We sometimes get questions like this:
"I just tested a brand new current production 12AX7 on a tube tester. And the meter on the tester says the tube is bad! What’s going on?!"
We can certainly understand the distress of having your brand new tube test as bad on a tester. Fortunately, the tube is likely just fine. And most likely, you are using a type of tester called Emission tester.
How do we know this? Are we clairvoyant? Well, at least not yet, but we just know. Let us explain.
There are two kinds of tube testers: Dynamic Conductance tester and Emission tester.
Conductance testers measure the gain (Dynamic Conductance) of a tube in an actual amplifying circuit and see if it performs as expected.
Emission tester, on the other hand, merely tests how much current a tube can pass through. This is to see if a tube can emit enough electron from its cathode. If it can’t, it must be worn or bad…. Right?
Well, that may sound like true, the problem is that tube manufacturers never published such thing as Emission rating of tubes. Emission Tester manufacturers instead tried to deduce good/bad emission ratings from tubes that were available back when these testers were made.
Now, that was more than 40-50 years ago. You know, when tube jukeboxes were playing, and we all had full hair - if we were born at all. Current production tubes were not available back when these Emission testers were being made. What this means is that classic emission testers don’t account for changes made to improve quality and reduce cost over the last few decades by tube manufacturers.
12AX7 is an excellent example of this. 12AX7 normally operates with 1/100th of current going through it compared to what Emission Tester tests for. Tube manufacturers optimized design around the actual operating condition. This has resulted in current production 12AX7 often showing as bad on classic Emission testers, even though the tubes are working 100% per spec.
Emission testers were useful back when every household had a Tube TV. When your TV stopped working, you checked the tubes on emission tester, changed the ones that tested weak, and it was all good again. Well, that and perhaps a slap or two on the side.
Emission tester is still useful for checking diodes and rectifiers. We do not, however, consider Emission testers adequate for testing new current production tubes, because of the reason stated above, and also since Emission testers do not test real performance, which is measured by gain (conductance) of the tube.
At Tubes for Amps, we only use dynamic conductance testers. We use top-of-the-line vintage testers that have been restored and calibrated, along with modern all-digital testers.