[regenfreak]

Apology for the late reply. I no longer get email notification of replies from this forum even I checked the option.

Can you post a circuit diagram of this valve version https://www.vintage-radio.net/forum/...7&d=1653147111
? Clever build.
From the photo I can only infer a rough picture as follows:
single tuned input -> 1/2 ECC85 gain -> double tuned BP -> 1/2 ECC85 gain -> double tuned BP -> 1/2 ECC85 mixer + 1/2 ECC85 LO. Right?

I posted the schematic in my first post number 1 of this thread. Indeed I replaced EC81 by ECC85. ( It is because the EC81 produces crazy harmonics with the plate B+ voltage over +19V. Below +19V it is stable and produces good oscillation. This is a mystery to me why EC81 misbehaves in this tuner as I used it many times in other DIY FM tuners.)

If you dispense with benefits of critical or sub-critical coupling, there is a point where a question arises whether a coupled BP still makes much sense.
For double tuned BP with a coupling factor of 0.5 or less the selectivity will approach that of two single tuned tanks of the same QL, separated some way, say by an isolating stage with 0dB gain, all that without incurring any insertion loss because of weak coupling. Additionally, such isolated single tank scheme would much better lend itself to alignments (no interacting).
BTW, for single layer air core inductors the optimal form factor D/L for highest possible Q lies in the range 0.7 - 1.
One valuable feature, exclusive to the magnetic coupling is symmetry, i.e. the -3dB edges of passband are equidistant to the centre frequency.

Indeed I spent ages to experiment and find the optimal form factor D/L that gives the minimal insertion loss for the 2nd order bandpass. The answer is not always clear cut. What works for HF does not mean it is the same at VHF. It seems the higher the the inductance of the coils, the higher the unloaded Q at FM broadcast band (which is opposite to what I expected). At VHF, everything is a compromise. Every component likes to couple to each other either inductively or/and capacitively. I always think of VHF bandpass filters as cavity resonators. Even I use partition walls to separate each stage of the bandpass filter, they are weakly coupled to each other (this can be easily demonstrated with a NanoVNA).

Before you start building another H-Z-not-so-Hi-Z probe be warned of versions like the widely spread on eBay "RF Active Probe 0.1 - 1500 MHz - 1.5 GHz Analyzer Oscilloscope", originally "Poor Man's 1-GHz" from Elektor (.pdf below).
Depending on the MOSFET type and layout it will show input capacitance of ~0.7pF (~1pF input "PCB trace" coupling capacitor) in series with the impedance Zg at the G1 (Zg = parallel connected ~1-2pF and ~10-50k Ohm), permanently mismatched output (Zout ~25 Ohm) and IL ~10dB (or worse, depending on output loading and frequency). Real part of Zg will drop with a 1/f² slope.
What you actually need is a probe with:
a) very low input capacitance (say <0.1pF) AND
b) well matched 50 Ohm output impedance AND
c) not too high and flat insertion loss,
d) usable BW ~1...100(up to 200-300)MHz.
This "Poor Man's" circuit, requires some mods and adds-on to cope.

Interesting, I have that cheap probe from Ukraine for two years but did not used it much. This mod is new to me. The hi-Z probe i mentioned have both adjustable input and output resistance using a series of three trimmer pots 1k, 10k and100K. It uses J310 and 2N3906. I built a few versions of it but the J310 always misbehaves in an unexpected manner at VHF, driving me crazy. I am in the middle of doing mod and optimising it for VHF

With good resonator design multi-section VHF/FM front ends can be a little narrow for the occupied channel bandwidth in terms of phase flatness

What are the physical meaning "phase flatness" and "group delay time" of a filter? For example, the data sheet shows group delay time plots of these 10.7MhZ Murata ceramic filters:

http://www.knap.at/datenblaetter/fil...-alt-p61e5.pdf



BTW I have been playing with a 5th-order Chebyshev bandpass filter of 0.1db ripple, cut-off at 90MHz and 110MHz. The attachements show my response vs the software simulation (https://rf-tools.com/lc-filter/).
My insertion loss is -0.6db versus -0.272 from ideal prediction. It is not bad! The practical difficulty is always the stray inductance and capacitance of the components. I probably do further fine-tuning by replacing the fixed coupling caps by piston trimmers.

Note that I got three S11 dips instead of fives dips in the ideal simulation. It is tricky to tune 5 resonators as the tuning is very sharp. The NanoVNA V2 plus 4 shows S21 dynamic range over 90db in the lower band stop region.

PS. here is a repair video and an interesting comments about the top-ended design features of the mighty 9-gang Kenwood KR 917. It uses saw IF filters and dual conversion.

https://youtu.be/orsah0Erv7w

Insane engineering designs!