[AMRadio] 12AX7 Modulators

Mike Dorworth,K4XM k4xm at arczip.com
Sun Oct 2 16:32:38 EDT 2005

>From Radio Amateurs Mobile Handbook, 1st edition edited by Bill Orr, W6SAI.
Page 85. The 12AX7 makes an excelent class B modulator for low powered
mobile transmitters. Operting at 300 volts it requires no bias with a
resting plate current of 10 ma. This is lower than ANY tube capable of
producing the same audio power. 6N7, 35 ma, 1635, 11 ma, push pull 6AQ5
draws minimum of 70 ma when the stage is OVER biased. The 12AX7 will deliver
7 watts of VOICE audio with negligible distortion at 300 volts . The plate
current will rise from 10 ma resting to about 35 ma on voice peaks. Two
12AX7 tubes may be used in push-pull parallel to obtain 15 watts of audio
power. The CORRECT load impedance for a single 12AX7 is in the vicinity of
14000 ohms. Two tubes in push-pull parallel require a load impedance of 7000
ohms. On page 92 of the same manual is a 12 watt mobile, 3 band transmitter
running 12 watts input which delivers 8 watts of FULLY modulated carrier
which was modulated by a single 12AX7 at 300 volts.

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