Interpreting Tube Matching

What are the important parameters to know in tube matching? The 8 Tung-Sol KT-120's came with numbers on the box from the factory as well as an additional sticker with more numbers on it.  One was Gm and another was IP. Probably other info too. I'm trying to determine what's important when matching up tubes on the M-125.

Thanks!

Frank111

http://www.mcshanedesign.net/tubes.htm

Great info on tubes...........if you did not get your tubes from him?





Thanks for the site Bob.................

Plate Current (PC) and Amplification (TC) are the biggies ... shorts and leakage tests as well.

A well tested tube should also be graded for microphonics and low noise.

Do you mean Ip and Gm?

sKiZo wrote:Plate Current (PC) and Amplification (TC) are the biggies ... shorts and leakage tests as well.

A well tested tube should also be graded for microphonics and low noise.

I agree with much of what you wrote, but...

In many - if not most - amps the bias current is measured by means of a resistor in the tube cathode. Ohm's Law is applied to the voltage measured across the resistor and the current is then known. In a number of other cases an ammeter is inserted in the tube cathode circuit to directly measure the current.

BUT... the current passing through the cathode resistor (or ammeter as mentioned above) in an amp is not just the plate current, it is the plate current PLUS the screen current (unless the power tube is a triode). Therefore, since the bias (a.k.a. cathode) current is not just the plate current then I believe it is better to match for cathode current than just for plate current - and that's what I do! I also check power tubes and cull out any tubes that have excessively high screen current.

And BTW, transconductance (abbreviated Gm) is not the same as mu or amplification factor - but I do agree that matching Gm is a real plus.

just my own opinion of course, and in general tube matching is a good thing. I've even paid Jim extra to do it, especially on phono tubes.
BUT, matching means the tube is matched at that exact voltage and current. Doesn't mean it's matched in the circuit you're using the tubes in,
that probably has different voltage and current then the "test" conditions.
Especially for an amp, the amp itself is the best test fixture! Buy a few extra tubes and "match" your own tubes!
Without changing the bias pot, plug in each tube and read the cathode current. That gives you the numbers.
Then pick the tubes that "match" closest to the current you want in that amp!