Failing dual gang pot?

[Esquirefreak]

Ah, so the amp started acting up... 😴

Weird, because I've had zero problems with it until this point.

When I switched it on today and started to play it made a faint distorting buzz on top of the note. I took it to my work bench where I verified grounding connections. 0.5Ω, which is what my meter leads measures. Also confirming that I had no continuity to ground where it's not suppose to.

Then I adjusted the LarMar MV dual gang pot and I heard the scratchy buzz on a few spots on the dial. As I turned it several times the scratch faded but now the pot does nothing. Full volume all along the track. It worked perfectly before.

I have taken it out of circuit and it measures good on both tracks, end to end.

Could the pot have failed even though it measures fine? And could it have failed because the bias supply is DC coupled to it?

Or are there any other suspects in the amp, so that the signal takes another path?

I have never experienced this behaviour on this type of MV and I've used it several times. Bias voltage and current draw is rock solid through out the amp.

/Max

[Latole]

IMO you may have a capacitor leaking / issue, not a pot issue.

Schematic diagram as built is a must

[Esquirefreak]

I thought about caps too. They're all new, just used for 1 month, but I know that's not a perfect guarantee. Are you referring to decoupling caps or electeolytics? No noise issues, as far as hum or background buzz goes.

No diagram, I'm afraid. It's built exactly as the schematic. I drew the schematic yesterday.

I just can't wrap my head around why the signal passes now when the pot worked flawlessly for a little over a month. Some component must have failed. I have some confidence in this build, because haven't made many mistakes in my other ones. Don't get me wrong, I'm not trying to be a smart ass.

By sunday I will try a couple of fixed grid leak resistors to out rule the potentiometer.

/Max

[Latole]

I mean coupling/ signal caps.

Schematic as built is a must to help you

[Esquirefreak]

It's built EXACTLY as the schematic, since I drew the schematic after the amp was built and working. Using mostly Vishay 1813 coupling caps.

/Max

[Latole]

It's built EXACTLY as the schematic, since I drew the schematic after the amp was built and working. Using mostly Vishay 1813 coupling caps.

/Max

Show it please.

[Esquirefreak]

It's in my original post.

/Max

[Latole]

I never follow it .

[Esquirefreak]

From reading up, I'm wondering if maybe my CF stage tube died. I have a Sovtek LPS in there and apparently that's a no-good-idea...

What symptoms would a dead/dying CF cause? It still doesn't explain the pot behaviour, right?

I post the schematic again below. The one that is also in my first original post in this thread.

/Max

[sluckey]

That cathode follower cannot be affecting your LarMar MV.

[sluckey]

Show it please.

Scroll to the top of this page and LOOK!

[Esquirefreak]

I swapped the dual gang pot out and all is fine now. I'm not sure why the old pot measured good, though.

/Max