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.


..... Phil