Tube testing for gain

[Tom Mc1]

I am expecting delivery of a Sencore TC142 Mighty Mite V tube tester so that I can test and possibly match some of the many tubes I have lying around. What I was wondering is if you can tell how much gain and how quickly it will start to breakup that you will get from a given tube by testing it. My understanding is that the V1 preamp tube has a lot to do with the amount of gain saturation. Also, do the power tube types just affect the tone of the amp or gain as well?

[WRGKMC]

No you cant tell those things from using a tube tester. The only things you'll be able to tell is if the tubes are good, bad or fatigued. It will let you know if the Filaments working, if the tube cap pass a signal and if the tubes become gaseous.

 

Tube gain is a matter of the type of tube, its physical design, How its biased and the circuit the tube is used in. A tube tester does not test a tube in its circuit so there's no way of it calculating what its minimum and maximum gains nor can it tell you what saturation levels it can achieve. All that stuff will change based on whatever amplifier circuit you use. All a tube tester does is make sure the tubes still operation and its most basic characteristics. If the tube reads poorly then its likely beat and should be thrown out.

 

Testing good doesn't mean the tubes completely good nor sounds good. It will not tell you if the tube is suffering from other faults like microphonics. There are faults that can occur or be heard that the tube tester simply wont detect. Red plating for example can damage a tube yet it may pass a tube testers basic testing functions. There are other tests like checking for matched pairs Measuring plate current draw under operating conditions a tube tester simply doesn't do.

 

I been an electronic tech for about 45 years. I never saw any sense in owning a tube tester. It doesn't measure the ley items you asked about so its limited testing functions are pretty worthless. Back in my TV repair days, we used to pull the tubes from old sets before throwing them out. We'd use the Tube tester to check the tubes and the ones that checked good we'd keep as used tubes. Even then we'd get some that tested good and simply didn't work right. Unless you have some business that requires bulk testing or was a field tech that did TV repairs at a customers home (completely obsolete job) there's no reason to own one. Tubes are cheap and always work best new. Why would you put used tubes in an amp unless its simply a temporary move till you order new ones? Keeping old tubes doesn't serve much of a purpose unless you're into repairs and need one to confirm a circuits working - even then a customer would expect to get new tubes, not used when they have a repair done.

 

 

[Tom Mc1]

That was very informative.

​I think that I will use this as a tool to figure out if the tubes that I buy meet specs or are substandard. I think that this would be a useful tool to check for balance if the tubes that I have. I realize that X brand and Y brand of the same tube could sound different in an amp. I wanted to know if there was a way to compare apple to apple without having to buy every brand on the market.

[WRGKMC]

Like I said, a tube tester tells you if they tubes are shorted or have become gaseous. A tube can read perfectly good on a tester and be completely shot for audio. Tubes aren't just for analog audio. Many can be used for binary switching and many other purposes

 

The key things that make for good audio like matched gain curves, bias levels and audio quality cant be tested with a static tube tester. You aren't passing audio tones through the tubes, you're simply heating it up and seeing how well the tube conducts. to test audio quality the tube has to be put in an actual amplifier circuit. You cant compare a tubes maximum bias using a bias tools and meter. You can see the saturation when you use an audio generator and view the tubes gain and amplification curve using an oscilloscope. You use different tools for different purposes.

 

A tube tester only gives you a few basic tests that will tell you if a tube has major fatigue, dead short or open filament. Its not going to tell you how well a tube words under normal or extreme operating conditions. You need to do more advanced testing under actual operating conditions.

 

One of the tests that can be done is a load line analysis. We used to do this when matching transistors to get (Near) perfect matches. A similar process can be done with tubes. This article from 1960 explains the basis. http://www.rfcafe.com/references/popular-electronics/load-line-story-nov-1960-popular-electronics.htm

 

This process while informative isn't overly useful to an end user or a technician. You'd need to be in the business of engineering or matching tubes. Since there's only a few foreign companies that make tubes, and because these companies already do these tests there's not a big need to do them other then for purely educational purposes.

 

But say you do have a set that are perfectly matched. You still need to put them in a perfectly balanced circuit to see any benefit. Most instrument amps are built with components that have huge tolerance variances, resistors an caps with tolerances up to 20% off. Older amps can have components that have fatigued and drifted to say 50% and still function fine.

 

Audiophile pursuits often build circuits with military spec components between 1~ 5% tolerance to get the highest fidelity possible. That's not usually the case with instrument amps that have a narrow frequency response, and especially since musicians like guitarists prefer low fidelity and driven tones the need to have high fidelity is low. A jazz guitarist may like pure tones with a wide frequency response but most other modern players like the ability to produce low fidelity and distorted tones. You don't get that from and audiophile grade amp. If you're into high fidelity, solid state is an easier way of getting there because its less expensive getting well matched components.

 

Even with the high tolerance components, tubes are biased within a range where the two work together. If one tube is slightly weaker you use its extremes to match the other. In the end that's all you get from them anyway. Like I said, a tube tester does a static test to tell you if a tube has major faults it functions and if it should be able to safely plug it into an amp and not cause the circuit to fail. Everything else needs to be tested under actual operating conditions. Even then it comes down to the particular amp and what any one person thinks sounds good to them.