military: Gm reading on TV-7 tube testers

TV-7 military tube testers are appreciated and widely used because of their versatility, ruggedness and simple operation. Unfortunately the meter just gives arbitrary values, from 0 to 120, and data tables indicate the minimum acceptable value for each tube.

But the arbitrary values of the meter can be easily converted to the actual mutual conductance Gm, expressed in micromhos, using the table below.

 

Intermediate values can be readily calculated by linear interpolation.

The table comes from the TM 11-6625-274-12 manual, covering models TV-7/U, TV-7/AU, TV-7/BU and TV-7/DU. Probably it should also apply to TV-7/CU.
 

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Beside the arbitrary values of the scale of the meter of the TV-7, BIAS control setting also uses arbitary values (0 to 100). However, the average user is interested in explicit grid voltages. I have compiled a chart that converts the arbitrary readings of the BIAS potentiometer to real world grid bias voltage.

But keep in mind that the above DC-BIAS voltage in addition is superimposed by an AC grid signal of variable amplitude. This serves to perform AC modulation (with line frequency) of  plate current and measurement of mutual conductance (= transconductance, Gm).

BTW, the amplitude of the AC grid signal is changed with the the range selector (B-F). Signal is lowest in RANGE F (highest transconductance).

The following table is an upgrade of the table contained in technical manual TM11-6625-274-12 as shown in the posting of Emilio and also covers the TV-7D/U. The TV-7D/U has an additional measuring range 'F' allowing the measurement of Gm up to 60.000 micromhos (=60 mA/V).

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Thank you for this important contribution. 

My question is: what is the "minimum value" standing for in terms of percentage of Gm? 

Second question: usually valve manufacturers indicate different Gm's according to different settings (Vg, Ia, etc...). What is the setting to be used to compare Gm reading on the TV-7 meters with Gm declared by the manufacturers? 

Many thanks again

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The column 'Minimum Value' just gives the minimum reading expected for an acceptable tube on an arbitrary scale. Tubes returning readings under these values should be replaced. It can be compared to the separation line between 'Replace' and 'Good' available on other tube testers.

Referring to the second question, the TV-7(*) is a tube tester, not a tube analyzer. It checks the Gm in a Hickok simplified circuit, where the tube is connected as a triode, regardless of how many grids it has. To perform the measurement you are asking for, you should have on hand a tube analyzer, like this TV-2.

Emilio

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Only one model of Hickok tests all tubes as triodes, the 799.  Others apply screen voltage, typically 130V, while the plate is at 150V.

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