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In Reply to: Re: Stupid question about tube testers posted by Mechans on March 15, 2007 at 16:25:52:
My TV7 has no "english" adjustment nor does it have a "green-yellow-red" zone on the meter. On mine you set it by the book and compare the reading on the meter to the reject value in the book. I don't know at what % of "new" the reject value is but on my Hickoks inc the TV7 (TV7 is a Hickok circuit) a new tube usually but not always tests about twice the reject value. Also, the scale turns out to be more than just arbitrary numbers. Jim McShane had a conversion table on his web site where the meter value can be translated to umhos. These tables also exist on some of the tube tester sites that can be found with your favorite search engine.
Follow Ups:
To convert TV-7 meter readings to umhos, multiply the meter reading by:25 if using the B scale
50 if using the C scale
125 if using the D scaleSo a 12AX7 that tests at 40 on the B scale has a Gm of 40 x 25, or 1000 umhos. A 6L6 that tests at 40 on the D scale has a GM of 5000 umho.
When comparing Gm readings on a tester to the specs in a tube manual keep in mind that the specs are listed under a specific set of conditions. The tester usually does not test the tubes under the tube manual conditions, so the readings won't agree. Plus tubes have a wide GM variation from tube to tube of the same type due to production tolerances/variances.
Thanks a lot to everyone. This is exactly the data I was looking for.
In an ideal word a tube tester would allow you to pick plate and grid voltage and then report the difference in current when the grid was changed by a certain amount. Plate and grid voltages would be well filtered DC and the supply would be regulated or stiff enough not to sag. Sadly such is not the case and it takes a degree of knowledge, skill, and experience to guesstimate the condition of a tube based only on a claimed tube tester result. With many testers the number has little meaning and is only a "realitive" number.Jim is an expert at testing tubes and a very helpful guy. If you didn't get the answer you need, you asked the wrong question. So don't be afraid to ask again but understand your tester might not do just what you want.
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