On 2017/11/10 1:32 AM, Phil Allison wrote:
John Robertson wrote:
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Shorts don't always show up until you load the tube (or tap it). You can
also get leakage and other odd things (microphonics, etc.) that the
better tube testers would show. Yes you can make some test gear in your
shop, but how do you get a standard unless you have several NOS tubes to
compare to? We do repair tube gear and so have to have a couple of tube
testers in our shop - mutual conductance is our preferred machine.
** I still do a lot of work with valves (tubes if you are a Yank) but have never thought buying a "Tube Tester" worthwhile. They simply do not carry out tests needed for audio service or production.
The best way to test a tube is to into plug it into a known working piece of gear and put it through it paces. Another way is to replace a suspect tube with a known good one and see if that changes things.
Recently, inspired by the schem of an old AVO163 "Valve Characteristic Meter" I designed and built my own tube tester that would perform all the needed tests at least as far as common 8 and 9 pin output tubes were concerned.
Using a very simple circuit plus basic bench equipment, it puts a power tube under realistic operating conditions and finds if it working normally.. Also matching the performance and idle bias settings for sets of tubes to be used in parallel is easily accommodated. The beauty of the method used is the tube operates under low duty cycle conditions so plate and screen dissipation limits are not approached or exceeded.
I find most use for the tester is with newly purchased tubes, to see if they are up to spec or not.
MY colleague, Rod Elliot, published the design on his web sight as Project 165 in February last year.
http://sound.whsites.net/project165.htm
The many warnings are justified, valve testers can destroy valves.
I think I want a AVO VCM163 Valve Characteristic Meter!
** That is a very ambiguous thing to post.
My design does a whole HOST of things the AVO does NOT - since it is specifically an output tube tester with used defined parameters that one to mimic or exceed actual operation.
The only similarity lies in the use of unrectified AC for screen and plate voltages.
Yes, Phil, it was ambiguous, however we use about forty or more
different types of tubes.
** Not power tubes you don't.
True - mostly 6L6, 7868, 6V6, 45, 6973, um...
And your process above, while it would be
great on high end equipment, is deeper than we would ever need to go for
our jukebox customers.
** My customers all own guitar amps, some of them ancient.
So the same need you seem to have.
These aren't Macintosh amps - and my staff just
can't take the time to learn a whole new set of testing procedures just
to get the best tube for the job.
** Not my quest either.
I just need to sort duds out of batches of new or used tubes when I do not have the target amp on hand.
I need something quicker and easier to use in our shop.
** Takes the same time as any tube tester, 60 seconds for the filament to heat and another 60 for the various checks.
Hence the MC
style tube tester which does a pretty good test for most tubes. And when
the results are inconclusive we have swapped tubes from our stock to try
and find the best sounding ones.
** Best sounding?? You on drugs?
No, mostly dealing with microphonics which the tube tester can miss.
6J5s are the biggest culpret, but I've even had them on 12AX7s...
The AVO looked like a step up from our current 1950s tester...
** If you can find one, it will likely cost you an arm and a leg.
And do nothing very useful.
Not likely to pick one up unless it was a very good deal. But it does
look impressive!
..... Phil
John