Quote:
Originally Posted by Alejandro Lieber[_3_]
In figu
http://1fcr.com.ar/classB.jpg
you can see a very interesting diagram of the audio modulator of an old AM
Gonset transmiter.
A 12AT7 as voltage amplifier. one 12AU7 with both triodes in parallel as
power amplifier and two 6DQ6A grounded cathode, grounded control grid with
audio input to the screen grids working push pull as pure class B.
R211 and R212 are only used for measurement purpose.
Why would they choose to use the screen instead of the control grid as input
? . Any suggestion ?
--
Alejandro Lieber LU1FCR
Rosario Argentina
Real-Time F2-Layer Critical Frequency Map:
http://1fcr.com.ar
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The short answer is that the standing current of the Class B power amp is very low, an important consideration in this kind of application. Fidelity is not important. Voice communication would normally be something like 30-3000 Hz with 12 db/octave slopes on each side.
Commonly in those tube modulators you would see this hookup using 807s. Often both G1 & G2 are driven.
A common triode modulator for mobile applications was the 6N7 & its relatives of which there are several. A 12AX7 makes a good Class B modulator for a low power rig.
Sometime a carbon mike would be used on the front end amplifier driving straight into the cathode. That way the mike got the necessary current for its operation.
Cheers, John