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: <[EMAIL PROTECTED]> To: "Discussion of AM Radio" <[email protected]> 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 !! > > > ______________________________________________________________ > AMRadio mailing list > Home: http://mailman.qth.net/mailman/listinfo/amradio > Help: http://mailman.qth.net/mmfaq.html > Post: mailto:[email protected] > AMfone Website: http://www.amfone.net > AM List Admin: Brian Sherrod/w5ami >
