| Meter reading | Range | Micromhos |
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DIRECTIONS:
- Enter your meter reading (0 – 120) into the “Meter reading” box.
- Choose your Range Switch position in the drop-down box.
- Press the “Calc Gm” button and read your micromhos score.
- Press “Reset” button to clear your entries or to start another calculation.
NOTES:
- calculator works for all TV-7 models: TV-7/U, TV-7A/U, TV-7B/U, and TV-7D/U
- calculator requires javascript
- Range A on TV-7 is only used for emission testing of diodes and rectifiers — no mutual conductance reading exists. Therefore, I have omitted Range A from the calculator. The Range A meter reading is an arbitrary emission score that is evaluated in relation to the “Minimum Value” notation in the setup book.
- Range B signal voltage is 5.0v ac with bias 0 to -40 vdc
- Range C signal voltage is 5.0v ac with bias 0 to -40 vdc
- Range D signal voltage is 1.0v ac with bias 0 to -40 vdc
- Range E signal voltage is 0.5v ac (500 mV) with bias 0 to -40 vdc
- Range F signal voltage is 0.5v ac (500 mV) with bias 0 to -4 vdc. Range F is 0 to 30,000 micromhos, not 60,000 as reported elsewhere. Click to “Read the rest of this entry” below for details.
- ©2014 TubeSound
Some sources incorrectly report that Range F on TV-7D/U as being 0 to 60,000 µmhos. This is incorrect. Range F is 0 to 30,000 µmhos, same as Range E.
Analysis:
1. The maintenance manual indicates that Range F is 0 to 30,000 micromhos.
2. The Field and Depot Maintenance Manual indicates that Range E and Range F give the same test result (40 scale divisions) from the “Calibration Test, aka… Simulated Tube Test”, applying the same signal voltage (500mv ac) and same calibration test voltage (50 vac).
Let’s establish exactly what micromhos score this combination will yield. Using a 50 vac source and 0.5 vac signal voltage, the math is:
50V source = I (current) x 10000 ohms (the resistance in the Simulated Tube Test) = 5ma. Take the 5ma / 0.5 vac signal voltage of Range E/F = 10. Multiply that 10 by 1000 = 10,000 micromhos.
If Range F was 0 to 60,000, the Simulated Tube Test would yield a result of 20, not 40, because the test parameters are identical.
3. The setup data indicate same Gm test scores for tubes testing on Range F and tested using Range E, using comparable setup data.
Examples: you can read the setup data for tube types 5842, 2C39, and 7289, which provides setup data for both Range E and Range F. These are three tube examples that I could find in the charts that have test data on Range F. The only difference in the setup data is a different bias setting; notice that the “Minimum Value” test result is the same.
On the surface, you may think that the bias in these examples is substantially different, but it is not. The difference with Range F is that the bias runs from 0 to -4, instead of 0 to -40. Hence, using 2C39 tube as an example, for non-7D models, the bias is set at “18” on the dial. This is largely equivalent to setting the dial at “78” on Range F, the difference being that Range F would be able to set the bias a little more accurately because it spreads 4 volts across the entire bias pot (instead of spreading 40 volts across the same pot for Ranges B through E).
Thus, the difference in Range F is that the bias runs from 0 to -4, instead of 0 to -40. The micromhos range is still 0 to 30,000. Since almost no tubes use Range F test data, the Range is largely meaningless.
Here are photos of a TV-7D/U that demonstrate that the technical manual specs are indeed accurate (not misprints or errata). The photos show the same signal voltage for Range E and F, same test result of the Simulated Tube Test on ranges E and F, and show that the only difference is the DC bias.
Range E performance specs:
Range F performance specs:
As you see, the Simulated Tube Test yields the same test result for Ranges E and F, exactly as the service manual indicates it would, using the same signal voltage and same test voltage. The micromhos are therefore the same; both ranges are 0 to 30,000 micromhos.