Signal leaking through with volumes off

[Chuck H]

In the future I will probably not use these types of eyelet boards anymore.

Right on there. But if this is your personal amp or if it's an amp that's going to remain with someone in your circle you might want to consider at least getting or making a normal fr4/g10 board in case it comes up later. This has been my own experience when amps that exhibited conductive board problems hang around a while. They don't get better with age and use. Already having the board on hand could take some bite out of the chore. Or, since you like the tone of this amp, you'll have a proper, non black board on hand if/when you decide to build another.

[Greg_L]

Right on there. But if this is your personal amp or if it's an amp that's going to remain with someone in your circle you might want to consider at least getting or making a normal fr4/g10 board in case it comes up later. This has been my own experience when amps that exhibited conductive board problems hang around a while. They don't get better with age and use. Already having the board on hand could take some bite out of the chore. Or, since you like the tone of this amp, you'll have a proper, non black board on hand if/when you decide to build another.

Yeah this amp is for a friend who lives several states away. I've kept him fully updated on all of these shenanigans and he's fine with whatever. His general attitude is "fix it if you can, no big deal if you can't". Like most people he's not too concerned with what the amp does turned down to zero. But as the builder I do want him to have a perfect amp though, or as perfect as I can get it. So replacing the board later is probably out of the question. He's not a dummy though. I could probably build him a new better board and he could replace it himself if he wants. I honestly don't think it will come to that. Not to be morbid or anything, but he is an older gentleman.

I'm gonna try the hairdryer and just see what happens. If that doesn't improve anything I'll just put it back together and ship it off. Besides this little hiccup, the amp does work wonderfully under normal use. We're both content with just accepting these findings and moving on to the next project.

[Greg_L]

I just had maybe a bad idea....is there a way to shunt these rogue AC currents drifting through the board to ground? Or will that be a path to ground that we do not want?

[Helmholtz]

Leakage currents (AC and DC) are caused by voltage differences between rivets. As long as voltages are the same, leakage currents will be the same. If you try to pull down voltages you will also pull down normal signal levels resulting in reduced gain.

[Greg_L]

Leakage currents (AC and DC) are caused by voltage differences between rivets. As long as voltages are the same, leakage currents will be the same. If you try to pull down voltages you will also pull down normal signal levels resulting in reduced gain.

I was just thinking kind of like we use shielded cable to collect noise in the shield and dispose of it to ground. I was wondering if there was a way to collect the leaked signal and get rid of it while preserving the intentional signal.

[Helmholtz]

I was just thinking kind of like we use shielded cable to collect noise in the shield and dispose of it to ground. I was wondering if there was a way to collect the leaked signal and get rid of it while preserving the intentional signal.

I understand what you mean, but the situation is different. Shielding helps with electrical fields but not with current. The conductive board acts as a high Ohm (distributed) resistor connected to different voltages. If you want to stop the current through a resistor you need to disconnect at least one end/terminal. Unfortunately a board has many "terminals"/rivets.

[Greg_L]

I understand what you mean, but the situation is different. Shielding helps with electrical fields but ot with current. The conductive board act as a high Ohm (distributed) resistor connected to different voltages. If you want to stop the current through a resistor you need to disconnect at least one end/terminal. Unfortunately a board has many "terminals"/rivets.

Understood. I'm just spitballin here. I'm sure if there was a simple remedy someone would have found it by now.

[g1]

Now that the location of the leakage is localized, is it still not feasible to lift some components from the area?

[Greg_L]

Now that the location of the leakage is localized, is it still not feasible to lift some components from the area?

I suppose it's possible. I could implement some turret strips or just let them float into the air. I don't know if it's worth it though, and maybe it could cause some other kind of interference. There's not a lot of room in that general area.

[Greg_L]

Ok a few more updates.

Hair dryer....nothing. No change.
Tape on the board in the problem area - this did reduce the leakage getting into the probe. But there was still signal getting picked up.

The problem definitely is radiating out of the V1a plate supply/tone stack node on the board. Leakage is strongest in that area and dissipates as you move away. Not that we didn't already know this, but I dug and probed deeper into that area. I completely removed the V1b grid wire so nothing is leaking into the grid and grounded the grid itself. The leakage still got into V1b. So, that's just how it is. Without removing things and setting up turret strips, it seems like it just is what it is.

[Helmholtz]

I think that that the leakage signal directly bleeds from plate A rivet to plate B rivet through the board (short distance).

[Greg_L]

I think that that the leakage signal directly bleeds from plate A rivet to plate B rivet through the board (short distance).

Yes it's not even an inch across.

I didn't see this before, but I did pick up some signal radiating out of the V1a plate wire. I swapped in a different type of wire - no change.

[Helmholtz]

If there's still any doubt about the conductive board you may switch your scope input to DC coupling (no radiation effect) and probe the board for DCV around high voltage points (moving baseline).
Board leakage resistance should be the same for AC and DC

[Greg_L]

Not to intentionally revive a dead thread, but just an update since so many of you volunteered time and brain power to this issue.

The amp is several states away in the owner's hands and he is very pleased with it. So thanks again everyone for your help.