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Zero Cool 31st May 2008 07:17 PM

Testing for tube shorts
How does the "shorts" function of a tube tester work??

Lets say i have a bunch of triodes and no tube tester with a shorts function. How can i set up a testing system???

IE: when a tube shorts. is it generally a direct short that could be measured with a multi meter?? or is it a low impedance situation that shows up with the tube filament is lit and HV is applied??

Lets say i have a test set up where i have a tube in a socket with a filament transformer and a HV Dc supply. How would you test??

Geek 31st May 2008 08:09 PM


Basically, it's a neon bulb in series with a capacitor, limiting resistor and the tube and AC is applied.

A good tube or tube section will have a diode effect, rectifying the AC to DC, which cannot pass through the capacitor, hence the bulb doesn't light.

A shorted tube or section won't have this diode effect and AC will pass, lighting the bulb.


Zero Cool 31st May 2008 08:22 PM

AHHHHH I see....Ok, Any suggestions on the size of the capacitor. I have one specific tube in mind to test that has a 250VDC plate rating. If just use 110v ac line voltage as the test with a neon and resistor rated for line voltage. what size cap should i use???

Geek 31st May 2008 08:48 PM


Never use the line voltage. Isolate it.

Here's the schematic for my DC grid-shift tester. Take a look at the shorts indicator circuit:

Isolated 120V, 0.1uF cap with 6.8K resistor (between the 6.8K and 0.1uF, current is limited to the bulb OK) and NE2H. The 220K across the bulb bleeds the DC out of the cap.


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