Days 27 and 28 Mixered Up

The JMD mixer promised to be a king-sized PITA.

First, I decided to use a 6JH8, affordably available, rather than the esoteric 7360 mixer. Swan did this swap in the 60’s to save some money, and the tube should be functionally identical. This meant wiring the 7360 socket with different pin assignments required for a 6JH8, a process which makes a neurotic a bit insecure about whether it’s all gonna be ok. If and when this is working, I’ll publish the revised schematic.

My second problem was what to do for the mixer input coil. As I mentioned before, the special high Q Miller input coils, meant to work with the Q multiplier, are unobtanium. I considered trying to make a substitute from Miller coils with a lower Q – 50 instead of about 100 – but decided I’d be better off skipping the Q multiplier concept completely.

Instead I wound a slug tuned coil that would resonate with a sweet 2 section vernier drive 365 pF capacitor, figuring that if I lacked sufficient image rejection, I could add the second coil and have a conventional double-tuned input circuit that could be made to work from 3 to 30 mHz.

I tested the circuit by listening on a general coverage receiver, injecting a signal, and peaking the coil. Seems to be relatively low loss and sharply resonant, so that’s about as good as I can do for now.

The mixer wiring was a bit tight but completed without much consternation.

I’ll remain a bit concerned about the 6JH8 mixer substitution until I see it work, but there’s really no way to test it until I finish the HF oscillator in the next few days. Fingers crossed, as they say.

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