[Hallicrafters] S-38

[John Schmitz]

Jeramy,
Good observation, I never caught this. I will say that I would not
necessarily trust the Photofact for accuracy. I have found many errors in
Photofacts in the past. I have the original S-38 manual and nowhere does it
say that V1 is a GT. And in fact, the chassis picture in the original manual
shows a metal tube, not a glass tube. I looked at the specs on the both the
tubes and sure enough, the glass tube has G5 internally connected to the
cathode and pin 1 is not even used (which I thought was rather odd as this
is a heptode and you would think they would make use of all the pins).
Anyway, It is now a curiosity with me as to how this circuit would work with
the GT. The oscillator injection is on the cathode. This would mean that
with the GT the injection would take place at both the cathode and G5.
Whereas in the metal tube the injection would only take place on the cathode
and G5 just acts as a suppressor, normally grounded. I know that many
pentode amplifier circuits tie the suppressor to the cathode but with the
oscillator injection in this case it seems it might have some type or effect
on operation. Hmmmm?

Anyway, I would go with the metal tube and G5 grounded as a normal
suppressor. By the way, I could scan and send you the original manual, it's
not that big. Let me know.

John


-----Original Message-----
From: hallicrafters-bounces at mailman.qth.net
[mailto:hallicrafters-bounces at mailman.qth.net]On Behalf Of Jeramy A. Ross
Sent: Tuesday, November 21, 2006 5:21 PM
To: hallicrafters at mailman.qth.net
Subject: [Hallicrafters] S-38

Hello all,

    I'm working on a Hallicrafters S-38 (original version) and am
restoring it back to as close to original as I can, but I've seem to run
into a problem.  Maybe it's just my lack of knowledge.
    On the schematic, documentation, sam's photofacts, etc. V1 is listed
as 12SA7-GT.  In mine, there's a 12SA7 metal tube.  In my wanting it to
be original, I thought I would replace the metal tube with the GT
version.  Simple swap I thought.  The only difference would be the
difference between the metal or glass envelope I thought.  However, like
the good little novice glowbugger that I am, I whipped out my 1959 RCA
Receiving Tube Manual and checked to be sure (feeling a bit foolish in
the process as I was sure there wasn't any difference).   Low and
behold... there were two, separate pinouts for the 12SA7 and 12SA7-GT!!
 I then checked online and online data showed the same differences...
The 12SA7 has G5 connected to pin 1 and the Cathode connected to pin 6.
The 12SA7-GT has G5 and the Cathode connected to pin 6 with nothing
connected to pin 1.
    Back to the S-38.. Pin 1 (G5) of the 12SA7's socket is connected to
chassis ground.  The cathode is connected a lug of the oscillator coil
(lug '27' per sam's).  The schematic shows this as well.  G5 and the
Cathode of the 12SA7 go separate places.. not connected together.. thus
not the 12SA7-GT.  So it looks as if it were wired for the metal 12SA7
in my novice eyes.
    What would the operational difference be between these two tubes in
this situation?  Was the glass version originally used in the S-38
(original version), and a later mutant of the original S-38 switched to
the 12SA7 metal tube?  Any knowledge, advice, wisdom?  I'd appreciate
any and all you guys could give!!

Thanks in advance,
Jeramy / W5XTL
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