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Hello, I have been a tube lover for a long time, but just recently bought a tv-7d/u tester. This tester has a reported minimum for each tube. I believe it is set at 65%. Does this mean:minimum/65% = test reading for a new tube.
I am trying to understand how tubes test relative to a new tube.
Thanks a lot for any help!!
Follow Ups:
When you mean set you are referring to the "english" good/bad delineation on a TV 7. The meter is pretty much self explanatory despite the fact that numerical values are provided. I don't think there is any microhmos reading available to give you a better idea of how well the tube is functioning. The scale I recall is 65 to hit the green zone anything less is supposedly marginal. When tubes were plentiful I am sure the intent was to get you to replace it.
I have used tubes that measure low. They sound good in some cases which throws the whole meaning of testing to decide on sonics into doubt. Ask Jeff Behr we have both had similar experiences using completely different testers. (they must match if using 2 channel dual mono signal pathways)
That said, many that test bad are truly bad- really dead or fried. The 100 on the TV DU 7 is not the new tube value that should be in the setting book. I have watched a dealer use his carefully maintained and frequently expert calibrated DU 7 to test batches of new tubes of all types. They are either duds or measure Green/Good from 70-90. Its really a yes /no deal.
If you want precision testing you need to climb into the stratospherically priced computer tracer devices.
I use a run of the mill 1961 Hickock 800 which needs to be calibrated at the moment but prefer the MTC microhmos values it reads. With those testers the nominal new or 100% value is given on the roll chart. Your setting booklet should give you the same type information but I have to see the next time I visit.
You might just love this one: the ARC dealer in town here has a service department with no tube tester!
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.
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.best regards,
Brian
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.
Russ
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