why 12ax7 may test weak on BK 600-606

A reader asked me why 12AX7 were testing weak on a BK 600 tube tester, but testing OK on a BK 707 (top panel Gm test).

The 600/606, and the bottom panel of BK 700/707, will overdrive a 12AX7 at plate(cathode) current levels that a 12ax7 tube is not really designed to handle.

[FYI – the bottom panel section of a BK 700-707 is basically identical to a BK 600-606. So that is why I am using the 707 for all tests in this article.]

The 707 bottom panel tries to drive a 12ax7 at around 20ma. Again, this is basically the same test circuit as a BK 600-606.

See photo of testing a chinese 12ax7 as tested on the bottom panel of a BK 707:

(a chinese 12AX7 being tested on the bottom panel of a BK 707. Notice that the operating current is 20ma.)

This Chinese 12ax7 actually passes the test easily, and tests strong (even with LIFE TEST engaged). (This is not advocating for Chinese 12AX7; it is simply an example of a 12AX7 that passes this test.) Notice that the operating current is 20ma.

Now… here is a Russian EH 12AX7, which fails miserably:

(Russian EH (aka Sovtek, aka Electro-Harmonix, etc…) 12AX7 testing very weak on the bottom panel of a BK 707.)

This is a classic example of where “you” (anyone) need to understand WHAT the test equipment is doing, and interpret it accordingly.

So, with the 600-606 or bottom-panel 700-707 regarding 12ax7 (or 6EU7, which is pretty much the same tube), if it tests strong, then the tube is an overachiever and most likely a quality tube.

…However, if it tests weak, you will have a harder time interpreting a WEAK result here, because driving a 12AX7 at 20ma current is FAR beyond what it was designed to do, which means the test is really INAPPROPRIATE. Ideally, a 12AX7 would be tested at an operating current of around 1ma to 3ma.

Datasheet for typical operating parameters of 12AX7. Max current 8ma, typical operating current 1.2ma.

Therefore, you will realize that every tube tester is a compromise in a ton of different ways…, which means that a number of tube types are either being over-driven or under-driven with the test parameters.

So, what to make of this? How should the typical user interpret this test result? The more that you learn about your tube tester, the more effective you can interpret its results. Using this 12ax7 situation as an example… if you purchased the tubes from a seller who is known to use quality test equipment and you have seen photos of the actual test results (for example, I almost always show photos of each section of a tube being tested that I sell on ebay), then you should trust the seller and realize the limitations of your tube tester.

If the seller has not demonstrated the tube tests good, then 100% return it, because a LOT of 12ax7 will pass this test FOR A BRIEF PERIOD. That said, if you try to test the tube for any length of time, you risk damaging the tube, which in the old days was known as “testing the tube to death” — destroying its emission because it is being significantly overdriven.

But like I said, in this example, just because a 12ax7 tests weak on a BK 600-606, it may not necessarily indicate a BAD tube because the test parameters employed by the tube tester will significantly overdrive the 12AX7 beyond its design parameters.