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-   -   high pitched squeal from tube amp (http://www.diyaudio.com/forums/instruments-amps/191034-high-pitched-squeal-tube-amp.html)

bassking 17th June 2011 04:26 PM

high pitched squeal from tube amp
 
Hi! I'm new to this Forum, but not totally new to tube amps. I've studied Physics, so I know the general things. And this one's got me stumped:

I bought a tube amp, Trace Elliot Bassamp, running six KT88s. It worked normally and was fully operational, but had ancient caps in the PS and had been tampered with. I did a couple of changes (new caps, new valve sockets...). When powering up all voltages are OK, although it's porely biased (I'm using old tubes for testing, but they're still OK).

Problem:
As soon as I turn ON from STANDBY it squeals your ears off. It only does this with preamp tubes inserted, without those it doesn't squeal, so I guess the power section is OK. Yes, I have tried different tubes. I've got loads of old tubes lying around (ECC83s). So it's not microphonics.

What really makes me curious is: When I switch it OFF (completely disconnect it from the power line) it squeals again, just for a short time, couple of seconds, then gets quieter and dies completely. As if some cap were unloading... Even with the power section powerless it manages the same volume.

WTF?

HollowState 17th June 2011 05:00 PM

Quote:

Originally Posted by bassking (Post 2609436)
I did a couple of changes (new caps, new valve sockets...).

Your amp is oscillating. The most common problem is that the amplifier's output stage is out of phase. This probably happened when you changed the sockets and inadvertently miswired the the output stage or output transformer primary leads. Assuming this amp uses global feedback, and that's what your description of the problem makes it sound like, try reversing the output transformer's primary leads that go to the KT88 plates. I'll bet this fixes your problem.

If you didn't change the KT88 sockets or mix the transformer wires, then something preceding them has mixed up the phasing. Recheck everything you did very thoroughly.

bassking 17th June 2011 10:25 PM

Hi HollowState,

Thanks for the answer! The leads from plate to output transformer were correct, but the leads to the grid of the power tubes are reversed (changed position of a circuit board for space reasons) and managed to get them the wrong way round (so the tubes were amplifying the wrong half wave all the time... Never had that before.
One minute of soldering got me my desired result, it works! Without the squeal...

Thanks a lot,

Max

DavesNotHere 17th June 2011 11:53 PM

make sure the sleeve on the input jack is not grounding out on the chassis.

most of his designs used a floating ground inside and .1uf caps from that floating ground to chassis.

there should be no dc ground on the chassis.

Ahole 19th January 2012 10:41 PM

Hello, new to this site but found this while googeling my issue. Same as above, guitar amp (EVH 5150 III) when switched on from standby a squeal starts, I get a squeal or a "transmisson interference" sort of noise. No work has been done to the amp recently and it worked perfectly last night!? Any help would be appreciated. aholley7@gmail.com

Enzo 20th January 2012 12:21 AM

Hi, really, you should start a new thread for this amp, rather than adding on to the end of a thread about a totally different amp. They may move this to the Instrument AMp section anyway.

DO ANY of the controls affect the noise in ANY way?

Does it matter which "channel" is selected?

Go down the row of small tubes, grasping each one tightly. Does gripping any of the tubes affect the noise?

Are we talking about a high pitched shriek? Or a loud hiss like a TV station off the air? (We call that "white noise.")

Gilgy 20th January 2012 12:25 AM

Quote:

Originally Posted by Enzo (Post 2870092)
Go down the row of small tubes, grasping each one tightly.


Not too tightly, please don't go breaking the glass envelope or burning your fingers on hot tubes.

You'd be a lot safer tapping the tubes with the eraser end of a pencil.

Ahole 20th January 2012 01:29 AM

Thank you and I appoligize for th hijack seems like other forums don't like repeat threads... its on all 3 channels no controls seem to alter it. And its a squeal in every sense of the word. Like a theramin (sp) on steroids!

Enzo 20th January 2012 06:38 AM

Yes, of course be cautious, but most of us won;t cruch a 12AX7 in our fingers. Use a glove or rag if they are too hot. I was indeed looking for microphonic tubes there.

Also, plug something into FX return jack to break the signal path, stop the noise?

And i forget if it has one, but kill the reverb if it does.

Even the resonance/presence controls have no effect?

kevinkr 20th January 2012 04:31 PM

:cop: Moved to Instruments & Amps per forum policy.

OK to discuss current issue in this thread since the title is very generic.


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