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Are my tubes microphonic?


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I have a question about my VTL ST-85 tube amp. I think one or more tubes may be microphonic. If I tap the amp while it's on, I hear a tinkle kind of sound from the amp and through the speakers. It doesn't seem to matter where I tap it, it's consistent. I first noticed it when I set my record cleaning brush on top while I was changing records.

I also hear the sound immediately after turning the amp off. I will hear it from the amp and through the speakers, until the tubes cool down too much to pass signal. At that point, I only hear it from the amp.

I have re-issue Mullard EL-34s and Electro-Harmonix 12AT7s. I am using the same EL-34s in my Carvin guitar amp, and have noticed a similar shut down sound, though not as much because it has a standby, which allows the tubes to cool more slowly.

My guess is that the shut down sound might not be anything to worry about? What I AM more worried about is the tinkling glass kind of sound when I bump the amp. My concern is if they're this sensitive, perhaps they're also adding microphonics to the music? I tried tapping each tube with my finger to isolate which one is doing it, but couldn't come up with a clear result.

I'm away from home right now, but when I get home, I'll pop the hood, turn it on, and see if I can do it again.

Any thoughts on this? I'm getting ready to place an order for some preamp tubes for a VTL TL-2.5 that's on it's way, so this would be a good time to order new tubes if I have to. I just want to make sure I'm not buying tubes I don't need.



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1. Turn on your system.

2. Turn the volume up about half way.

3. Lean over and speak into a tube and say "Hello, is this thing on."

4. If anyone says "yes," your tubes are microphonic.

Seriously, just tap on one. From what you say it sounds like at least one is microphonic.


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Yes. Your have at least one microphonic tube. There is not necessarily anything wrong with it if it sounds good. I have one 6H30 in my Peach like that. Touch the unit and boing....oing.....oing.....oing. Sounds just fine when you leave it alone and I am still using it.

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If you are using the pencil test you will probably find almost all tubes will produce a sound out of the speakers if you tap on them. I ony consider a tube truly microphonic if you have issues like are being discussed here. If you touch the unit or the equipment rack and you here it out the speakers, you have a very microphonic tube. Still, you have to be the judge of when too much is too much. I ride with them until there is a real problem.

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I have a pair of EH 12AX7 in my TT preamplifier. They have made a similar noise since day one, but more of a koinnnnng. But of a similar type of decay, just a lower frequency. It is only noticeable on quite passages and high gain settings. Besides that problem, they sound nice to my ear, so I have not had the impulse to change them out. I would say if the noise is not objectionable, don't change them.

The Tube Depot will check for "microphonics and low noise" on replacement tubes for a $4 charge. I have not used this option, so I can not vouch for the effectiveness of this check. That brings the total to only $20 a tube, might be worth the experiment.

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It is my undestanding that practically ALL output tubes are microphonic..?


Preamps and phono preamps will have a much higher instance of microphonics then an amplifier. I could hit any tube in my amps with a screw driver and still not hear a bit of noise out of the speakers (unless I hit the tube hard enough to break it)... rarely will an output tube be microphonic at all.

Also technically speaking a microphonic tube is ALWAYS bad for the perfomance of the product it is used in.... unless you happen to like that little bit of extra reverb and distortion [;)]

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  • 1 month later...

Looking back over my old posts, I found this one interesting. I didn't know the true meaning of microphonic tubes til I got my used VTL preamp. It came with a set of Ei 12AU7's that were so horrendously microphonic as to be totally useless. You could be standing 5 feet away and speak, and hear yourself through the speakers! Plus, everything had a ringing sound added to it. Chucked 'em, bought some Mullard CV4003's from Kevin at Upscale...problem solved. It was definitely an educational experience. One I'd prefer not to repeat. I rarely throw tubes away, but those things were useless.


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I don't recall if it was the preamp (Artemis) or the power amp since I've switched them both out several times lately...but one of those is so microphonic... I was listening to something one day and I heard "oingggggg oingggggggg oinggggggggggg" as the dog walked across the floor!!! This thing picked up her footsteps and oinged in unison with her walking by.

I'm beginning to think it was the preamp because I think I yanked it right after that

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If you play at moderate levels 10-15 hours a week

Power Tubes will generally last 1 - 1.5 years.

Preamp Tubes will generally last 2 - 3 years.

Signs of Wear

Power Tubes:

• Low end response is not as clear and distorts earlier.

• You turn your amp’s master volume up to hear the level that

you are used to hearing.

• Eventually you will hear 'crackling' 'popping' sounds, often accompanied by:

• 'Swelling' volume levels.

• Buzzing.

Preamp Tubes:

• Hissing, 'shooshing'.

• Sputtering, 'grumbling'.

• Microphonic squealing (feedback type sound).

• Loss in signal in one or more modes of the amp (channel, reverb, fx-loop).

courtesy Mesa Boogie


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