Someone with more time than me may like to research the above valve and construct a receiver using it. Its a very unusual valve originally intended as a FM limiter/detector, but I reckon is good for many more things.
My thoughts are, to build a simple RF amplifier centered on a ham band, the output then going to the EQ80 first control grid. For full limiting, the grids need a 14 volt p/p signal as I understand it.... easy to acheive with a regenerative amplifier, the EQ80 grid can simply be connected to the top of the tank.
The second EQ80 control grid is connected to a VFO also centered on a ham band..... this becomes the main tuning.
The EQ80 is a limiter, in which case it will supress any AM modulation, ie noise and much sideband interference. Limiters, by their very nature will "lock" onto the strongest coherent signal in the passband, in our case, the tuned one. RF sensitivity is increased, and RF bandwidth is narrowed with increasing regeneration.
As SSB demodulation is a mixing process only, the audio will appear as a PWM signal on the EQ80 anode, and can be recovered by a simple LPF. Any AM station will cause only a heterodyne.... BUT, by filtering the recovered DC from the EQ80 anode, and using this error voltage to control the VFO frequency, it will be possible to synch detect AM with selectable sidebands. This will require another mixer in tandem....
Because very good isolation is achieved between RF and VFO, there will be minimal, if no pulling with tuning.
One knob tuning and perfect tracking is easily achieved as VFO and signal circuits are identical in frequency.
In essence, we should have a stable, regenerative , sensitive, potentially quiet direct conversion RX, with good frequency stability, no requirement to have fiddly threshold settings, good post detection filtering with a suitable audio filter not requiring huge and generally problematic audio gain.
The EQ80/6BE7, can be thought of as an analog NAND gate, where for the anode voltage to go lower, the two control grids must be positive together. At 90 degree delta, the anode is at high voltage, as the phase rotates between the two inputs, anode voltage drops to a maximum at zero degrees. Between the two, the "pulse width" varies as a function of the phase difference., or the "overlap" of input voltages.
I bought a box of these on ebay.... they are very cheap.