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

[Scott Gravenhorst]

jpdesroc at oricom.ca wrote:
>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.
>
>What do you think?

Hmm.  I played with tubes as a kid (because that's all there was for
consumers) and my dad had two tube testers.  There were buttons and
rotary switches on the panel that you had to set according to a book
that came with the unit.  These controls would insert correct circuit
paths with correct resistance values and apply correct voltages to the
tube's electrodes.  Then you would press and hold the "test" button and
watch the meter.  There was a red zone (bad), yellow zone (poor) and a
green zone (good) on the meter.  The manufacturer taylored the
resistors for each tube listed in the book so that if the tube is good,
it would read green if the tube was up to spec.  Some tubes, even when
new, won't make it, so there was a note in the book indicating that the
tube is good if the meter reaches a value of XX.  With this system, any
tube could be tested and there was never a need to modify the tester to
make the meter go higher.  If meter didn't go up much and was in the
red and no exception was noted in the book, the tube was just bad.

Especially a tube like a 12AX7, it should have an easily tested
emission and plate current rating.  I would not mess with the shunt or
other internal workings of the tester.  

Are you sure the tester is in good condition?  

I had a couple of known good tubes I used just to be sure, tubes I had
already verified at a radio shop with tester (yes, I know how rare
those are now).  

Do you have the tester manual?  Perhaps there are clues there.

But it shouldn't be difficult to build a tester just for 12AX7, there's
a published spec for it's performance at different plate voltages and
different grid bias values to give a specific measurable plate current.
 I believe that should tell you what you want to know.


>J-Pierre
>
>
>
>_______________________________________________
>Synth-diy mailing list
>Synth-diy at dropmix.xs4all.nl
>http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>

-- ScottG

-------------------------------------------------------------

-- Scott Gravenhorst
-- GateMan-III - FPGA Based Monophonic MIDI Synthesizer with SVF
-- PolyDaWG/8 - FPGA Based 8 Voice Polyphonic MIDI Synthesizer
-- FatMan: home1.gte.net/res0658s/fatman/
-- NonFatMan: home1.gte.net/res0658s/electronics/
-- When the going gets tough, the tough use the command line.