Q&A regarding different micromhos scores

Hey Tubesound,

Hoping you can answer a question for me. I have an Amplitrex and a TV-7D/U tube tester.  The Amplitrex tests a 12ax7 as new at 1600GM.  According to your TV7 GM calculator, I believe that should translate to 64 on the TV meter.  However, when I test the same 12ax7 on my TV-7 the meter only hits 42.  Does it sound like my TV7 needs some calibrating or am I missing something? Hope you can help and thanks for the great calculator.

 

Reply —

Yes, you are missing the fact that there is no relationship between those readings.  Tube tester Gm readings cannot be compared among models that have any difference in signal-bias-plate-screen voltages.

Gm is based upon bias, signal voltage, plate voltage and (where relevant) screen voltage.  In regards to a tube tester, those voltages make up the test parameters of that particular tube tester.

  • There is no such thing as one “standard” Gm value for an amp tube.  Gm test scores vary based upon signal voltage, DC bias, plate load, and plate/screen voltage.  Different testers (even among same manufacturer) can use different voltages, therefore results are different.

Since there is NO SUCH THING as one standard mutual conductance value for an amp tube, you should NOT expect your tester to read same (or even similar) to the nominal Gm value listed in a tube characteristic book, or any other comparison base you are using (such as a different model of tube tester).

To emphasize this important point, I will offer you this quote from “Latest Instruments for Servicing Radio-Television”, ©1953, Coyne Electrical and Radio School, pp 270-271:

“If mutual conductance readings are to match those given on tube manufacturers’ data, all of the applied voltages as well as the plate circuit load must be exactly as listed in the same ratings…. The fact that there is no one standard mutual conductance or transconductance is evident from Fig 2.”

gm

Each tube tester manufacturer determines a minimum acceptable Gm score based upon the combination of the test parameters of their own machine.

On a TV7, the minimum Gm for a 12ax7 is 800, which is base for most Hickok-based circuits.  A typical new 12AX7 will test between approx 1100 and 1400, with 1250 being considered “nominal” or average for a new 12AX7.

Of course, putting too much importance into a Gm score is misguided.  Higher is NOT better in any practical way — meaning that you think higher would, in any way, correlate to “better sounding” or even “have longer life”.  Of course, we are assuming a tube that is within a reasonable ballpark of “nominal new”, a reasonable ballpark would be roughly +/- 15% of nominal new.

As an example, using the standard Hickok nominal new for a 12AX7 to be 1250 micromhos, it would be misguided to assume any superiority to a tube reading 1400 vs a tube reading 1100.  Either of those tubes could easily be NOS and any assumption that the 1400 tube is “better” (will sound better or will last longer) is simply ignorant of the realities and complexities of vacuum tubes.

regards,

bob


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