This was my original prototype design to see how well a simple superhet could be made to function, and is still the "guinea pig" for testing new ideas.
In order to derive the maximum performance, i decided to go for a fairly high IF frequency to eliminate images. The usual 1700kC is low enough to achieve reasonable gain, but selectivity suffers. Both these issues are nicely avoided by using IF regeneration. Its also out of the range of interferring AM broadcast stations.
The best and latest in valve mixer technology was in the 60s when Philips created the ECH81 triode/heptode. This valve had little frequency pulling between mixer and oscillator, and is fairly tolerant of oscillator voltage level into grid 3. By using a variation of the Vackar type oscillator, thus keeping the Oscillator level constant over the tuning range, the effective gain is also kept constant over a wide tuning range. The triode oscillator section is fairly low gM, and has to be driven hard to get into the higher SW bands.. The ECH84 came later, and although designed primarily for synch seperation and gated AGC in TV service, appears to exhibit slightly better performance as a mixer in this application.
Because of the very high impedance of the dynamic load on the mixer anode, a result of the "Q" multiplication effect of the regenerative IF, the conversion gain is increased dramatically, therefore, an RF attenuator is required to limit overloading in the IF. The high conversion gain also allows a relatively lossy passive bottom coupled dual tuned bandpass filter on the input, again improving mixer performance by narrowing the RF bandwidth, thereby reducing out of band noise. This is quite noticeable when comparing this to my simpler RX having just the one RF tuned circuit.
Using the Pentode screen voltage to vary the IF gain, results in very smooth and controllable regeneration with little frequency shifting. I did try a second IF bandpass filter between mixer and IF in an attempt to improve the IF shape factor. This worked to a point, but something I had not envisaged was the "double humping" when adjusting the regeneration. As the "Q" increased, the effective coupling between the two coils increased, causing extreme interaction and effectively splitting the signal into two several kCs apart at the critical threshold.
I would like a 3:1 transformer in the detector anode to gain a free voltage stepup to increase the audio gain, have yet to find anything suitable.
This radio I used 5 pin DIN sockets , and PVC plug in coils, an easy way to change bands and makes superhet tracking design slightly easier. Air cored coils and air spaced tuning capacitors are the ultimate for stable and "drift free" oscillators. The best tuning capacitors are made from brass, these have less thermal expansion than aluminium vanes, good luck finding these. Polyvaricons are abysmal, only slightly better than varicap diodes.
Having no tuning slugs improve the thermal effects on the coils, but also makes adjustment very time consuming, needing to add or remove turns to get it just right, but its worth it to maintain the high "Q". This set of coils covers 5 to 15megs, tuning is touchy. More coils and less tuning range would be the next goal with this one.
Some may find this approach interesting, I know back then i had lots of fun building and experimenting with it.