[Milsurplus] 'splain me this 6L6 transmitter circuit

[DeWitt Clay]

I agree that the second stage is a doubler on the high band and the cathode circuit provides some regeneration to boost the output. You could try shorting out the cathode on the high band and see what it does to the output signal level.
N4QNX


________________________________
From: Hue Miller <kargo_cult at msn.com>
To: tetrode at googlegroups.com; milsurplus at mailman.qth.net 
Sent: Friday, May 17, 2013 12:10 PM
Subject: [Milsurplus] 'splain me this 6L6 transmitter circuit


Maybe I have asked this question already?
Sorry, I tend to forget, but I don't recall any definite answer.

Imagine a basic 6J5 + 6L6 MOPA. Xtal controlled Pierce osc.
untuned plate, driving 6L6.  B+ under load, 340 volt. The
frequency range is 2 bands, 4 - 7.5 and 7.5 - 14 Mc/s. Okay?
However, in the cathode of the 6L6, is a "transmitter bandswitch".  All it does is, one band, cathode goes straight
to keying relay. I think this is the low band. Schematic does
not say what band the switch positions are, but I am guessing
low band, because the rceceiver section is set, in the drawing,
to the matching low band.
So, in the high band, the 6L6 cathode is switched to go thru
a parallel 2.5 mH choke and 240 pF capacitor.
Okay, consider the choke infinite for RF.
So why is the 240 pF capacitor in the RF path?  Is this possibly
to provide RF degeneration?
NO word in the manual explaining this.
BTW, rig is US Navy "MBM" radio, apparently built by Jefferson-Travis, to be supplied to guerilla forces in occupied
Philippines 1945. It could either be powered by handcrank, or
by separate power supply from a built in 6 volt wetcell battery.
According to the book "You're No Good To Me Dead", at least one using team found the radio a total loser. I am
guessing that this was from the wet battery not for whatever reason not having a full charge, and maybe the
antenna tuning procedure being not clearly marked.
And, as the book describes, you would not want to walk
thru downtown Japanese occupied Manila lugging two
huge unique looking suitcases, one for the radio, one for
the power supply. In fact on one mission, the team sent
into Manila was spooked enough that they just threw
the suitcases into the bay. They said they felt they were
carrying a neon sign that flashed, "I am a spy".
Your take on this circuit?

The receiver is also a little strange. Right out of the mixer
is a single tuned circuit. Why not a standard IF can here,
or did they NOT want good selectivity?
The radio looks odd, too, in a BC- 312 random controls
location kind of way. I have long felt that Jeff-Trav always
tended to produce radios that looked kind of odd.
tnx  -  Hue Miller ______________________________________________________________
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