting with what Enzo suggested

[Greg_L]

I'm with Enzo. Divide and concur. You can use basic trouble shooting techniques to isolate the matter. Enzo's brass tacks practices have worked for me and many others here on more than one occasion. Just look at his rep! One step at a time... Starting with what Enzo suggested

I believe I've found where it's happening. It happens on both channels in the same way. Ground the second grid of each channel and a little signal from the previous stage is still getting into the plate of "b" and onto the rest of the amp. Now why that's happening is still a mystery.

[g1]

You could have poor filtering (or rather de-coupling) and may not necessarily see ripple on the filters.
Not sure if you tried paralleling good caps across the front end filters.
I just had a unit that only hummed when reverb was on. Fine with reverb off. Oddly enough it was the front end filter caps.

[Helmholtz]

[QUOTE=g1;n913828]You could have poor filtering (or rather de-coupling) and may not necessarily see ripple on the filters.
Not sure if you tried paralleling good caps across the front end filters.
QUOTE]

All this has been adressed earlier in the thread. If there is not a single mV signal either between different ground points or on the supply node (as stated by the OP), there must be a different reason. That's why I'm now suspecting a conductive board.

[Greg_L]

[QUOTE=Helmholtz;n913834]

You could have poor filtering (or rather de-coupling) and may not necessarily see ripple on the filters.
Not sure if you tried paralleling good caps across the front end filters.
QUOTE]

All this has been adressed earlier in the thread. If there is not a single mV signal either between different ground points or on the supply node (as stated by the OP), there must be a different reason. That's why I'm now suspecting a conductive board.

True. I looked hard at the B+ rail and the ground side. No signal in either.

[strato56]

If AC-grounding the grid from V6B kills the signal, maybe you should try AC-grounding the R27/C21 node.

[Greg_L]

If AC-grounding the grid from V6B kills the signal, maybe you should try AC-grounding the R27/C21 node.

I'll go back and re-check, but I think I've tried that already. What will that tell me?

[Helmholtz]

Okay back to this....clean signal at V1b cathode with probe grounded at C20

I read this as no signal at V1b cathode, correct?

[Greg_L]

I read this as no signal at V1b cathode, correct?

Correct, no signal. Zero signal. By "clean" I meant no unwanted AC signal in there.

[Helmholtz]

Did you try lifting the complete node which connects V1A plate wire, R15, R16 and C6 from the board (for normal input) ?
If your problem is caused by a conductive board, this node will be the point where the leakage enters the board. So the V1A plate signal must not connect to the board.

[Greg_L]

Did you try lifting the complete node which connects V1A plate wire, R15, R16 and C6 from the board (for normal input) ?
If your problem is caused by a conductive board, this node will be the point where the leakage enters the board. So the V1A plate signal must not connect to the board.

I have not tried that yet. I had a busy weekend and will be getting back on it today.

[Greg_L]

Here is a trick. Get a small piece of metal, maybe the size of a business card or even a playing card. COnnect a wire to it. Now cover it with tape so it can't touch anything live. Ground the wire to the chassis. et voila you have a moveable shield. You can stick it between parts or under wires or wherever looking for possible sensitive areas.

Okay I tried this. I made a little shield, soldered a wire to it, covered it with tape. I grounded the wire and carefully probed and prodded the shield between wires, under wires, everywhere I could. No change.