Volume won't turn down all the way

[Smokebreak]

I'm goin a little crazy over here. This amp, which started as an express last year is a JCM800(emplexador spec) today, PPIMV, no normal master. As an express, I now remember that the volume would never actually go silent, when turning the gain knob all the way down. It was minor, and I never though much about it. Now, with an added stage, it's basically a little louder than bedroom volume, when the gain is turned all the way down. PPIMV works fine, and kills the signal btw.
I took out the 2nd stage and put it to plexi specs, and the volume still won't die, though granted it's a little quieter than it is with the added stage.

I can't think of anything else to do, or how the signal is getting through, but figure there is a common cause of this. I've replaced the pot, measured all the pots>ground, changed a bazillion tubes, reflowed joints. Something else I noticed is that as an 800, it had a boatload of gain at the beginning of the gain knob, no headroom at all. The pot was a Bourns D1M, I believe that's a 10% at midway. I replaced it with the same.

So basically, this amp has been 3 amps, different pots, and exhibits the same behavior no matter how it's wired. Any ideas?

[Leo_Gnardo]

Crosstalk within a dual triode - your first stage triode is "talkin' to" your second stage within the 1st pre tube. Common in Fenders for instance. And that extra gain stage just makes it more obvious. May have to add yet another pre tube to escape this problem, so that the first & 2nd preamp triodes aren't within the same tube.

[Zippy]

The pot was a Bourns D1M, I believe that's a 10% at midway. I replaced it with the same. So basically, this amp has been 3 amps, different pots, and exhibits the same behavior no matter how it's wired. Any ideas?

You may have replaced the pot, but if it is the design of the pot that's getting you, it's still the same.

Measure the resistance between the middle and outer legs of the pot. I'm guessing that it doesn't go close enough to zero to dump all the signal to ground.

I'd be interested to learn whether the crosstalk option is working here. Have you switched tubes around to see whether that is an effect?

[Smokebreak]

Crosstalk within a dual triode - your first stage triode is "talkin' to" your second stage within the 1st pre tube. Common in Fenders for instance. And that extra gain stage just makes it more obvious. May have to add yet another pre tube to escape this problem, so that the first & 2nd preamp triodes aren't within the same tube.

Well that's what's got me scratchin my head. Wired as a plexi, I'm only using one triode of V1. 2nd stg/CF is V2a/V2b . I do have V1b's cathode and plate wired up still, but no grid. Plate "goes" nowhere.

[Smokebreak]

The pot was a Bourns D1M, I believe that's a 10% at midway. I replaced it with the same. So basically, this amp has been 3 amps, different pots, and exhibits the same behavior no matter how it's wired. Any ideas?

You may have replaced the pot, but if it is the design of the pot that's getting you, it's still the same.

Measure the resistance between the middle and outer legs of the pot. I'm guessing that it doesn't go close enough to zero to dump all the signal to ground.

I'd be interested to learn whether the crosstalk option is working here. Have you switched tubes around to see whether that is an effect?

That's true. I'll try an Alpha. Yep I've tried nearly every tube I have including my #1 quiet, goto for high gain. I actually have a piece of tape on it that says "#1"
Bourns is 10ohms > ground

[pops]

Are you using a single cathode resister? Try using separate for each side of the tube.

[Smokebreak]

Are you using a single cathode resister? Try using separate for each side of the tube.

They're split. I had non-elytic .68s and changed those to elytic, but that didn't change anything, except that I put 1uf on which btw is nice right there

[Smokebreak]

Ok I put in a mojo 1M and that did the trick! Thanks for the tip, I would have torn apart the rest of the thing. So 2 Bourns pots no bueno for volume control...The taper is MUCH better on the mojo, too. Much more gradual, even though I just measured both Bourns, and the taper was indeed close to 10% at midway, which I would think would mean a more gradual taper.

[Leo_Gnardo]

Crosstalk within a dual triode - your first stage triode is "talkin' to" your second stage within the 1st pre tube. Common in Fenders for instance. And that extra gain stage just makes it more obvious. May have to add yet another pre tube to escape this problem, so that the first & 2nd preamp triodes aren't within the same tube.

Well that's what's got me scratchin my head. Wired as a plexi, I'm only using one triode of V1. 2nd stg/CF is V2a/V2b . I do have V1b's cathode and plate wired up still, but no grid. Plate "goes" nowhere.

Shoots that theory in the ... well well well. A volume pot that doesn't go to or near zero will also do it. Glad you found a fix!

[Zippy]

Ok I put in a mojo 1M and that did the trick! Thanks for the tip, I would have torn apart the rest of the thing. So 2 Bourns pots no bueno for volume control...The taper is MUCH better on the mojo, too. Much more gradual, even though I just measured both Bourns, and the taper was indeed close to 10% at midway, which I would think would mean a more gradual taper.

What do I win?

Thanks for having the faith to give it a check.

Now, answer me this... Why would you ever turn an amp like an Express or Marshall to zero?

Enjoy!

[Gaz]

For others that come across this thread, a simple way to figure out if it's your pot is to simply ground the wiper with a jumper cable. If that mutes the sound then it's the pot or some other cross-talk earlier in the circuit. You can further isolate the pot by simply muting the signal at the input of the pot, if that also mutes the signal, then it's definitely the pot!

[cbass]

Also try cleaning the pot even if its new

[Smokebreak]

Thanks erybody! Zippy, Today the prize is a gently used Bourns 1M Pot with built-in, patent pending, "bedroom level zero-set"!

[sepulchre]

Maybe it's just been my bad luck but I've never had a Bourns I liked. Every one had some problem or other. But then I haven't tried one in awhile (4 or 5 years).