Tubes Asylum

New Production tubes often fall outside factory specs

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This Post Has Been Edited by the Author

In Reply to: RE: Any Current Production 12xx7 Tubes Worth Buying Yet? posted by TubeAcolyte on March 27, 2017 at 11:27:17

Hi TubeAcolyte. Per your mention of new production tubes falling outside spec right out of the factory. I have related details from 2 well-known tube sellers. Likely I have more details, but like you, I have buried where I filed that information.
1) From one well-known tube dealer:
New production tubes can range dramatically. I get new production power tubes of a certain brand and type that ranges from 35% to 180% of expected value. I've had to learn from experience which portion of that range the tubes I want lie in. But the Maximatcher which is the industry standard tester for power tubes accepts them all as good tubes. And in most circuits, they will bias up just fine and work well. The life (in hours) of the tube is based on the number of electrons available in the cathode coating, not the measured strength of the tube on a tester. If biased the same the 35% and 180% tubes should last about the same amount of time. That is an extreme example, but the point remains.
Brand new high grade 12AU7's from a reputable source regularly run in the high 60% to mid 70% range for mA and Gm.
So, no, new does not mean extremely close to 100% value. That is the target, not the actuality. Sorting tubes to be very close to 100% will leave you with not so many tubes.

2) This is a Jim MCShane quote likely soured from one of an Asylum thread entry:

I think even more relevant is the fact that even brand new tubes test over a wide range of parameters. I have new 12AX7s that test 25-30% apart, that's very normal. And the tube that tests higher is not "better", just different.
Jim MCShane

Edits: 03/27/17

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