Oscillating Preamp?

I've got a preamp using 4 12au7 (ECC82) tubes.

When I use my preferred combination (RCA 12au7A's) there is a high pitched whining every so often. Must be above 16Khz as I can hear it but the old man can't!

If I use other (less good sounding) tubes it completely goes away.
If I use the same 12au7A's in other preamps they are silent in them too.

Are these particular tubes sending the preamp into oscillation? I can't really think of any other logical reason why they would do this and only in this particular preamp. The tubes are pretty much NOS and not microphonic at all.

It's really rather annoying as when they are not whining the sound is sublime, which is the reason I want to persevere with this combination rather than resorting to other tubes!

Cheers

Chris

In about '69 I was asked to build an amp using a ELL80 as the O/P valve, which I did using point to point wiring and aluminium screens.

When I had finished it the asker said that he wanted it all to look 'neater', and he rebuilt it on circuit boards, but it then had an 8Meg oscillation which was constant at the O/P. I declined his request to rebuild it for him.

Valves are high impedance devices, and more prone to feedback oscillations, and perhaps your higher grade valves have a wider bandwidth and/or sensitivity, and are showing up a tendency of the amp because of its layout.

Its posible that one or more are microphonic, sometimes you can find the offending one by gently tapping the glass of each valve when the preamp is on, and hopefuly you will find the one that's causing the problem, as tapping them can often cure, or at least change the way the valves behave as far as the noise.
I hope this helps.
A...

Ditch the valve, it has a problem. I had the same thing with a very expensive Black Treasure. High pitched whine from the tube after about 10 mins.

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Thanks chaps,

just strange that it is perfectly silent in other preamps under the same conditions though.

Fairly sure its not microphonic either, I can tap on it less than gently with a screw driver and no sound through the speakers at all even on high gain.

Replacing the valve would surely cure the problem but I don't have another 12au7A grey plate spare . . . . Should have some clear-tops arriving soon though . . .

Quote:

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Originally Posted by Opti-cal

Thanks chaps,

just strange that it is perfectly silent in other preamps under the same conditions though.

Fairly sure its not microphonic either, I can tap on it less than gently with a screw driver and no sound through the speakers at all even on high gain.

Replacing the valve would surely cure the problem but I don't have another 12au7A grey plate spare . . . . Should have some clear-tops arriving soon though . . .

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I'm not an expert but the voltage seen by the valve in one preamp is different to another. It is not microphony. Its a voltage induced oscillation in the tube which has a flaw.

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Quote:

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Originally Posted by farflungstar

I'm not an expert but the voltage seen by the valve in one preamp is different to another. It is not microphony. Its a voltage induced oscillation in the tube which has a flaw.

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Yup, that sounds about right, I suspected it was just a mis-match between pre-amp and tube. A real shame as 85% of the time it sounds top notch . . .

Probably worth searching out a 12au7A which does not whine. The one on the other channel is fine!

Sometimes placing the offending valve in a diffrent position in the preamp ie; a lower gain section can remedy the problem, without knowing the preamp your using, i have no idea if this is posible.
A...

Quote:

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Originally Posted by Opti-cal

Thanks chaps,

just strange that it is perfectly silent in other preamps under the same conditions though.

Fairly sure its not microphonic either, I can tap on it less than gently with a screw driver and no sound through the speakers at all even on high gain.

Replacing the valve would surely cure the problem but I don't have another 12au7A grey plate spare . . . . Should have some clear-tops arriving soon though . . .

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Quote:

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Originally Posted by Opti-cal

Yup, that sounds about right, I suspected it was just a mis-match between pre-amp and tube. A real shame as 85% of the time it sounds top notch . . .

Probably worth searching out a 12au7A which does not whine. The one on the other channel is fine!

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Having owned a lot of valve gear you learn along the way - which is part of the fun. A failing capacitor doesn't give such obvious clues there's a problem. At least it isn't such an expensive valve and is relatively easy to replace with another.

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Attachment 27220

Preamp is a Musical Paradise MP-701

Offending tube is at the rear. I have rotated it and the issue occurs on the other channel.

I have to use it in the main driver position (top or bottom) as the other tubes are Sylvania's (now Gold Lion's) and this configuration sounds far superior.

If I had another good RCA 12au7A I'd just swap them and I'm sure it would be cool!