Amplitude Modulators


1. The following circuit is an example of a low level amplitude modulator. It may be used in a multimode transmitter to avoid the need for unbalancing the balanced modulator and is not critical in its adjustment or operation.


2. The following circuit provides good amplitude modulation to around 90% depth before significant distortion becomes apparent. The ECH81 is particularly useful as the triode and heptode sections are not connected internally although they do share a common cathode which should be decoupled to RF. Decoupling the cathode and G2/G4 to audio did not appear to make a significant difference to the modulation level.

The preset potentiometer should be adjusted for the best modulation envelope shape and this typically occurs when the unmodulated output level is around 40-50% of the level without any bias applied. All supplies should be stable and well decoupled. My application uses the triode section as a unity gain inverter to provide push-pull audio to the balanced modulator - in this mode the heptode section is not used so there is no interaction. Other mixer valves like the 6BE6 should work in this modulator application but do not have the triode section and to date have not been tried.

The 3.395Mhz tuned circuit connected to pin 6 consists of 30 turns 9/47 strand Litz wire on a one quarter inch diameter former with a dust iron tuning core and a tuning capacitor of 270pF. Litz wire is not mandatory but does give a higher Q than single strand wire at LF and low HF. The choice between high and low impedance outputs will be determined by each users individual requirements.

Other heptodes should function in a similar manner. The author noted considerable envelope distortion when the AF and RF inputs were swapped over and this was not pursued further. A similar circuit was tried using control grid modulation with an E180F pentode but the distortion on the modulation troughs was worse than with the heptode.