AVO Tube tester MK3/Mk4

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Output for digital meter

We added an output to the tester, to which you can connect a normal digital voltmeter.

The anode current is 10x the voltage you read.
So if you read 7.13 Volt, the anode current is 71,3 mA.

There are a few advantages with this.
  1. The analog meter works just the normal way. Connect or disconnect the digital meter as you like. It doesn't affect the analog panel meter, or measurement in any way.
  2. More precision. The output is extremely precise. Any difference with the analog panel meter, the digital meter wins.
  3. No need to set the mA knob any more. The digital meter works independant from this. So if the tube has 9.15mA, you will always read .915 on the digital meter, regardless if the tube tester panel meter is set for 10mA, 25mA, or 100mA full scale.
  4. Transconductance measurement is faster an easier. No need to do the backing off, for tubes in the 2.5mA range. For the higher ranges, just enough to prevent heavy needle movement is fine. For larger tubes, you do need to do it roughly, in order to pass the 2,5mA range. Once you go to the Transconductance setting, the digital measured plate current will step up. The Transconductance equals this step. This requires no precise setting of the back up, and no precise setting of the Transconductance dial. This gains very much precision, because the whole system with backing off and Transconductance dial is bypassed.
Numeric example for ECC83 test.

Data sheet says 1,2mA at -2V grid, 250V anode. Gm=1.6mA/V (or 1600 mMho).

Reference value for quality: Multiply -2V with 1.6mA/V. This is 3,2mA.

Measurements
  1. Set the tube for 1,2mA by changing the grid voltage. Read this 1.2mA digital, it wll say 0.120 on the digital output.
  2. Without backing off, go to transconductance test. Ignore the analog meter. The anode current at the digital output will now jump from 1.2mA to 2.8mA. The difference is 1.6mA, or Gm=1600
  3. Quality of a tube under test. Measure grid voltage needed, and Gm result. If the above value of 3,2mA is not met, this tube has less emission. A loss of 15...30% is tolerable.