• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Test of ECC81 to 83


I have close to 100 pcs ECC81 to ECC83. Some used, some new but no box. :xeye:

Now I want to test all the valves. First, I want to remove all the bad valves. Second, I want to look after balanced valves.

Now I have a question. Is there some standard test condistions to do that?



I am assuming that you do not have a tube tester.
So for 100 tubes you want something quick and easy to make with a minimum amount of parts and measuring equipment.

So what I would do is make a test circuit consisting of 250 V dc supply, 100K ohm resistor from Vdc to plate, 1k resistor from cathode to ground, and grid connected to ground. And 6.3 or 12.6 volts to the heaters. With a working 12AX7 (ECC83) you should measure about 145 volts at plate. With 12AU7 (ECC82) about 55 volts. With 12AT7 (ECC81) about 95 volts.

Equipment required : tube socket, 2 resistors, DC supply, voltmeter, AC or DC heater supply.

This gives a quick indication that the tubes are basically working. You can see it the heaters light up. If the voltage reads 250 the tube is completely dead. If the voltage is much higher than listed then you have low emmission. If the voltage is down around 2 volts you have a completely shorted tube.

As for balanced or matched tubes you could select pairs that have the closest matching voltages, but be aware that this is only matching based on one particular dc operating point and says nothing about ac characteristics. Although it may be better than nothing.