Microphonic ECC83 12AX7

[SoundCity85]

Hello,

I was wondering, at what point does an ECC83 become too microphonic? I have a tube tester with a line-out to listen to the tube, and most of the tubes respond to some extend to tapping with a chop stick (I use a thin paint brush). Some seem to transfer the brittle sound of the glass, while other gives more a knock or thumb sound. One louder than the other, but most of them make a sound. So surely, V1 is more of an issue than the PI spot. But is there consensus on what degree of microphonics can be used in V1 and V3? It seems very difficult to approach this in a somewhat objective way.

Cheers D

[sluckey]

Your ears will tell you when the tube becomes too microphonic.

[SoundCity85]

I tried to make clear that this was not the point of my question   

[sluckey]

I think it's exactly the point. Would you prefer a meter that has a scale marked red, yellow, green?

[SoundCity85]

No, just something a bit more than "try and listen" when you have expensive measuring equipment at hand

[JB]

Surely an ECC83 or any other valve is only "too microphonic" when it's not suitable for a given application.  Microphonic valve may be troublesome in a combo that's vibrating from the speaker but fine in a head or a hifi.  Also not sure how you'd measure and quantify it without a given force or sound pressure hitting the glass!

[shooter]

while playing, how often does something mechanically thump a tube?
if the tube guts are loose and wobbly, it should show up on a scope as you play, swap tube, problem goes with bad tube.
I've had 2 tubes that i could "prove" microphonic, both Gibson amps, both V1, I could whistle at the tube and it amplified my whistle.  to be on the safe side, I swapped both socket n tube, rewired for a modern AX7, problem solved.


As to fancy test equipment, I find in tube amps anyway, the amp itself is my best test equipment, the Tektronix and fluke just help me get'r done faster 

[shaun]

I think it's exactly the point. Would you prefer a meter that has a scale marked red, yellow, green?

Yes - have you got one? :)

[sluckey]

I do. It also measures soil ph.

[shaun]

In my experience, a 12AX7 will have only minimal or no microphonics if everything is running smoothly. When chopsticking, you might hear small microphonics, but if I have a loud or obvious microphonic issue with a 12AX7, then it will usually be either a problem with the wiring - ie., I have wired something wrong which is causing the tube to become microphonic - or the tube itself will have a problem.

I have bunches of old 12AX7s from old amps, which I wanted to use (cause I like to save cash where I can), but they are quite often microphonic; I assume it is because they are old and tired, and when I replace them with a new tube, voila, no microphonics. So I keep a set of new 12AX7s around just to test where the problem may lie.

I'd love a decent tube tester, but the newer ones can be pricey.

I just re-read your question: "I was wondering, at what point does an ECC83 become too microphonic? " So Sluckey was right. Some people like microphonics in a preamp tube because it might encourage feedback, which some guitarist want.

[SoundCity85]

My tester measures RMS noise in dB. The tapping is surely picked up by it, besides the fact that it has a line out and you can amplify the sound just like in a tube amp. I was indeed thinking of a way to somehow quantify the tapping with a chop stick, or at least do it in a controlled manner so it becomes reproducible. If you have hundreds of tubes, it makes more sense to go for a approach that allows comparison to some degree.

Thanks for the great discussion

[PRR]

For military acceptance, there is a thing with a rubber hammer hanging like a pendulum. You pull it halfway back and release. It gives the tube a repeatable knock. But still "how much is too much?"

Put the amp all together and set the gain high or max. Does the amp howl? That's too much. Kick it. (Or make a big rubber pendulum.) Is it almost at the howl point?

RDH 4th has some comments, too many to post here.

[glass54]

PRR

Love the AF Microphonics test.
I assume the (max) 1/2 ounce cork comes from a bottle of Red Wine and correctly wetted (wine bottle stored on side) to suit the bandwidth in question?
Kind regards
Mirek

[SoundCity85]

Wow those are some nice sources. The howl is something is noticed for the first time yesterday. I only just started "listening" to the tubes, and unfortunately my Philips mCN deltaD tube from 1954 howls on one side of the triode. I didn't notice this before, as I pulled it from a PI slot where it was not possible to hear it. Too bad because it measured like new, and was very nicely balanced.