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Old 4th May 2011, 06:39 PM   #1
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Default Is it possible to dampen a partially microphonic 12ax7?

I just 'finished' an amp similar to a fender pro jr. It sounds fantastic, but the preamp 12ax7 is slightly microphonic. I swapped it out with a few others, killing the microphonics, but they all sounded flat to me in comparison with the original. The original is beauuuutiful! So where I didn't have a grid stopper on the second stage, I put in a 10k, and it could be my imagination, but I think that helped a little. I also double-checked that all the connections are good solder points.

The question: Is tube microphonics entirely determined by the construction of the tube, or does the circuit that it is in, to a degree, affect it. If so, what can be done to cut it back a bit? Change the operating point? Use a big grid stopper? A cap somewhere?
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Old 4th May 2011, 09:01 PM   #2
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Originally Posted by DrStrangelove View Post
The question: Is tube microphonics entirely determined by the construction of the tube, or does the circuit that it is in, to a degree, affect it.
Yep, determined by the construction of the tube. They have to be filled with vacuum instead of embedding the internals in a block of plastic. Even a solid state circuit will be microphonic if the gain is high enough. I did a longwave xcvr project that was a TRF design. All the gain occurs at audio frequencies, and that 100db of gain makes the audio strip microphonic. It's not a problem unless you turn the volume up too high. In cases like that, about all you can do is to make everything as mechanically stable as possible. I doubt, though, that it's a problem here.

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If so, what can be done to cut it back a bit? Change the operating point? Use a big grid stopper? A cap somewhere?
Why worry about it at all? You already said that the microphonic 12AX7 sounded better than any others you tried. Perhaps part of the tone is caused by that very microphonic-ness? Something to consider, I think.
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Old 4th May 2011, 09:22 PM   #3
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Thanks Miles,

Indeed I wouldn't worry about it, and I DO think that the tone is in large part do to the microphonics, but I'm mic-ing the amp in order to run effects to another amp, and even with the volume all the way down on the project amp, it starts to squeel a little. If I could knock it back just enough to cut that, I'd be a happy camper. It gets dropped out when I'm playing, so I may just live with it anyway.

Also, I ask for the sake of my curiosity/education. I'm running one of the stages with higher that usual current. (68k load resistor and 680ohm cathode) I thought that may have something to do with it?..

You mentioned that running it at high gain has some relevance? In this case, the volume doesn't have any affect on it. Are you referring to the gain potential of the circuit, or the actual signal amplification?

Thoughts?
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Old 4th May 2011, 09:25 PM   #4
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BTW: I already checked that it's not second amp feeding back. I'm pretty sure the slight squeel through the second amp is resonating in the tube in question.
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Old 4th May 2011, 09:32 PM   #5
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Microphonics
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Old 4th May 2011, 09:33 PM   #6
M Gregg is offline M Gregg  United Kingdom
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I guess you have tried dampers!

Eg:


tube damper | eBay UK

Regards
M. Gregg
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Old 4th May 2011, 10:14 PM   #7
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Thanks trobbins. Great article!

And no, M Gregg, I suppose I haven't tried dampers yet. Guess I'll try that next.

The article confirmed something I was worried about, which is that it will probably get worse. Oh well. I'll enjoy the tone while I can.

Thanks all.

Tim
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Old 4th May 2011, 10:36 PM   #8
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Originally Posted by DrStrangelove View Post
Also, I ask for the sake of my curiosity/education. I'm running one of the stages with higher that usual current. (68k load resistor and 680ohm cathode) I thought that may have something to do with it?..
Has nothing to do with microphonics. A 68K load is heavy for a 12AX7, and will make lots of distortion that you wouldn't want anywhere near an amp you're using for music reproduction, but which will help with guitar amp tone, which is largely distortion.

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You mentioned that running it at high gain has some relevance? In this case, the volume doesn't have any affect on it. Are you referring to the gain potential of the circuit, or the actual signal amplification?
Actual signal amplification, but microphonics become relevant only for something like a TRF, where all the gain occurs at audio frequencies, as opposed to a superhet where gain is distributed between the IF strip, audio strip, and sometimes a pre-mixer RF strip as well. I've never heard of a guitar amp that had 100db of voltage gain.
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Old 5th May 2011, 12:19 AM   #9
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Quote:
Originally Posted by DrStrangelove View Post
Thanks trobbins. Great article!

And no, M Gregg, I suppose I haven't tried dampers yet. Guess I'll try that next.

The article confirmed something I was worried about, which is that it will probably get worse. Oh well. I'll enjoy the tone while I can.

Thanks all.

Tim
Instead of ordering and waiting for your parts, you could try using a couple of O-Ring washers applied to the tube first. This should give you an idea if its microphonics or not.
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