How to test tubes accurately on a service tester

Test tubes to the highest degree of accuracy using a service tester with the following instructions:

With the exception of laboratory and quasi-laboratory models most all other tube testers and the bulk majority of testers in use today are of the service type. This means they were designed to locate weak and bad tubes, not to test the quality, or the specification of new tubes. Additionally the selling price of these models were a key issue for the TV/Radio and electronic shop. This created a series of technical trade offs in producing a good tester that could locate bad tubes effectively, but still be in a price range that service shops could afford.

The average service tester has an accuracy of between 10% to 15% (depending on make & model) when properly calibrated to the manufacturer's values. This can be improved through tight, or precision calibration, but not much beyond 7% to 9 % on average. In comparison most laboratory models have accuracies of 1% to 5% (as specified in their own literature). These testers having more precision components and had regulated voltages, where service testers have fewer precision components and do not have regulated voltages.

The following are steps that will apply to most all service testers:
  1. Allow the tester to warm up for at least 15 minutes before testing a tube (transistor models as well).
  2. Set up all switches before you insert the tube to be tested.
  3. Set the line and bias voltages controls initially.
  4. For greater accuracy in test results allow the tube to warm up for 3 to 5 minutes, longer is better.
  5. Always test for shorts and leakage first - Failure to do this could damage your tester!
  6. If the tube has a short/ high leakage discard it and DO NOT make a Gm or other tests.
  7. Just before testing, re-check the line voltage and bias voltage level setup.
  8. Perform the test - hold the test switch/buttonfor at least 30 seconds. This is required as most testers only apply plate voltage during the test and only then is the tube conducting. As it conducts it will settle in as the plate current reaches is correct operating point. Be aware that tubes do drift, so holding the switch down long enough for the test result to stabilize will help. If the test value continues to fall by 500 to 1000 Gm it is possible the tester has a defect, or the tube is drifting due to a serious emission problem. Perform a life test on the tube per the testers instructions.

One of the typical trade-offs in design and price was the use of the line voltage control. This devise allow the tester to be set to a specified line voltage which allows the inter relationships between several operating values to be reasonably maintained based on the predetermined line set value. This function also crates a line sag which when a tube is inserted causes some of the operational voltages to drop under load. The heaver the load the greater the sag and the greater the reduction in key operating voltages/currents. This reduces the accuracy of the test result to some degree which will vary depending on the make and model tester and type of tube being tested.

To increase the accuracy of the test you can use an external DMM (Digital Multi Meter) which will allow you to set the line voltage accurately to the correct level for your tester under load making sure the signal voltage is correct, and the bias voltage as well. These three items will have a large effect on the Gm result.

Key issues in accuracy of testing:
  • A working and correctly calibrated tube tester
  • Correct AC line voltage
  • Bias voltage
  • AC heater (Filament) voltage
  • Proper settings of, Sensitivity, English, Meter Shunts, and Calibration controls
  • Test signal level

By keeping these values within a tight nominal range you will be able to measure your tubes with a grater degree of accuracy in Gm value.

DO NOT turn off the tester when switching between tubes to be tested! Just remove the tube, re-set the setup switches, re-check the setup, and then insert the next tube to be tested.