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Old 30th April 2003, 12:07 PM   #1
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Default 12BH7 is microphonic

After changing the voltage and bias values to support a 12BH7 tube I removed the 12AY7 tube from my "Joel's lineamp". The B+ plate voltage is sitting at 225 at the plate and the bias is at -8.5volts, which I assume is a working level. The substitution seems like a very worthwhile one as it seems less colored, and more lifelike and the hum dropped to nothing. Note: I have two of these line amps that I constructed, one is for final mods and the other is for modifying weekly/daily. The problem is that the tube is microphonic, if I touch it or tap the chassis I hear the noise thru the amps when they are at idle. I have tried several NOS GE tubes as well as an RCA and a Westinghouse. Everything displays this behavior. Is there something I need to do to correct this situation?

After trying other tubes in this line amp(with the correct plate and bias settings) 12AY7,12AX7,12Au7,12AT7, and 12BH7 the 12BH7 is my tube of choice.

The tubes checked fine in the tube checker.

Any ideas here would be appreciated.


Joe
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Old 30th April 2003, 01:07 PM   #2
EC8010 is offline EC8010  United Kingdom
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Default Far too sensitive.

12BH7 has mu = 16.5, so I expect that you achieve a gain of about 12. If you couple your pre-amplifier to a power amplifier with a sensitivity of 500mV (not untypical), you have a sensitivity of 42mV at the 12BH7 grid. Any valve will have a tendency to microphony at that level. You have too much gain.
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Old 30th April 2003, 04:45 PM   #3
Joel is offline Joel  United States
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As far as I know, the 12BH7 was never designed to be the input tube of a preamp. I'm not surprised it's microphonic. Tubes like the 12AY7, 6EU7, 7025, etc have internal structures to guard against microphonics.

You do not have too much gain.
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Old 1st May 2003, 01:10 AM   #4
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Default Microphony.

Hi Joe,

More often than not, I find microphony circuit dependent, not tube dependent ( there are exceptions).

What EC8010 has posted is correct in that most tubes would exhibit microphonic behaviour under those cricumstances.

Most of my experience with the 12BH7A is as some sort of cathode follower service where it naturally won't show any microphonic behaviour.

I recall building an SRPP stage around a 12BH7A and that one wasn't microphonic either.

There are alot of ways to combat microphony, shockmounting the sockets using rubber grommets is just one of them.

From an engineering POV the 12BH7A is a vertical deflection TV tube, this doesn't preclude from any audio service though.

Cheers,

P.S.
Quote:
Tubes like the 12AY7, 6EU7, 7025, etc have internal structures to guard against microphonics
Any of the above types can be made to exhibit a tendency to microphony depending on what circuit they're used in...so, no guarentee there.
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Old 1st May 2003, 04:45 PM   #5
Joel is offline Joel  United States
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I'd like you to explain to us all, Frankie, how a tube's tendency to be microphonic can be physically changed at all by what circuit it's in. Perhaps you want to revise your statement?

Can a tube's microphony be more audible in a high gain circuit? Sure. So can interference. So can a lot of problems. The gain of the circuit is not however, in any way, changing the source level of microphonics inside the tube! This should be self-apparent to the rational amongst us.

That circuit is not a high gain circuit. It also does not utilize low-level signal sources, ie. it's not a phono stage.

And, just because a particular tube is the flavor of the day for DIY folk does not mean that it is particularly good for that type of service.
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Old 1st May 2003, 07:13 PM   #6
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Yep, can only be reduced (like other noises and distortions) by NFB, not something to use where you need gain.

I'd really like to see his zero-microphony circuit for 6W6 outputs.

Tim
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Old 1st May 2003, 10:36 PM   #7
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Default READ PROPERLY.

Hi,

Quote:
Can a tube's microphony be more audible in a high gain circuit? Sure. So can interference. So can a lot of problems. The gain of the circuit is not however, in any way, changing the source level of microphonics inside the tube! This should be self-apparent to the rational amongst us.
So? Where did anyone say otherwise?

Cheers son,
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Old 1st May 2003, 11:35 PM   #8
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Those cathode followers ought to get a life of their own.
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