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.
The one EQ80 advantage over a mixer, such as the heptode/hexode, I think is the limiting action. To get an accurate voltage change per radian in a phase detector, a limiter is required to prevent errors caused by amplitude changes being wrongly interpreted as phase errors. This is automatically taken care of in a digital PLL
The EQ80 is a weird construction that appears to "funnel" the electron beam through slots in the accelerator electrodes, so its "all or nothing" that arrives at the anode. With just 6 parts to wire one up, it seems to me an easy experiment to add one into an old AM AA5 for instance, and run it at the existing 455kc IF.
To synch the radio, a 455kc reference is needed, and after filtering, the DC anode voltage of the EQ80 would control a simple varicap across the main AA5 oscillator. The oscillator would remain at 90 degrees phase locked to the received signal, and , as the phase changed, the EQ80 anode voltage would rise or fall and pull the VFO one way or the other. Of course, at "zero beat" where the same reference oscillator is fed into a second mixer at 455kc, we would have synchronous detection . Obviously there will be no 7useful audio at the EQ80 anode because of its limiting action.
The 455kc reference oscillator could use the triode section of the ECH81, and enter one port of the EQ80.. The AA5 final IF signal would enter the second port of the EQ80. Both these voltages are more than adequate to drive the EQ80
The error voltage at the Q80 anode then feeds a varicap across the AA5 oscillator section, a few Kc change here would be more than sufficient.
The Heptode section of the ECH81 would also have one grid to the same reference osc at 455kc, the second grid at the 455kc signal frequency. The audio at the ECH81 anode would then feed back into the existing AA5 audio section.
This could be easily expanded to include interstation muting, simply by monitoring the audio on the EQ80 anode for noise or heterodynes .....either would not be present when locked. This would be nice to remove the severe whistle when tuning. . This experiment would easily prove the concept.
I will add a schematic outlining the concept.
Quadrature FM Detectors: Function and Failure, December 1959 Electronics World - RF Cafe
At the link above, some more insight given about 'quadrature detectors'. The article states that a quadrature detector offers high AM limiting action. I gave a quick look on Ebay right now, and actually the EQ80 are looking not that much cheap, being the lowest price I saw about 10 Euros/each (1Euro more ore less = 1 USD).
My understanding is that 'dual control pentodes' can work as 'limiters'.
'Dual control' means, If I understand correctly, that the effectiveness control of both grids over the electrons flux is more or less the same, with same voltage swings applied to them.
6DT6, 6AS6 and also ECH200 or ECH84 looks exactly 'dual control'.
EQ80 has a 3rd control grid, and I often think how to use it in a receiver.
Tens of yrs ago, I found one 'EQ80' in a valve TV. I think I still have it. I was 8 yrs when I found it and I was surprized also at that time for the unusual tube name.
What's the 3rd grid possible use in a homodyne ?
Very nice and interesting to develop an 'Automatic Frequency Control' to achieve a synchronous AM detection !