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

[René Schmitz]

Hi JP,

jpdesroc at oricom.ca schrieb:
> In fact all I want to check is the "on test" 12AX7 gain
> and compare it to a 'standard, verified, normal' value.
> Since the most regular B+ voltage I see in
> the musical tube amps I fix is around 300-400VDC
> I'd choose 350vdc stable supply.
> Then, correct me if I'm wrong but all I need is a
> standard triode stage circuit driven with a suitable
> 1khz stable source then monitoring the plate
> swing voltage (that would swing around 2/3 of B+ value) 
> rectified and fed to a small solid state meter driver..
> Using a "known reference gain" tube I'd
> adjust the meter so its needle is around
> 50% of the meter span (and put a mark there) 
> so lower means the on test tube starts to get weak
> and upper means the on test tube is GOOD
> and maybe better quality than average
> if needle get higher.


Tube parameters tend to stray quite a lot. You could of course do what 
you propose here. But I'd recommend using one of the quiescent currents 
and resistor values that the manufacturer publishes data for, so you 
know what plate current and what gain you can expect. And then compare 
the values you obtain with those from the datasheets. That way you don't 
even need a reference tube, but compare to the idealised "mean value 
tube". Tube testers often don't measure amplification/transconductance 
but rather just "emission" that is how much plate current for a given 
(Up, Ug, often they just use low supplies, hence low currents, so 
sometimes the tube still fails when it sees the actual higher supply. 
(arcing, bad vacuum etc.)
So makeing a rig for testing just 12AX7s makes sense to me.

Cheers,
  René