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Questions about tubes and gear that glows. FAQ

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Re: Hickok testers and older triodes

Posted by PhilF on September 6, 2002 at 02:04:18:
One of the biggest problems with testing directly heated triodes like the 45 and 2A3 is that most testers give you an emission test only, not a Gm (mutual conductance) test. This is because the 60 cycle AC that is used to heat the filament can get into the Gm bridge circuit, causing erroneous readings. So, the designers of the Hickok Cardmatic line of testers, for example, decided a more accurate test would be one that just measures plate current only (an emission test).
This would normally be an acceptable compromise, except that there is another problem. Many testers simply do not provide enough voltage to the filament, especially when it comes to low voltage high current tubes like the 2A3. You might be surprised what you'll find if stick a 2A3 in the socket of your tube tester, set your tester for 2.5V filament voltage, then measure the actual filament voltage AT THE PINS of the tube under test. The 2A3 pulls 2.5 amps of current, which is quite a bit. At 2.5 amps, dropping a volt or so in the the wiring and switch contacts of a 40 year old tester is something that should be expected. But when a volt or so equals almost 50% of the rated filament voltage, you have a problem. I shudder to think how many perfectly good 2A3's have been thrown away just because a tester that was supplying only 1.5 volts to the filament showed the tube was weak. As an example, the Computerized Cardmatic software actually selects 4.3 volts for the filament voltage of a 2.5 volt 2A3 to make up for this deficiency, quite a difference.
So, the "authority" is correct in stating that not all tube testers are to be trusted, especially when checking older directly heated triodes, but probably not for the reasons you might think. Just make sure the filament is getting its rated voltage and you will already be two steps ahead of the game.
-Phil