I see what you are getting at, David. In the description of the BT9121, The vision IF is stated to be 6MHz ±2.5MHz giving the LO frequency at 39MHz (as the sound IF is 2.5MHz. With the BT8161, the LO frequency is 41MHz with the IF at 4MHz. The band limits of the 6MHz IF strip are 3.5MHz and 8.5MHz with the BT8161 IF strip’s being 1.5MHz and 6.5MHz. Thus if everything else was the same (valves etc.) then the same IFTs would cater for both as you stated, David. However, both the valves and the IFTs are different!
Firstly, the valves:
|Valve||Cg pf||Ca pf||Valve||Cg pf||Ca pf|
Secondly, the IFT DC resistances:
|BT8161 Manual||BT9121 Manual|
|Detector ||5.2||15.5+15.5||4||8 + 8|
The difference in resistance is quite marked, and this is a bit surprising as, doing some 4/6MHz resonance calcs, with 25pf and formers similar to those used in the BT9122, gives a turns ratio between the same mechanical coil for the 4:6 frequency ratio is 2.1:1. This is about right for the secondaries but not the primaries. I obviously need to do a bit more work bringing in the valve capacities as well!
Thanks for the kick, David
Oh dear! I forgot about the table problems! I'll see if I can find Terry's table maker.
PS The W in the second table were Omegas in the original spreadsheet!!