Intermitent squeal not affected by MV settings

[Silvio55]

Hi! I'm having some troubles with my JMP 50 clone the last days. I have used it for a couple of years (not exhasutive using) without problems, but 2 or 3 days ago, while playing at home it started to do an intermitent high pitched squel, not very loud, it did for 10 seconds or so and it stopped, today I experimented the same issue, I quickly unplugged the input cable, and lowered the master volume (it has a PPIMV), and neither of these affected the squeal, after a few seconds the squeal was gone. I tapped the power tubes to see if something happended but nothing, checked the speaker cable and everything seems fine.
Both times the amp did it, the amp was cold, the first minutes after turning it on.
What could it be?
Thanks!

[loudthud]

It could be noise coming in over the power line or from a radio transmitter like a cell phone. Have you tried the amp in another location? When the amp is making the noise, pull the line cord out of the wall. Does the noise stop immediately, or fade out over a period of a second or two?

[Silvio55]

It doesn't sound like the tipical cell phone noise, wich is frecuent if someone has a cell near the amp, this sounds like a cold solder joint-intermitent, but it's a squeal similar of the "reversed OT primaries" and getting positive feedback.
I could try pulling the line cord, cause it's gone in a few seconds, and the amp doesn´t do the squeal every time I play it.

[g1]

You said you tried tapping the power tubes, did you mean preamp tubes?
Squeal is more likely to come from preamp tubes, power tubes tend to give more of a rattle type noise than a squeal.

[Silvio55]

yes, the power tubes, since the squeal was not affected by the MV settings, and the MV comes after the PI, I believe the problem is after the PI. I also tapped the preamp tubes without succes either.

[Silvio55]

UPDATE: The squeal IS affected by the MV!! I though it didn't because when I turned it down to 0 it still squeled, but turning the volume up makes the squeal louder, wich is also strange, the volume makes it quieter but doesn't kill it when turned to zero.

[Chuck H]

Microphonic preamp tube... I think.

Pull V1. Does the squeal stop? Try a different preamp tube there.

[Silvio55]

I will try it, when the amp start squealing again, one of the most frustrating things is that the intermitent squeal appears for a couple of seconds and then it stops, most of the time the amp works fine.
I´ve had problems with microphonics tubes before, but it was nothing like this.

[Justin Thomas]

I agree - it's something in the preamp. If you get no luck with tubes, check for bad solder joints. Give it the "Enzo Whack" and see if it changes anything. Heck, whack it while it's squealing, too... Maybe chopsticks is in order...

Justin

[Silvio55]

I think I found the problem! It's something in the second preamp tube socket, probably a cold solder joint, it makes some loud pops when I move the tube a little with my hand, it doesn't do it when I tap the tube.
Tomorrow I will get the chassis out and see what's going on with this socket and report.

[Silvio55]

After spending some time with the amp in my work bench (living room table), I discovered that the second preamp tube was not the problem, it made some popping noise but I fixed it, it was a loose contact in the socket, but the real problem was one of the power tubes. it made the squeal by tapping it with a pencil, I checked the socket (contacs, wires and components soldered to it), replaced that tube, put that tube in the other socket, and it was indeed the tube it seems that there's something loose inside.
I have to replace both power tubes I guess, but at least I solved the problem, thanks guys!!!

[Chuck H]

Huh... Microphonic power tubes don't often squeal. It's usually more like they grunt. So, one problem solved and another potential future problem thwarted. I'd say you're winning today

[Justin Thomas]

I've had power tubes with internal intermittent shorts make some higher-pitched sounds... that EH 6V6 in my SF Champ shortly before it met its fiery death comes to mind... while it didn't whistle or screech in the normal sense, it was definitely higher in pitch than the grunt my SED 6L6s make, which simplify every little bump to the cabinet... I'd get that tube out pronto, because an intermittent short is just waiting to become a permanent short, which is maybe waiting to take out some other much more expensive stuff... I can't speculate on what parts are shorting to what, but I wouldn't trust the fuse to save my amp.

Justin

[Silvio55]

Yeah, it was a productive day today! The amp needed some cherishing after all the rehearsals and gigs.
The tube was not microphonic, it made some popping noise when tapped and sometimes it started to squeal abruptly and didn't stop until tapped again, it was like something loose inside, like a bad contact of some sort or something like that.