96.234.46.168
In Reply to: RE: Agree... posted by Jim McShane on May 30, 2009 at 12:34:23
JIM McS -
A bit more data re: your post -
From a database of the first 400 US NOS 6SN7 tubes, the following
added info -
A - 19 sections tested below 2200 gm, with an average plate current
for the lower testing section of 5.33 mA (4.4 mA min @ 2175;
6.1 mA max (Gm not noted)
B - 10 sections tested > 3000 Gm, with an average Ip of 8.3 mA,
with one section testing at 4800 Gm / 9.8 Ma Ip
Some of the test variance would probably fall within the
2% reject rate factories allowed distributors in the 1960's.
Among a group of older Japanese (13) and USSR (5) tubes (guess
late 70's to mid 80's production), test results were about the
same as the USA tubes.
However, the section to section Gm and Ip were closer. Japanese
tubes tested mostly 2400-2600 at about 6.6 mA. USSR ran closer to
2750 at about 6.8 mA
How current production tubes would compare I leave to your
experience, as I have none to sample.
The USM-118 (cardmatic) has regulated supplies and a wider range
of test voltages, as does the Triplett 3444 series, the New London
901A and the over-large RCA Lab unit, to name a few.
A BOGEY (design center) tube should test at 75% on the USM-118
(does NOT mean that the tube is at 75 % of the NEW value - I know
know this, but I add the comment for others who may read this
post).
Test data is more reliable using these units - I just happened to
use the Hickok unit for this test series, as Ip is easier to sense.
When matching section to section, a TEK 570 and a computer are
very handy. Alternately, a stairstep generator and a scope or
"sample and hold voltmeter" system would work.
Alternately, a properly configured DIFF AMP circuit with the
proper instrumentation should work well to sense balance and
perhaps tube noise as well.
TUBOS
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