High pitches squeal after about 10 minutes of playing

[MRX2099]

My rack mount systems consists of a
(1) Mesa Recto Pre Preamp,
(2) Mesa 2: One Hundred Power amp,
(3) t.c. electronics G-major,
(4) 482i sonic maximizer and lastly
(5) a Furman M-8L power conditioner.

I turn that system on normally then in a few minutes I switch the standby on the preamp then power amp to on, play, then about 8-10 minutes or so when it starts getting warmer there is a real high pitched squeal coming from somewhere. I though it was where the system is on the same room as my computer, maybe the monitor was interfering, but when I switched off the monitor it still does it.

Everything is plugged into the Furman but I don't have the Furman plugged straight into the wall, its goes into another power strip then that goes into the wall, would that cause any type of interference or no? I would try it now to see, but all the tubes are out still packaged up to take back to have checked one day this week....

I unhooked the 482i and ran it without that, then pushed the BYPASS on the g-major to bypass it, but still get the squeal. I tried tapping the tubes with a pencil eraser to see if any certain tube was bad , but did not get anything different. I live in such a PODUNK town now, there is one small music store not so close by (about an hour away.) I took all the tubes out and was going to get them checked today, but their tech guy ( whom does not even work for them just stops by often to see what they need checked of fixed, was not there, and no-one else there could stick their finger up their........errrr......... I mean knew how to test the tubes........................

any ideal if it may be something else of could it be just one of the tubes going microphonic, I purchased everything used , but the PREAMP was only about a few months old so I know those tubes should be still good.......

Thanks
Jeff in PODUNK USA

 

[Jackie]

Try removing the Furman. Someone had a terrible ground loop over at the JVM forum and he removed the power conditioner from his chain - fixed his problem.

 

[MRX2099]

Hrmmmmmm I was putting the tubes back in and when I got to the Power Tubes, I see 2 disturbing things, looks like 2 of the tubes have the center black pieces broke off that go into the center of the tube socket (don't know if that happened when I was taking them out or they were like that already and that was causing the problem, or) ...... then I see one other of the sockets itself has black around 3 of the pole pieces where the tube legs go in...... so either way I have to get 2 new tubes first and try that...

what would cause the black like burns on the tube socket itself??



Also on the back of the Recto Pre around the right of the tubes there is another INPUT plug. what is that input for????

Thanks
Jeff

 

[elvis]

The plastic key just helps get the right orientation for the tube. Mine break kind of often. If you keep the orientation right, that shouldn't cause a problem.

Nonetheless, you may have put a tube in in the wrong orientation and had it arc at the pins. That would burn the socket. I can't think of anything else offhand that would do it.

It's possible that you did some damage to the tube when you broke the key, and it may have lost vacuum, arced, or something else bad.

Usually a second input jack on the back of a rack preamp is wired in parallel with the front jack OR one overrides the other. It's just so you you can wire it all from the back in the rack.

 

[sotek]

Actually, I've got the same for about 1 year on B channel only. I've replaced all tubes (3 12AX7 + full 6L6 set) but no positive results. The problem has to be elsewhere.

Recently I've took it off the rack, opened it and it was full of dust, esp. the pots. After cleaning them with compressed air, I realized, that volume pot for B channels is cracked/wanky a bit. I need to order a replacement to fix this and then I will see if it helped. Damm, there is one of the output trannys just right behing the pot, so it needs to go out as well to replace the pot.

Anyway, if you take it for repair, please share the results.

 

[crane]

Could you post a picture of the burned socket without the tube ? It may help identifying the problem.

 

[MRX2099]

OK today i took the cover off and its all looks fine inside everywhere even on the other side and between the tube socket board. I started inspecting the tubes all very carefully and , voila, the 2 with the center "Key" for orientation that were broke, actually were just the plastic "key" piece not the glass, so I super glued those 2 back on.. however I saw one tube that has a large crack all the way down the tube, so I am guessing that is the one that was in the place of the arched socket...and causing the squeal....( it probably leaked out the vacuum and was arching since it was the bad tube) so over all I am only out 1 tube, hopefully my local shop has Mesa tubes or they can order them.


Thanks everyone I'll keep you informed on how it goes



Jeff

 

[elvis]

I think it's funny the way people talk about vacuum "leaking out". Actually air leaks in. But both adequately describe the situation.

Glad you found the bad tube. The squeal was likely an oscillation due to the bad tube causing the circuit impedance to change and lose stability. Oscillations are often a very bad thing for the amp (either causing damage directly or as a symptom of a problem that will cause damage), so never let them squeal for longer than it takes to shut off the amp.

 

[MRX2099]

Yeah that is true, air would have to leak IN since the vacuum actually "vacuums" everything out. I did shut it down soon as it began to squeal. Also mine is loaded with 6L6GC SRT430 are those like older since all I can find on ebay now are the Mesa Boogie SRT440 are those basically the same? would it hurt to install one Groove tube 6L6 with all the rest of them Mesa tubes? or does it matter that much?


Thanks

 

[elvis]

I have no idea if the 430 vs. 440 will be audible. I generally can't hear the diff.

However, you really want to use matched pairs of tubes. The power amp is push-pull, so you want symmetric gain. Also, going to another tube brand could require rebiasing the amp unless it's been tested to show it will have proper DC current at the existing bias point of the amp.