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In Reply to: Re: How do you clean 40+ year old tubes? posted by brobertson on September 04, 2003 at 17:03:14:
you said - "Too bad they do not end up testing new afterwards..."It's funny that you mention that because there are 27 of them and they all test as new on my tester. My tester doesn't have the ability to test the individual triodes so it sums the two halves. The standard transconductance of one triode is 2,200 MicroMhos so the summed reading should be 4,400 MicroMhos for a new tube. Yah, I know this isn't the best tester for dual triodes but the bottom line is that there are 27 tubes and they all test between 4,700 and 4,950. This tester has indeed picked out other bad tubes as was evidenced by how they sounded, or didn't sound, in my preamp and subsequent tossing of bad ones into the trash can. I'm guessing that since they were used in the tone generation section of the organ that they just weren't used anywhere near their maximum capacity. I don't know. Any ideas?
Follow Ups:
Hi Bob ,
Testing both sections at once is not a good idea , after all one section could test 150% and the other 50% ! Ever thought about making an adapter with a plate switch ?
Oh, oh! I think that ain't right, FB. I am going to guess our fellow Bob owns a B&K 500. They tell you in the manual that the combined transconductance is 'carefully' calculated for. If the one section is at 150% of gm, it is probably leaky or gassy. The 'Shorts' test or the 'Gas' test would show that. Besides; having one section 'gassy' is probably impossible. I can't recall if the B&K 500 has a 'Life' test, but a triode at only 50% of gm wouldn't pass a 'Life' test, either. These are all guesses; I may be full of crap.
and it tests for shorts, combined transconductance, life and gas.
All of the cleartops tested great on it but the best test available to me is when I try them two by two in my Foreplay preamp. The Foreplay has separate volume controls for left and right and has a very thin top chasis plate which means that microphonic tubes will be easily identifiable.This is kind of fun, maybe I'll run an ad in the local paper:
"Old tubed ogans hauled off for $10." :-)
fatbottle,"Ever thought about making an adapter with a plate switch ?"
Nope! But now you got me thinking. I'll go to Duncan Amps tube data page for a 12AU7 and print it and get out the pencil. If I can figure it out it will sure be cheaper than looking for a better tester.
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