TR-4 transceivers and other tube transmitters that employ controlled
carrier AM are susceptible to excessive microphone drive. Referencing Figure 1, with no
modulation the left triode of the 13DE7 fully conducts and the anode
voltage is very low: no carrier to the antenna. When the modulation
level is increased the audio signal is rectified by the grid of the
left triode, this gives negative bias to the same and the average anode
voltage increases. The audio signal is superimposed to the anode
voltage with full amplitude. This is controlled carrier.
Figure 1 -
Existing TR-4 AM Modulator
The problem of the circuit is that the
“average” anode voltage should stop increasing,
with increased modulation level, at about 1⁄2 of full voltage
(full voltage is 250 V in the TR4). This does not happen in the TR-4
circuit and it is very easy to go from low modulation and low power to
very severe “full distortion” and asymmetric
modulation, eventually to a situation in which the output voltage is
full drive with nearly no audio.
The design in Figure 2
corrects this behavior. When the average voltage to the 6JB6 screen
grids go above 1⁄2 of full voltage (250V) the
“average” voltage is limited by feedback, like we
have in a stabilized voltage source. The audio signal is not limited at
all and the overdriven modulator behaves now like a overdriven full
carrier screen modulator. In practice it works much much better than
Modified TR-4 AM Modulator
IN3IEX - Giorgio Fontana, Trento, Italy (JN56NB).
is a PDF version, suitable for printing.