Tube noise and gain test fixture

[Matthews Guitars]

I've gotten it into my head that I want, NEED, a special test fixture so I can pre-qualify preamp tubes (regular 12ax7 types) for performance. Use it to pick out tubes that are best suited for high gain and low noise applications.

First, is anybody making this already? I don't want to put a lot of effort into making another thing some people call a "wheel".

I want it to be a self contained device requiring no additional test equipment to be connected. I think all it needs is a meter to show a noise figure value, and a speaker to hear the noise
at different selected drive/gain levels. Also it'd be good for checking for harmonics.

Being able to externally monitor via an oscilloscope would be quite easy to manage. I'd include that.

So is anybody producing something like this already?

[martin manning]

The closest thing to what you are describing that I can think of is the now out-of-production Vacuum Tube Valley Dual Triode tester. https://www.usaudiomart.com/details/649 ... r-ser-006/

[R.G.]

I don't know of any that are in production. I've cobbled a couple together over the years. You're right, it's a simple enough design: power supply, socket, input arrangements, "typical" circuit using parts chosen for low intrinsic noise, followed by your choice of arrangements to read the results. For really low noise tubes and careful evaluation, you might want a super-low-noise opamp arrangement after it to bring any internal noise up for study. A +20/40db arrangement ought to help categorizing things. The back end analysis could be scope, amp/speaker or something as fancy as an Audio Precision setup. All would have their uses.

An interesting first test would be to measure the tube's output noise with a nominal-ish 1M grid resistor and a short to ground. The difference ought to be the resistor thermal noise times the tube voltage gain.

I'm not clear from your post whether you want to build one for yourself or to sell to other people.

To a very accurate first order, the cost of parts can be done by designing the enclosure, switches, pots, sockets, etc. with no attention paid to the inside electronics at all. I've been amazed at how true this approximation is, over many projects over the years.

[trobbins]

Perhaps well worthwhile looking through how Scott Reynolds has worked through that concept:
https://tavishdesign.com/

[Matthews Guitars]

My objective is to obtain such a device, for my own use. Not to build them to sell to others.

If I have to build it, that's within my capabilities but as I said earlier, I don't want to reinvent the wheel. I'd like to see an established design to use as my basis to start building.
Better yet, if the price is fair, just buy it. I'm not as into DIY projects as I used to be. I've got enough that are going....SLOWLY...to want to add more. But I could use a tube noise and gain test fixture, for sure.

[neskor]

This small Vacuum Tube Valley Dual Triode tester looks great.
Is there any more info?
Is this a project from Vacuum Tube Valley magazines?

[trobbins]

Given something like what you'd want would be quite niche, you could check with Scott to see if he has/would make a jig and gain unit for you.

Ronald's uTracer has a distortion assessment tool which may get you the information you are after, but not for noise comparisons, and is a commercial partly made kit of parts, so again not what you are immediately after.

Given you are just going to test one preamp tube type, then buying a low noise amp that uses that tube would allow swapping and comparison, as it provides power, connections, controls and amplification. You'd then need to provide any one of a large range of signal/audio analysers made over the decades to provide 'noise and distortion' measurements, or do a bit of diy and use a soundcard and software and follow the many setups used by the high-end modern audio testing fraternity.

[martin manning]

This small Vacuum Tube Valley Dual Triode tester looks great.
Is there any more info?
Is this a project from Vacuum Tube Valley magazines?

It was offered as an assembled product, and it was quite expensive, IIRC.

[martin manning]

My objective is to obtain such a device, for my own use. ... I could use a tube noise and gain test fixture, for sure.

What tube testing gear do you have now? IMO a curve tracer will tell you a lot more about the tube (emission, Gm, matching of dual triode sections) than a single point test. I have over a decade of experience with R Dekker's uTracer, and have found it to be very useful for evaluating any type of audio tube. Noise testing is another matter. An accurate quantitative assessment would require a fairly sophisticated device. A qualitative assessment could be made using a really quiet plate and filament supply, and a simple CC gain stage circuit. A pair of them would allow direct comparison of dual triodes\ sections. Looking at the outputs on a scope with the input grids grounded would be an easy way to get a measurement of relative noise level at the output.