Tube noise listening device

[rjuly]

I'm interested in building a device similar to that showcased in this YouTube video on the AudioTubes channel:



It is a simple device they built to help listen to to tubes for hum and microphonics. It allows for switching between triodes and has inputs for both AC and DC heater current and plate voltage from an external power supply.

I am thinking the key to it would be to create a circuit that is itself as low noise as possible.

Have you seen a schematic for something like this?

Would you have any thoughts about the best way to design a simple quiet circuit for this purpose?

Many thanks,

- Richard

 

[mad.ax]

I don't see the point in such device...
You'll need a different one for different tubes and/or pinouts...
You still need an high voltage supply, plus an heater one...
And the result will not neccessarily match what you'll get once the tube is plugged in its final destination because many parameters might be different...

Much simpler imo is to plug the candidates in the circuit...

Or if you have a tube tester, add a simple transformer into the anode circuit, like they do in the Roe Test
RoeTest - Röhrenprüfgerät Röhrenmessgeräuml;t

Axel

 

[RuudNL]

For measuring tube noise, I use the following setup:
I have a microphone circuit without a microphone capsule (but instead a fixed capacitor) and a socket for the tube under test.
The output of this test circuit goes to a 60 dB amplifier.
This amplifier is followed by an "A" noise weighting filter and a RMS meter.
(Noise weighting filter and RMS meter are combined in my Nakamichi T-100 audio analyzer)
This gives me an objective indication of the tube noise.
Of course there are different methods, but this works for me.

 

[ruffrecords]

Interesting box. I built something similar many years ago. They are not really testing for noise but for microphonics which is something completely different. I do agree though that the ruggedized tubes do generally have much lower microphonics than the regular versions but you would expect that anyway. Mt LINDOS audio test set has a built in speaker. In noise measuring mode it had a built in 50dB amplifier and if you turn up the speaker volume you can clearly hear the tube noise and its microphonics. It is fun to listen to tubes warming up and colling down. You get some nice pings and boings.

Cheers

Ian

 

[Matador]

https://tavishdesign.com/pages/audio-tube-noise-measurments

 

[Tubetec]

Yes Ive also noticed these low level artifacts in tubes warming up , thats why I always insist on at least 30 minutes warm up time for tube gear before pressing record or any kind of noise testing . My guess is its the anode is mainly responsible for these ping sounds , cathode gets up to opperating temp relatively quickly by comparison .
Has anyone got a IR thermometer to see how long it takes for the anode of a tube to stabilise after power up ?

 

[CJ]

The guys at vacuum tube valley used a weight on a shaft that they dropped from a fixed height to measure microphonics.

 

[Tubetec]

I'll normally hold a tube close to my ear and tap it with my finger , if it sounds like a bag of bolts rattling around or it has a pronounced ping at a certain frequency chances are it wont do as an early stage preamp .

 

[merlin]

I am thinking the key to it would be to create a circuit that is itself as low noise as possible.

Have you seen a schematic for something like this?

Would you have any thoughts about the best way to design a simple quiet circuit for this purpose?

The tube is set up as an ordinary gain stage so the noise at the anode will already be fairly large, typically around 100uV, or even more if there is hum/microphonics, so there is no need for a special low noise amp, a simple FET opamp will work fine (TL071 etc). Don't forget to add add clamping diodes to protect it from voltage spikes though. The opamp provides the gain and buffering, then use that amplified signal to drive an ordinary power amp (or add your own little speaker driver or whatever).

 

[trobbins]

So just to reiterate, this is a simple uncalibrated shock-induced clang test jig, almost exactly the same as any high-gain amplifier provides with its input stage. The only form of measurement is what you hear through the speakers.

If you want to augment that measurement you can take the amp output signal through audio software that provides a frequency spectrum, and watch the spectrum as the valve is 'clanged', and note whether the response has dominant resonances and/or a whole cacophony of microphonic frequencies, and whether the frequency range is limited to above a certain lower frequency (eg. the EF86 is typically silent below 1kHz due to internal construction optimisation). You could also try and set up a repeatable 'clanger' and measure the peak dB level.

A more exotic test would use a vibrator table, and a sweep of vibration frequency - then the meter can be a dumb rms meter.

Some background reading at Microphonics in valves.pdf