Newsgroups: alt.guitar.amps
Subject: SOVTEK TEST REPORT
Date: Thu, 21 Sep 2000 01:41:49 -0600
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Xref: geraldo.cc.utexas.edu alt.guitar.amps:276885
Lord Valve Speaketh:
Just got samples of two new tubes from Sovtek...
the 12AT7EH and the 5751. Both of them look
just like the 12AX7LPS/EH inside and out, so
obviously the plates are made with the same
tooling and the bottles are the same. I only
got two of the 12AT7EH and four of the 5751s.
I did preliminary testing on them with my Hickok
752, and came up with some curious numbers; as
a control, I pulled two each of the NOS JAN 12AT7WC
by Philips and GE (at random from my untested stock)
and four NOS JAN 5751 by Philips. It seems that
the transconductance ranges on the Sovteks don't
match up all that well with the original American
tubes. Here are the numbers for the 5751s, in
umohs, per section:
SOV #1 - 1840/1900
SOV #2 - 2020/1880
SOV #3 - 1800/1860
SOV #4 - 1920/1840
NOS #1 - 1500/1460
NOS #2 - 1400/1420
NOS #3 - 1280/1280
NOS #4 - 1380/1340
It would seem that the Sovteks have higher
transconductance numbers than the NOS parts...
and that the NOS tubes are more tightly matched
>from section to section. (Close section matching
is a specification for this particular tube.) In fact,
the lowest TC measurement for the Sovteks is higher
than the highest TC figure for the NOS by a considerable
margin. Of course, this doesn't matter; you just need
to bear in mind that the Sovteks will have more gain
than the NOS will. I tried all of the Sovteks out in
my test rig...two of 'em had noisy sections. (One
case of Niagara Falls Disease, and one Harley Davidson
Infestation.) The two good ones were fairly free from
microphonics, and very quiet. Many of you who have
used NOS 5751s have found out that they tend to hum
in some circuits, especially high-gain front ends;
the Sovteks were remarkably free of this. Since
they're "hotter" than "real" 5751s, they will produce
more gain than the NOS; depending on what you're
looking for, this may or may not be a good thing.
When I get a few in the field, I'll have more to
say on the subject. So far, the reject rate (50%!!)
is not encouraging. Of course, the sample was very
small, so it could have been plain bad luck. We'll
see.
The 12AT7s tested as follows:
SOV #1 - 3340/3340
SOV #2 - 2840/3440 (See below)
NOS GE #1 - 3800/4400
NOS GE #2 - 3800/3700
NOS PH #1 - 4080/4300
NOS PH #2 - 4000/3700
(NOTE: SOV #2 initially produced a reading of 3200 umohs
in the first section, but this reading dropped off to
2840 within ten seconds. The tube displayed this behavior
every time it was tested. I usually reject tubes which
test like this during my screening process, but since I
only had two to work with, I went ahead and did audio
testing on both of them.)
In contrast to the 5751s (above) the Sovtek 12AT7EHs
produced lower readings than their American NOS
counterparts. When installed in sockets which called
for 12AX7s (front end gain stage) they were *really*
microphonic; if these are typical, they will be
useless in that application, and probably fairly iffy
for reverb drivers, too. When used in the PI position
in one of my Fender test rigs, they passed with flying
colors, even the one which displayed bogus readings
on the tube tester. Since these cost considerably
more than the (still available) NOS JAN 12AT7WCs, I
can see no reason at this time for me to stock any.
Of course, when the NOS finally run out, it's nice to
know that there is another choice on the table.
While I'm at it, I got a couple of pre-production
samples of the soon-to-be-available Sovtek GZ-34/5AR4.
These have straight-sided bottles, with no outward flare.
The ones I got have only four pins on the bottom; I
suggested to Mike Matthews that he add the fifth pin
than NOS 5AR4s have so folks won't cry about it. He
allowed as how that might be a smart notion, and said
he'd see what he could do. They tested just fine on my
equipment, and sounded fine in several amps...HOWEVER...
when used upside down, they rattle a bit. It seems that
the filament wires are vibrating inside the cathode
sleeves. This is nearly inaudible when the tubes are
right-side-up, but fairly annoying when they are inverted.
Since 90% of the amps that use this tube use them upside-
down, I suggested to Mike that he might like to pass this
info on to the boys back in Mother Russia. He allowed as
how that might be a smart notion too. Of course, this
wouldn't be a problem for amps used on stages with live
rock bands, but that little jingle could worry the hell
out of the bedroom jammers, and might put the kabosh on
a recording session to boot. Time will tell.
So Sayeth the Lord.
Lord Valve
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