For Triode mainly..... he is to blame for this ....
After researching that 6BN6 you mentioned, and the homodyne you mentioned, here is a circuit that im very sure will work first time. Because its a new topic, I have made it a, ummmm, new topic
The RF input to detector and mixer is presented to the control grids of both. The 6BN6 is configured as a limiting quadrature detector as per the data sheet. The ECH81 is also the data sheet
The VFO is also presented to the control grids of both the ECH81 mixer and 6BN6 detector.
The error signal from the 6BN6 anode is used to control a varicap diode in the common VFO.
The varicap needs its anode lifted from ground to cancel the standing DC voltage on the 6BN6 anode, this hovers from around 1/2 supply to 3/4 supply, this accomplished by the adjustable screen dropper tap, this puts the varicap in its linear capacitance slope, needs to be say +2volts to 30 or so.
As the tuned frequency deviates from the Fin, this error voltage will force/drive the VFO back to center.
The 6BN6 limiting action should improve locking performance on modulation peaks.
Both oscillator and RF circuits use similar tuned circuits , cover the same frequencies, so perfect tracking is assured.
With approx the +20dB conversion gain of the ECH81, audio level should be sufficient to drive a simple triode/pentode output stage directly to a goodly volume, ECL82 or similar.
Locking range is dependent on the ratio Varicap C/Tuning C. Too much will make tuning "snappy", tending to jump from one close station to another, too little will make tuning "fiddly" to lock.
Adding a tuned RF amplifier stage will improve weak signal performnce.
A simple passive audio LPF will likely be required to reduce adjacent 5kC hets.