How to interpret the measurements on nos tubes

[jcr1234]

When buying tubes, often the seller will say "these test at 76 where new is 70 and minimum is 40." What do those numbers represent and where can I find the information that tells me what is good and what is nos and what is minimum?

Thanks,
Jon

[passfan]

When buying tubes, often the seller will say "these test at 76 where new is 70 and minimum is 40." What do those numbers represent and where can I find the information that tells me what is good and what is nos and what is minimum?

Thanks,
Jon

I believe you need to find out what kind of machine they were tested on and what type of test was run. This is information that can only be obtained from the seller. Some are transconductance , some are emissions. Just because a tube tests good does not mean it will work at all in an amp.

[Phil_S]

I think, mostly those ratings 40, 70, 76, aren't very meaningful. If new is 76, test is 70, and used up is 40, I guess the tube should be good, but I don't think it tells you much more. Even if you get a "pair" that test 70, it is unlikely they are matched.

There are more sophisticated testers that do give meaningful results. Like the other guy said, ask the seller what it means.

[jcr1234]

Right , I understand that much. What Im curious about is are those measurements micromohs, mv. ma? Also, where can I find the information that says xx is good, xx is new?

[Structo]

As said, every tester is going to show a different reading so unless you have the same exact tester calibrated exactly the same, the numbers don't mean anything.

I think it is more of a sales gimmick than anything.

The ultimate test is when you plug the tube into your amp.

I've had tubes that test strong that sound like arse in an amp.

[billyz]

You have to know what model of tester was used. Those numbers sound like a TV7. Which is a military model. The numbers are arbitrary and abbreviated for field use by GI's. Most of them were made by Hickock and measured gm or mutual transconductance.

If I remember right, the minimum number represents 60% of typical new. So a tube could measure say 105% of new. It just means it was a little above typical. Or look at it this way. If the manual said new was 70 and a tube measure 76 it is 108% of typical new. minimum good would be 60% or 42 on the TV7 scale. That does not mean to tube is bad. just less than Military standards.

You can convert TV7 numbers to GM. see attached file
Use linear interpolation to calculate between numbers.

All this is only for a TV7 meter, other meters used different scales and some measured different parameters, like emissions other tested life.

Meters are good for testing for shorts and finding very weak tubes.
Some poo poo them but I find them very useful if you know their limits.

[Structo]

I've been scanning ebay and man, those TV-2 and up testers are crazy expensive.
I know military stuff is good but some of those go for over $800!

I was trying to get one that does transconductance but most seem to be emission testers.

A Hickok 6000 went for over $250 the other day.
I'm watching a bunch but there is usually a frenzy in the last few minutes and the price jumps.

I didn't realize old tube testers were that popular. Used to be the HAM radio guys that traded all that old test gear.
Now that hobby is waning, are the Hi Fi and guitar amp guys going gah gah over these testers?

[PlinytheWelder]

There's still a lot of tube guys out there.
FWIW, I have a Heathkit TT-1A tube tester and I at least find it good for weeding out the real stinkers.

It's a real good tube tester!

[img:800:1067]http://www.kw.igs.net/~knickerson/img/TT1_lrg.jpg[/img]

[Structo]

One of those went for $465 the other day!
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... K:MEWAX:IT

Man, I couldn't believe it.

I stopped bidding at $150.....

I did get a Simpson, we'll see how good it is, pretty clean.

There is a TV-2A/U Military here:
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... K:MEWAX:IT

It will end very high.

The problem I see with some of the Gm testers is that most of them use tubes inside either for calibration or for meter amplifiers, so if those aren't any good.......
Then the calibration part of the equation can A) either cost you an arm or leg or B) not be feasible.

Regardless, I just wanted one so I can test a bunch of pulls I have and to have a way of comparing the good ones to the weak ones.

[David Root]

I have a VTV tester for preamp tubes and a MaxiMatcher for power tubes. Both are modern digital machines.

The VTV only gives relative numbers, not mA, on double triodes, and even though I may get 100/100 on a tube, indicating current balance, when I do the triode to triode balance test, sometimes they are not voltage balanced, and vice versa. This is just the nature of the beast. Still very good for picking PI tubes and eliminating microphonics, which it also tests for.

Because it doesn't give you an actual mA value, you cannot tell what the current draw will be until you put it in the amp, and I have found for example that the late '60s early '70s Amperex 7025s I have will drop a stage plate voltage by about 10V compared with an RCA 7025 or 12AX7 of similar vintage. This can be useful in a Dumble build.

The MaxiMatcher gives mA and Gm. It is limited to only two plate voltages, 325 and 400, would be nice if it had 450 too, but it's a good tool.