"Ozzy 2005" wrote in message
...
Description:

Pair of NIB GE JG-5751 triple mica black plate military tubes.Test
1250/1325 and 1450/1450 where new/nominal is 1250 gm and minimum is 812 on
calib. Hickok 533A tester.

Question:

5751 being mil spec do you have an idea why they would test so
differently?


They seem to test Damn close for transconductance to me.


Answer:

Yeah I do: Mil-spec doesn't imply matched sections, in fact it doesn't
really imply anything. The 5751 is less noisy and microphonic than a AX7.

Exept that a 5751 is not a 12AX7, the 5751 has less gain by some 30%


You should really understand that you couldn't hear a difference betwen
tubes that were say 950gm and 1500gm.

This is a True statement



Just the result of tube dealers trying to one up each other etc..

mostly a bunch of crap.

Theoretically you want tubes relatively close in gm so they get used at
the same rate and last as long as possible, but it's not reality. They
NEVER wear evenly, so again it's just BS.

Tubes do not wear out in a linear fashion. A well used tube will test near
nominal for most of its life. When the emmisssive coating is depleted from
the cathode the tube in question will then rapidly drop in transconductance.


Perhaps you can enlighten some others in your situation of these
realities. BTW: those tubes test about as good as they get 1250 is high,
sometimes new 5751's only test 1100, so those should last as long as
anything if you follow, (or have the probability of doing so.


I have NOS 12AX7 tubes that tested significantly below nominal right out of
the box. The same tubes are still in use, 5000 to 7000 hrs later, testing
right where they tested out of the box
Transconductance is a very poor indicator of a vacuum tubes "newness".

Most tube testers transconductance (or emission) test cannot be used to
judge the "match" of pairs or sections of tubes (yes, there are expetions
but I am willing to bet you or the seller don't own such a tester) They
don't test a tube even remotely close to normal operating conditions. Only
making a mesurement of current while the tube is in actual "in circuit"
use, passing an actual real world signal, will match up pairs reliablely.

The REALLY IMPORTANT tests a tube tester performs are the shorts and
inter-element leakage tests.
A tube with a short can put an end to your equipment in an instant.


Reply:

Your detailed response debunking some of the myths surrounding tubes is
very much appreciated. I will keep an eye on your auction and place a bid
if I can afford to.

Thanks

A tube tester is a quick-N-dirty way of checking a tube for basic functions.
Most are not accurate current matching devices or accurate gauges of how NOS
a particular tube is. Service testers like most Hikkoks, Sencores, B&Ks and
so forth are tools to weed out bad, shorted or worn out tubes in a service
call situation.

If you want to match tubes, buy and learn to use a curve tracer or build
your own.

--
Alan Gallacher
Born to Tinker!