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

[harrybissell at wowway.com]

Mesa-Boogie used to show a picture of a tech hitting their amps
with a rubber mallet to induce (if present) microphonics or loose
connections.  I've tried it and it works very well.  Just remember to wear
earplugs becuse ~some~ loose connections can generate hellacious LOUD
artifacts.

I think the blows are typical of what an amp would encounter in transport
anyway. It will show up microphonic tubes nicely.

H^) harry




On Thu, 17 Jan 2008 13:23:10 +0000, dragon.servicing wrote
> 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.
> >
> 
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Harry Bissell & Nora Abdullah 4eva