Looking for information on testing for micophonics - especially schematic of a specialized unit for testing various audio tubes like 12ax7, 6l6 etc. Willing to build a test set.

A simple tester for microphonics would be any audio device that uses the tube in an audio circuit, particularly a preamp or amplifier. High power tubes seldom exhibit microphonics however, so. you might want to start with a simple preamp circuit for tubes like the 12AX7. A microphone preamp would be ideal, or a phono preamp .... something with a lot of gain. All you have to do is ground the input, and tap on the tube while listening to the output with the volume turned up. (all the way up if you dare, and have a great speaker or headphones)

Some tube testers do have provision to "listen" to the tube while you tap on it. Off hand, I don't recall which.

I think the TV-7 has pin jacks for tube "noise" testing, presumably with hi Z phones. Never tried it. I would surmise that any sort of "microphonics" test you could perform would only be relative, in the instance you had multiple tubes to compare. If there was some sort of quantitative test for that, you could not afford the gear. Presumably for self generated noise besides microphonics, there is a well defined quantitative test, at least for a specific type. Probably the only practical test for microphonics is in the actual gear that uses them and swapping tubes to see if there is a reduction or if one in particular is notably worse due to internal mechanical construction or a defect.

Many tube testers have a noise test. My Hickok 533 does it by connecting the noise pin jacks to a receiver, the TV-7 manual suggests connecting them to a receiver or to an audio amplifier and speaker. High-Z headphones would not be sensitive enough. I will have to try using an audio amplifier with the noise test on my Hickok, as that should work just as it does on a TV-7 - in both testers the noise jacks are capacitively coupled across the shorts lamp.

In wilder days, I tested Grundig TK 46 tape recorders, by starting a tape on record.
unplugging the microphone, then screaming into microphone jack.

If the scream was heard on playback, at least we knew the amplifiers were
working.

All tubes are microphonic......the "test" is to find ones that are outside the normal distribution.

Gather your inventory of 12AX7s and plug them one at a time into an amplifier. The "test" is to tap on them and listen to the result. After you test a few, you will clearly hear if any are exceptionally sensitive. If the better ones behave "acceptably" in the target equipment, then you are done. If not, buy some more and return to step one.....

But:
The last time a screened a pile of 12AX7s, I also found quite a few with excessive grid emission. Between the two "features", I was rapidly running out of tubes.

So I have one of those eye tube grid emission testers, but I have rarely used it. So what do you think causes grid emission in a 12AX7 and what sort of issue does it cause in an audio amplifier stage?

I might suggest tapping a rare premium 12AX7/ECC83 , judging by the price,
made by German Monks during Starkbierfest during low lunar gravitational
influence, with a mink hammer.

There were no industry-wide standards for microphonism in tubes. Manufacturers and some organizations like the military or the aerospace industry might have had their own internal specifications but they would have had to develop their own test procedures and equipment. You're probably not going to find any commercially made, general purpose microphonism testers out there. In the field, technicians just rapped tubes gently with rubber covered screwdriver handles and listened or looked for the noise in the output. With a little experience they'd know what was normal and what was not.

We did the same with tubes in TV's. Tap on a tube, usually with a fiberglass tuning wand or the back of a fingernail, and observe the picture to see if the 'intermittent' problem showed up.

Half the time in my experience it was the tube, the other half, the socket.

DOD's test procedures are specified in "Test Methods for Electron Tubes," the most recent revision of which, MIL-STD-1311D, is dated 19-Apr-2011. The noise and microphonics test for receiving tubes is Method 2201 in that document.

there are tube socket plugs what are mounted with rubber stripes, special made for micrphonic sensitive tubes.
Additional use soft wires to connect them. You can build that plug self, I have pictures made how to build them

Martin

Tape recorders had them especially the ones with their own speakers.

Grid emission occurs when the grid gets contaminated and starts acting as a cathode. This in turn causes it to go more positive, thus turning the tube on harder. The biggest problem comes in a high-impedance, high-gain circuit in the front end of an amplifier. The immediate symptom is a plate voltage that is way too low.

From the early recognition by Lee De Forest , that a radio tube needed to burn out
thus depriving the radio set owner the ability to steal performance rights to
entertainment, tube makers used up all get-out-of-jail cards their physicists gave them.

The biggest laugh was when tubes were forced to be made to last decades by undersea
cable bandwidth, they were obsoleted by transistors, and Sputnik.

Siemens Munich yes. they are manufactured to be finest spare for heerenveen made Tek scopes
Philips SQ are the same but for channel islands made Tek scopes.
Telefunken fits Rohde&Schwarz

Basically, this is what I do... but first, I carefully take out the audio NFB to get the largest gain.
You can tap on plate and grid resistors, too... if they crackle, replace!
Cheers,
Roger

i would love to see these pics as i think that would save some very good testing 12ax7's i have
Cheers Don

hello Don,

www.wellenkino.de

there: forum, community
there: Audio
there: RIM Imperator projekt

it is full with picture from a very special homebrew amplifier

greetings
Martin

Thanks MM looks very interesting , wish i could read German
i didn't look threw all pages ,your amp looks fantastic
Cheers Don