Tube Myths- Tested Tubes

By the many emails, thanking me for the info, I have decided, to not let criticism, of my supposed intentions, stop me from posting, information.
Almost all customers, of mine, asked if a tube, has been tested. Most ask for the best testing tube, thinking, that it would mean, that it is , the best tube, as far as longevity, sonics, noise, etc. Testing tubes, will tell you, whether a tube, is within specs. It will give you a reading, that you can correllate, with the manufacturers specs, to determine if a tube, is good, and useable, or defective. It will not tell you, how good a tube, will sound. It will not tell you, how long a tube, will last. Noise testing, will not guarentee, that a tube, will not be noisy, as some circuits, place demands, on tubes, that no amount of testing, can predict, how they will act. Don't get me wrong, testing is invalueable, because it narrows the field, and gives the user, a much better chance, of a tube, that will meet their needs. It just not a carte blanche guarentee, that a better testing tube,ie; balanced sections,emissions, will be a better tube. Because a tube, tests higher, will not mean, that it will have a longer service life, than one that tests lower.
There are many tests, that a tube can be subjected to. The more testing, that is done, the better chance, of predicting, how the tube, will act. But none can tell you, how it will sound. That can only be determined, by the tube, being placed in the intended circuit, allowed to breakin, and listened to. And because circuits, run tubes differently, it's difficult to determine, that operation, in one circuit, verifies the same operation, in another. For instance, a tube operated at 240v, might be just fine, but at 310v, it shows some problems. Or a tube at 290v, is perfect, but at 210v, it exhibits problems. In a perfect world, it would be just fine, throughout it's entire operating range, but this is the tube world, we live in. Enjoy.
Flethc - Interesting post and pretty relelvant to a nagging question on my mind. One of my CJ amps appeared to have blown several EL34's due to a volatage surge last month. The bias lights turned red on 5 out of the 8 tubes, and the amp shut down. I replaced the tubes but kept the old ones to test when my newly purchased B&K 747 testes arrived. Well, the tester sez they are fine! There are no shorts, no gas leaks and the Gm and Tube Life tests are fine. The tester is a fully refubished, like new unit bought form B&K Test Equipment. I double checked the calibration and it is indeed pristine. Sooo..why did the amp declare the tubes "bad"?
Which brings up another point: what faith does one place on "test results". Yes, there are many tests to subject upon a tube, but what can you glean from the results? Some vendors will state "no shorts or leaks" or "76/38" yet you have to rely that the operator has the proper procedures, a calibrated tester within accepted standards (do you know what the standard or standards are?) or that the results are within the capabilites of the testing device.

Another point, relating to Alexc's post, is that tube amps run at voltages from 200 to 600V. Tube testers, except for the ones found at laboratories, do not even come close to this - so how can one predict or correlate real world service in an amplifier? Also, I don't see how you can diagnose a power tube without subjecting it to the equivalent HT mains voltage. So it's very possible to pronounce such faulty tubes as "fine". (Imagine a buyers' reaction to someone like Alexc stating - and later insisting - that the tubes he's selling test fine: the tester says so!).

What I'm trying to get at is that if you buy from someone you know, you can use stated test results as a GUIDE. I think, in a lot of cases, tube test data, as published in ads on ebay and here, is in no way 100% accurate - but since most hobbyist use testers such as Hickock, Avro, Eico, etc, the test results are on a similar plane and, therefore, can be used as an accurate guide. All things being equal - of course they are not. Caveat emptor.
this is a very helpful and informative post for the "new to tubes crowd." thanks.
AlexcThe B&K will not apply operating voltages, to the tubes. So at the 240v? it applies, everything is fine. But the amp applies operating voltage to the tubes, so at 470v?, the tubes show problems. Gs5556 Now you know to ask, not just is it tested, but how was it tested, what voltage, what the manufacturers spec, etc. This will give you a much better chance, of getting the most useful answers. Maxcast Welcome to the wonderful world of tubes. Above all, trust your ears!