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In Reply to: RE: Tube strength and readings posted by Maddnotez@gmail.com on September 07, 2014 at 16:59:56
Info like that on ebay is all over the place from a technical stand-point.The 12AX7 has a center design target of 1600gm. Some NOS 12AX7s measure 2000gm. Thus, a real 1250gm measurement may be a highly used tube. An exception are some new manufacture tubes such as the Sovtek 12AX7LPS that is normally 1250gm.
A life test that reduces the vintage 12AX7 filament from 6.3 volts to 5 volts is a better indicator of tube health. At 5 volts, a 12AX7 healthy tube will not drop test readings at all where as tubes with little life left will drop at least 5-10% or more. However, no one wants you to know about that test on used tubes on ebay.
So, buyer beware.
Edit- some NOS tubes such as 6AQ8 and some 6922 drop like a rock at 5 volts filament. Thus, I am referring to 12AX7s above. It takes time testing tubes to get a feel of this life test.
Edits: 09/07/14 09/07/14Follow Ups:
The "spec" for Gm varies - 1250 is the spec with 100 volts on the plate, 1600 with 250 volts on the plate.
And as I just posted a few days ago there is a lot of variation in Gm due to typical production variation.
???
The expected reading for 12AX7 on all three of my Hickok testers (533A, 600, and 600A) is 1250 per section.
1250 micromhos per section indicates a "typical new 12AX7" tube result; with a typical Hickok tester circuit...
which means that tubes that test around 800-1000 sound noticibly worse in my gear than NOS tubes that test 1250 per section.
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