[Milsurplus] those T2X3 tubes (was VRC-19)

[D C Macdonald]

I am assuming that loctal tubes such as 7C5, etc.
are the 5-7 volt filament tubes to which you refer.

73  ---  Mac, K2GKK/5



----Original Message Follows----
From: ejones at hiwaay.net
To: milsurplus at mailman.qth.net
Subject: [Milsurplus] those T2X3 tubes (was VRC-19)
Date: Thu, 24 Jun 2004 22:27:26 -0500 (CDT)

de N4TGC Eric

After foaming at the mouth a bit, I decided to examine why that "1.4 volt
filament" mistake keeps getting repeated, when the filament of the T2/T2X3
sub-miniature tubes are not now, have never been, and never will be,
anything but 1.25v.  I thought of at least two reasons (besides the fact
that people will repeat all sorts of dumb stuff without bothering to check
if it's true ...):  One, 1-1/4v does resemble 1.4v; two; there is actually
a supply line in the TRC-28, etc. rigs that is 1.4v - altho the fact
there's TWO lines clearly marked 1.25v (one of them regulated) seems to
have escaped much notice ... worse, the 'point' of 1.4 on my TRC-28
schematic is so dim, the value looks like 14v!

Here's the seeming mystery: since the filament of these "wire-pin" tubes is
actually 1.25v, why IS there a 1.4v supply in the VRC-19, and even 1.5
volts in the PRC-6?  I'll address the PRC-6, since I'm more familiar with
them: the first seven tubes in the receiver string all have a choke in
series with the filament.  Looking at the schematic, it's not obvious that
the value given for them is not micro- or milli-henries as you'd expect,
but is DC ohms!  So these tubes all have a voltage-dropping choke - but why
doesn't the tx tubes? Because the 3B4 PA tube hogs the filament voltage.
Similarly, the 5672 audio out filament is ballasted by the carbon
microphone element.

In checking these sub-min tubes, it's best to NOT depend on the tube
checker to supply the correct level - even if it has a 1.25v setting (which
mine does not).  Instead, I plug a DMM into the appropriate pins of the
loctal socket BEFORE inserting the tube to be checked, and preset the
filament volts using the line reostat.  (It shouldn't be necessary to
compensate for tube loading)  It's interesting to watch tube performance
soar up and down as the filament supply is tweaked ... which is what
the"LIFE" test does, BTW.  Filament voltage can be so determinant that
there are special tubes designed just for the varying supply in automotive
applications - their filament will be given as "5 to 7volts" rather than a
fixed 6.3

So keep 'em glowin' - but not TOO bright!	e


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