How to Determine if a Tube is Microphonic?

[GAStan]

How do YOU determine if a tube is microphonic? What test(s) do you use?

I've been rolling tubes in my ODS build and ALL seem to be microphonic to a point. I am beginning to suspect the problem may be my inexperience, that I am being too critical or performing invalid tests.

First I let the tubes run for 10 minutes or more to get up to operating temperature. I use V1 to test. With the OD channel OFF I very lightly tap the tube with my fingernail. The current tube in the amp is a JJ ECC83S. All I hear from it is a dull "thud" through the speaker, I consider this as good, not microphonic. I also tap the amp on the side of the chassis. I hear nothing from the JJ. With other tubes I have varied results-most have a ringing similar to tapping an empty wineglass from the speaker, some only when I tap the tube others also when I tap the chassis side. Some sound off when I operate the small switches such as the BRIGHT switch.

To be clear, I VERY LIGHTLY tap the tube with the tip of my fingernail, much lighter than tapping it with a chopstick would be. I'm just wanting to test the tube, not damage a hot tube.

Am I performing valid tests? At what point should I consider a tube microphonic?

[Raja_Kentut]

My wife worked in a store that sells and selected tubes. The microphonics test consisted of exactly what you do: tapping the tube with your fingernail. This made the tube much more expensive There were no real measurements, only the hearing of the tester.

[xtian]

Tubes are very robust while in operation--remember they flew in spacecraft! No worries about tapping too hard. The difficult thing is, you can tap one corner of the chassis and a microphonic tube on the other side will ring, making it difficult to find the offender. The trick is to MUTE a bad tube by applying pressure with your fingers, or oven mitt or whatever. You can often STOP the ringing by applying pressure to the tube to dampen the vibration.

[GAStan]

My wife worked in a store that sells and selected tubes. The microphonics test consisted of exactly what you do: tapping the tube with your fingernail. This made the tube much more expensive There were no real measurements, only the hearing of the tester.

This is good to know. Thank you

Tubes are very robust while in operation--remember they flew in spacecraft! No worries about tapping too hard. The difficult thing is, you can tap one corner of the chassis and a microphonic tube on the other side will ring, making it difficult to find the offender. The trick is to MUTE a bad tube by applying pressure with your fingers, or oven mitt or whatever. You can often STOP the ringing by applying pressure to the tube to dampen the vibration.

Also good to know about their robustness. Thank you also.

All other tubes in the amp have been checked and have no microphonics. I am swapping the tubes I test into V1 one at a time.

The tubes I am most concerned about are still within their warranty period so I want to return them if they are actually considered microphonic. This is why I posted this question.

[bepone]

tapping at the most sensitive place - V1, because microphonics is amplified by max value.

[Reeltarded]

loud *thub* is also microphonic, or even a wiring issue at the cathode ground... usually.

Chopstick the cathode caps and use the chopstick to do single taps with a little meat behind them about an inch from the tube. Trying not to ring the chassis, but hit it like a tiny dead-blow hammer.

If it thubs, it is activated when playing and it disturbs the lows when the note resonates through the rig.

With just the clean up loud you should be able to beat the shit out of it with hardly anything ringing.

[TUBEDUDE]

When instead of a thud, it rings a little. High or low frequency.
For octal pre's I isolate the socket from the chassis with 2 mil. Sorbothane rings.
Some of those elastomer rings on the glass might help. The hifi crowd loves 'em.
I've seen some chassis that have an isolated sub chassis with the sockets on it.
Combos are harsh on tubes.

[R.G.]

Once upon a time I thought-designed a tube microphonic measurement device. It was a chassis with a small 300V or less power supply, tube socket(s) and an audio output jack. Also, the secret weapon: a surplus pager vibrator motor and a way to drive it. Plug the tube in, let it heat up, and then turn on the vibrator motor. Watch the output signal on a scope (back when this was done...) or just listen to it. The vibrator motor produced a local-standard vibration that didn't change much between tubes and a reference for comparing from tube to tube, which is hard to do with just a calibrated flick of a fingernail. I reasoned that there was some info to be gained from the spectrum of the output signal. This is fairly easy with modern electronic scopes that contain FFT software.

Never built it, as I only had a dozen tubes to do, and tapping was faster. Someone who uses a lot of tubes - a tech for instance - might like to cannibalize some old radio or hifi chassis for their own microphonics tester to offer pre-sorted tubes, or just to weed out the really bad ones.

[Kagliostro]

Ciao GAStan

May be you are interested on this

Microphonics in valves.pdf

1958 - Bruel & Kjaer - Microphonics On Vacuum Tubes (Petersen).pdf

Franco

[GAStan]

Ciao GAStan

May be you are interested on this

Microphonics in valves.pdf

1958 - Bruel & Kjaer - Microphonics On Vacuum Tubes (Petersen).pdf

Franco

Hello Franco,

EXCELLENT information! It really helps me to understand exactly what microphonics are so i can more accurately look for them. I even learned a new technical term, "clang"

Grazie,
Glenn