[AMRadio] Minature Mobile Transmitter

Mike Dorworth,K4XM k4xm at arczip.com
Tue Oct 4 20:41:00 EDT 2005

OK, from the top. There are several dozen examples of mobile rigs in QST, CQ
magazine and both the ARRL Mobile Book and both CQ Mobile Handbooks all of
which use a 12AX& in class B, zero bias. So they must work OK. I have run
mobile AM with the 12AX7 modulating a 5763 so there can be no doubt it
works. In the 14 th Edition of the Radio Handbook edited by Bill Orr on page
452 is a Modulator using a 12AX7 that modulates a 2E26 at 12 watts input.
Using a plate to plate load of 18000 Ohms and 300 volts the 12AX7 delivered
almost 7 watts of SINE WAVE audio. As stated earlier, the inexpensive power
supplies of those days furnished 300 volts at 100 ma. A Class B audio system
could produce 6 to 8 watts of audio to run a 12- 15 watt input mobile rig.
There was then, and it remains true today, that no known tube other than the
12AX7 had such a  a low resting current  of 6 - 10 ma. With a peak of 40 ma
the modulator will use about 50 ma total to run  mudulator including the
pre-amp, usually a 12AU7, this leaves 50 ma at the 300 volts to run a
oscillator, buffer if needed and drive a final and furnish it's screen grid.
This is another 50 ma. So then the 100 ma is used up. The ARRL Handbook for
1960 clearly shows this to be the correct tube for the job. Without looking
it up, I seem to remember about 70 ma idle for a pair of 6AQ5s which pump
out FIVE watts?  This all started with a question if the ARRL Handbook was
misprinted. Several dozen successful rigs printed in standard Mobile Manuals
, QST, CQ etc makes it a correct choice under the required circumstances..
As a extra bit of information, all of the old farm radios for the AM band of
the 20s,  30s and 40s all used TRIODES in Class B to conserve battery power
in the audio driver and output circuits. Ditto all of the original
transistor radios of the 50 and 60,s used class B audio to keep down the
current drain.

----- Original Message ----- 
From: <bcarling at cfl.rr.com>
To: "Discussion of AM Radio" <amradio at mailman.qth.net>
Sent: Tuesday, October 04, 2005 5:21 AM
Subject: Re: [AMRadio] Minature Mobile Transmitter

> On 3 Oct 2005 at 0:39, Bob Bruhns wrote:
> > That's amazing.  Can't argue with success.  And
> > the 7.5W-300V-16K p-p adds up.  The published
> > 12AX7 plate dissipation spec must be overly
> > conservative, and the cathode must be pretty
> > efficient.
> >
> > I always thought of the12AX7 as an audio preamp
> > tube.
> It is... There HAS to be a good reason why manufacturers
> used things like 6AQ5 and 6BM8 for 4 watt audio output
> stages instead of a 6C4 or 12AX7 !!
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