[sdiy] A simple 12AX7 vacuum tube tester schematic..

[Dan Norris]

> I need a simple schematic for a tester 
> of the popular ECC83/12AX7 vacuum tube
> used in lots of audio/musical amplifier
> these days. I already have 2 tube testers
> but because this type of tube draws very little
> plate current my testers meter's needle 
> doesn't go up very far so readings are
> not quite acurates.
> 
> All I need is a component list of the best values for
> a standard triode tube circuit section that would have:
> 
> The VDC supply value for the test schema (ex: 350VDC)
> The plate resistor
> The cathode resistor (if any needed) to the ground
> The grid resistor value (to the ground ?)
> The negative bias test voltage (ex: -1.3v)
> The uAmp meter value for readings (hooked in series with the plate
> resistor)(ex: 0-100uA)
> The threshold meter reading value for a GOOD ECC83/12AX7 (ex: 60uAmp
> or more is GOOD)
> 
> A simple SPDT switch will select what section of the tube is tested.



Bad 12AX7's might look like gold on a meter. The best test is a little amp like a Fender Champ. Plug in the tube, turn on the amp, and listen. Noise is bad, silence is good. I know that sounds simplistic, but that's where it's at with preamp tubes.
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