Home build mildly fluctuating volume faint and end-of-note distortion

[Fred G.]

Hi All,

I have an Allen Accomplice I built in 1999. After years of modding and tinkering with this amp, I decided to do a major modification and turn this little amp into a JCM 800 clone. I love the results! I've also done a cap job on it, about a year ago, after (gulp) almost 20 years on the original Illinois E-caps.

A problem has surfaced - the volume has started fluctuating, VERY mildly, but just enough that my bandmates and I can hear it. There is also a very faint, extra distortion when a note's volume fluctuates "low". It's minor, and a non-musician may not notice it, but it's there, and it's a new phenomenon. And it's driving me NUTS.

The power tubes are old RCA's I've had in there for a while. They test "good" on Sencore and B&K tube testers (yeah, I know...), but I haven't swapped them out, as yet. The newer E-caps are Nichicons and Illinois.

I'm suspecting power tubes or one of the newer filter E-caps, but before I start digging it, I thought I'd float this in front of you guys.

Suggestions?

Thanks,

Fred G.

[g1]

Does it happen at all gain settings? Being that the JCM800 is probably higher gain, there is a chance it is intermittently getting unstable or oscillating.

[Fred G.]

Does it happen at all gain settings? Being that the JCM800 is probably higher gain, there is a chance it is intermittently getting unstable or oscillating.

Well, it's most noticeable when I turn the guitar volume down, to be honest. When the guitar volume is cranked, you don't notice it as much.

I'm a blues-rock guy, so I tend to set it and forget it - no pedals, I like a simple set-up and a good master volume works well for me. Once I find amp settings I'm happy with, it stays there, other than to turn the volume up or down. My tone is in the 70's Dickey Betts zone. This amp is a 2-6V6, 5AR4 rectified "marshall-ized" combo, nothing really fancy.

There was no problem for several months, so I'd be inclined to rule out any high-gain oscillations or instability unless a failing component is at fault - we're talking a pretty simple master volume combo here.

Thanks!

[The Dude]

It wouldn't surprise me if an Illinois cap was bad- new or not. I might try clipping known good caps across the existing ones one at at time to see if that gets results. If you have a good cap tester, that'd work, too.

[Fred G.]

It wouldn't surprise me if an Illinois cap was bad- new or not. I might try clipping known good caps across the existing ones one at at time to see if that gets results. If you have a good cap tester, that'd work, too.

I'm thinking I used the Illinois caps in the bias circuit (without having the amp out of chassis in front of me), because Mouser didn't offer the values I needed in the Nichicons. I used Nichicons this time around because the Spragues were ridiculously priced, and I was reading really good things about Japanese QA regarding the Nichicons, plus run-of-the-mill feedback about F&T et al....

A big issue here is that, right now, the effect is very subtle, and I don't have the luxury of dismantling my amp in my drummer's basement and playing around with it during rehearsal. The problem (of course) won't manifest itself enough in my basement shop for me do do any effective troubleshooting...

Hell, I guess I'm just basically whining, at this point. I'll play the amp at a low-exposure gig I have coming up and "stress" it, as well as the next rehearsal or two, and see if I can get something to break. I have an O-scope a friend gave to me recently, but I haven't played with it yet, nor have I built a dummy load for it yet, so that's another story, plus I have to self-educate on that, if it still functions (sat in a box in his old bedroom at his mother's house for 30 years...).

Thanks, guys.

[Justin Thomas]

Is there a ceiling fan in the room? That's all I got... Otherwise I'm ducking out.

Justin

[g1]

Ok, if it was ok for months after the 800ization, that should rule out the mod. I asked in case the problem might have been right after the major change.
Which 800 and does it have an FX loop?

[Fred G.]

Ok, if it was ok for months after the 800ization, that should rule out the mod. I asked in case the problem might have been right after the major change.
Which 800 and does it have an FX loop?

Definitely wasn't the mod, the amp sounded KILLER for months. It was a pretty abrupt change. The amp still sounds pretty good, just "not as good" and is showing subtle signs of an issue developing as described.

It's gotta be a tube failing or a cap...I'm a wee bit suspicious, but skeptical, of the channel volume pot going bad (I've been having recent, and consistent, problems with CTS pots, believe it or not).

[g1]

I've had a bad heater connection on a pre-tube do something similar. Just a little drop out once in awhile.

[TimmyP1955]

I once had something similar - might even have been in a JCM800. One section of the preamp would sometimes oscillate at about 32kHz (it's nice having a scope with FFT to find such things). A wee cap across the plate resistor solved the problem.

[Fred G.]

I once had something similar - might even have been in a JCM800. One section of the preamp would sometimes oscillate at about 32kHz (it's nice having a scope with FFT to find such things). A wee cap across the plate resistor solved the problem.

OK - I've brought the amp home to my shop and managed to recreate the "issue" - here's what I've found so far:

The noise is most easily re-created when playing an open E chord, specifically plucking the E note on the D string. It is an intermittent, distorted, fuzz-tone type "note" riding on top of the true note - sounds like a fuzz-tone, and sounds like it's a musical third (chuckle, the noise has a good ear and a sense of humor). And as I said, it is intermittent (but easily induced), but most definitely there where it sure shouldn't be;

I've also noticed an intermittent crackling, as well.

I swapped out all tubes, preamp, power, and rectifier, without any effect. It's not a bad tube. I'm suspecting a problem with a filter cap, at this point, even though I recently replaced them all.

Ideas?

Thanks in advance!