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In Reply to: RE: Agree... posted by Jim McShane on May 28, 2009 at 16:18:09
Having just tested about 500 known NOS 6SN7 series using a HICKOK 1575B (Mil version of the 539B/C), some interesting results came to light -
A - Transconductance generally ranged from about 2200 uMhos to
3000 uMhos, with the center about 2500-2600 uMhos-
data minimum being 1700 uMhos per roll chart.
B - Among the 6SN7WGT/WGTA types, the older tubes ( 1950-1965)
were consistently higher than the late 1970-1980's MIL stuff.
C - Test data quite dependent on the AC voltmeter setting, as
line voltage changes caused a shift in Gm and Ip readings.
This is probably linked to changes in filament voltage
and plate voltage during test (triodes sensitive to plate V)
D - Even when the tube sections Gm matched, plate current varied
by as much as 1 ma(generally closer to 0.4 Ma or less). Typical
plate currents were in the range of 5+ mA to about 8 mA at
the indicated 4.5 V grid bias point.
The point being that just a Gm match is close, but not perfect.
E - Jim McS is correct in that a tester can only do so much.
A few obviously used tubes were also tested, with many at
the low end but with some testing at the high end even with
significant visual indications of use present.
TUBOS
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