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Re: Stupid question about tube testers

To convert TV-7 meter readings to umhos, multiply the meter reading by:

25 if using the B scale
50 if using the C scale
125 if using the D scale

So a 12AX7 that tests at 40 on the B scale has a Gm of 40 x 25, or 1000 umhos. A 6L6 that tests at 40 on the D scale has a GM of 5000 umho.

When comparing Gm readings on a tester to the specs in a tube manual keep in mind that the specs are listed under a specific set of conditions. The tester usually does not test the tubes under the tube manual conditions, so the readings won't agree. Plus tubes have a wide GM variation from tube to tube of the same type due to production tolerances/variances.



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