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

[dragon.servicing]

Hi,
well I would be most interested in a design that would allow the common 
pre amp valves ( tubes ) to be tested.
just a couple of comments, mainly from various discussions I have had 
previously with people not on this list.
if you are checking gain of the valves then the heater voltage needs to 
be regulated, as changes in the heater voltage can change the emission 
characteristics.
I agree about the comments  about being able to listen to the audio 
signal being fed through the valve as noise/distortion and microphonic 
valves (tubes) might not be detected otherwise. another point is that 
you need to be able to check both halves of the 12ax7 valves 
independently and also check for cross coupling between the two sections.
( I had problems repairing a Peavey amp when I used some EI vales ( 
tubes ) as replacements. you got a quiet distortion sound when the 
volume control was turned down to zero. it was down to the new but 
faulty Ei valves coupling the signal between the two sections.
I think there is much more to testing valves ( tubes) than first meets 
the eye
I think you can buy a computer controlled tube tester if you have enough 
spare money....
regards Peter



> Hi,
> 
>> The problem with this tester is that
>> for the 12AX7 the manual says:
>> GOOD if it reads 22 or more on
>> a 120 scale!! Most tested 12AX7's go
>> from 15--23..! Not quite acurate.
>