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Old 25th October 2007, 07:34 PM   #1
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Default Test pentode grids without a tester

I have three rare EL34 and want to test their grids for sorts and/or melting.

My ideia is to power the filaments and with a variable low voltage supply at the anode put a small amount of current (20mA for example) through the tube and them meassure the voltage at the grids.

This method will work?

Thanks for any help.
Diego
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Old 25th October 2007, 08:54 PM   #2
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Hi Diego,
The method you propose may detect some voltage on the grid (G1 or G2) due to space charging. But this will be inconclusive as to whether it's leakage between elements or not.

Don't heat the tube. Leave it cold and apply a variable DC voltage through a series resistor and a milliamp or microamp meter between the grid of interest and the element next to it. (Your DC supply may already have a milliamp meter?) A good tube, when cold, should have no leakage between any elements and should withstand at least 100 volts on G1 and more on the rest. (NOT heater to cathode)

This test is called a DC hypot test and will reveal any leakage or shorts that a regular low voltage ohmmeter will not. Polarity is not really important in this case since the tube is cold.

Victor
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