With lots of time to think, but little time to build, and as we close out 2024, my thoughts turned to "why not a vacuum tube version of a Direct Conversion Receiver".
Friends N2CQR and KK4DAS caused this thought aberration as they embark on a 2025 community build of a solid state DCR. Mind you this is just N6QW having a lot of time to think.
Firstly, FOAM is still valid as a driving concept: Filter, Oscillator, Amplifier and Mixer. I see four tubes would be needed. Two 12BA6's, one 12AX7 and one 12AQ5.
One 12BA6 would be at the RF Amplifier spot with the second 12BA6 used as the LC Local Oscillator. The 12AX7 would be used as the Product Detector and Audio Pre-Amp. Finally, the 12AQ5 is employed as the Audio Output Stage.
The beauty of this specific topology is that much of the circuitry already exists for these FOAM elements. Some of these circuits are sitting in my Hybrid SSB transceiver design. No High School Science project involved here. The toughest part is the Local Oscillator and keeping that stable. But that is the same problem with a solid-state version.
We would have to use 12VDC on the filaments to address any hum problems and the plate supply can be something as simple as a 125 VAC to 125 VAC Isolation transformer in a full wave rectifier circuit with a hefty filter on the output. The current draw for the plate/screen would be something less than 50Ma and a 100 Volt Zener would stabilize the supply voltage to the LC Oscillator. For the front end we could use a 40M Band Pass Coupler (design exists).
Now a potential slick trick for the LC Local Oscillator would be to run it on 80M and using a diode doubler create a very stable 40M LO.
[A 17M SSB transceiver design of mine which used a 4.9152MHz HB filter needed a 23MHz LO. I built a 11.52MHz Super VXO and diode doubled that to give me about 40 kHz right in the middle of the 17M phone band, so I know that the doubler solution works!]
Here are some of the circuits that would be used in the Thermatron DCR. These work! We could entertain using a third 12BA6 as the audio amplifier stage which would give an output on par with a LM386. I investigated the 6BA6 as an audio amp stage and that exits in hardware.
So, there are two burning questions that surface. The first is why the Vacuum Tube DCR and the second involves serious psychotherapy for N6QW. The first question is that while I haven't done serious deep diving on the internet, I haven't seen any tube DCR's. Possibly since the DCR hit the streets in 1968 and was solid state it never started as a vacuum tube design that migrated to solid state.
The portability of the solid-state is a huge factor when you think about a POTA op using CW and a DCR. A $15 DC to AC inverter from Lauren Sanchez' boyfriend would resolve that problem -- but more hardware to lug to the picnic bench.
N6QW has always been a few corn flakes short of a full bowl so no psychotherapy required or needed.
A project like this could be the seed for an evolutionary development like adding a mating transmitter. That 12AQ5 would be a Jim Dandy 5-watt transmitter tube and with some intermediate circuits (also existing) the DCR can be turned into a superhet with a HB crystal filter.
I see a project like this as a means of "coping with the chaos" that soon will infest this once great nation. Those that chose that option, what have you done?
Happy New Year.
73's
Pete N6QW