[Milsurplus] [Glowbugs] 'splain me this 6L6 transmitter circuit

[Richard Brunner]

This one is easy...  With the cathode grounded it is a tuned plate 
oscillator which depends on grid-plate capacitance for feedback, and will 
operate only on the fundamental frequency.  With the cathode above RF ground 
it becomes a Colpitts oscillator which depends on the 240 pf to ground and 
the cathode-grid capacitance for feedback, and the plate can be tuned to 
harmonics for output.  This is usually a multi-grid tube.

Richard, AA1P


----- Original Message ----- 
From: "Hue Miller" <kargo_cult at msn.com>
To: <tetrode at googlegroups.com>; <milsurplus at mailman.qth.net>
Sent: Friday, May 17, 2013 3:10 PM
Subject: [Glowbugs] '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 receiver 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
>
>