Tubes with identical parameters, 0% tolerance!!!!! Very rare and difficult to select such pairs. Still, it’s possible to select such pairs only from a very large batch of tubes, all with the same date of manufacture:1986.
The tubes are carefully selected after accurately testing a batch of 1000 tubes – all manufactured on 1986, using a tube tester well calibrated, which has as factory settings an accuracy of 1.5%.
Each tube is heated at least one hour before testing, for an even better measurement accuracy (In the description photos one can see both the tubes, and the tube tester, during the heating and the testing process)
Conditions for tests: heater=6,3V, plate=250V, g1=-14V, g2=250V.