26-10-2016, 11:31 AM
Almost any AM Local oscillator circuit can use Octode/Heptode OR Hexode + triode, as long as similar HT, LT, etc as long as two of the sections of a Heptode/Octode/Nonode Mixer/Osc are wired in a triode type oscillator using cathode and first grid as feedback.
Then if you replace IFT mixer out anode load with a tuned MW loop and the RF in with AF in, you have an AM modulator.
A two valve version using as the mixer almost any Pentode (Osc Anode to G2, audio to G1) , Hexode, Heptode, Octode etc and any valve as the triode Osc (tie G2 & Anode) will often work in the same circuit.
Above 30MHz it's better to use a triode or triodised "self mixer oscillator" with an AF driver and RF out buffer amp. (dual EF80, ECC85 and maybe an EF80 or EF86 driver).
The ECH81 is readily available, as is the NOS Russian equivalent. It will work down to 14V HT (use a car radio circuit) with positive bias. I'd probably use two so as to have an AF preamp and RF buffer/Driver. A single 12V or 24V PSU is viable.
Then if you replace IFT mixer out anode load with a tuned MW loop and the RF in with AF in, you have an AM modulator.
A two valve version using as the mixer almost any Pentode (Osc Anode to G2, audio to G1) , Hexode, Heptode, Octode etc and any valve as the triode Osc (tie G2 & Anode) will often work in the same circuit.
Above 30MHz it's better to use a triode or triodised "self mixer oscillator" with an AF driver and RF out buffer amp. (dual EF80, ECC85 and maybe an EF80 or EF86 driver).
The ECH81 is readily available, as is the NOS Russian equivalent. It will work down to 14V HT (use a car radio circuit) with positive bias. I'd probably use two so as to have an AF preamp and RF buffer/Driver. A single 12V or 24V PSU is viable.
