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12ax7 tube testing

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Not really. All you could test with a multimeter alone would be the heater filaments for continuity.


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12AX7 has a higher gain than 12AU7 and if you want to measure this value, you need to put it into a pre-amp.


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Do they ever fail like that?, I've serviced huge numbers of items using ECC8x series valves over the years, I don't think I've ever seen (or heard of) such failures.
Just mostly from memory of those drug store tube testers that had a red lamp labled shorted element, I assumed this was done by some kind of conduction detector, but no I haven't personally troubleshoot that much tube equipment in my life. Do seem to recall ham radio failures in power output tubes running class C could draw excessive control or screen grid current and shorted element was one possible failure mode.

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The circuit you built in the other thread can help you test the tube.
Put a 2k 1/2 watt resistor from cathode to ground.
At the moment you have the cathode going to ground.
Put the resistor in series.
Put the multimeter ground lead on circuit ground.
Put the meter on the highest DC volt range.
Put the + lead (if your meter can read a voltage as high as 400 volts) on the plate pin and record the voltage.
Put the + lead on the cathode pin and record the voltage.
You might want to decrease the voltage range one step for this one.
Report to us the voltages you record.
If you have a plate voltage that is as high as the high voltage and a cathode voltage of zero the tube is not conducting any current. It is dead.

Edit: No 2k resistor - use any resistor you have between 50R & 2k for the test.
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