Looking at the chronology of the D&W solid-state FM front ends, it is known that the silicon bipolar version of the D&W 341/AFC FM front end was used in the Radford FMT1 FM tuner. This became available early in 1966, as shown in this excerpt from the 1966 Audio Fair catalogue.
This timing would have made it a reasonably early application of silicon planar transistors.
The two dual-gate mosfet version of the same front end, the 341/AFC/FET/2 was used in the Rogers Ravensbourne 2 FET FM tuner, which was shown for the first time at the 1968 Audio Fair.
The same 341/AFC/FET/2 front end was used in the Radford FMT3, which was listed (at that time alongside the FMT2) in HFYB 1970.
A FET-based front end, presumed to be the 341/AFC/FET/2, had been offered as an option on the Radford FMT2 from early 1967, as shown in this excerpt from the 1967 Audio Fair Catalogue.
If the presumption is correct, then this would have been a very early application of dual-gate mosfets in an FM front end. What appears to have been RCA’s initial commercial release of a dual-gate mosfet was the TA2644, announced in Electronics World 1966 October. TA2644 was in fact the development type number for what became the 3N140, although when the latter name was applied I do not know. RCA had used the TA-series development type numbers quite often when discussing mosfets in its publications, and they were shown in early edition datasheets under the definitive type numbers. The RCA 4060n series of dual-gate mosfets (3 for TV applications, 2 for FM) were in the 3N140 group, although the three TV types had their own TA-numbers as well. Anyway, it does appear that the basic type of dual-gate mosfet used in the 341/AFC/FET/2 was commercially available by early 1967. Of course, it is possible that D&W had used jfets in 1967, such having been used in American FM tuner front ends since late 1965.
On the basis of circumstantial evidence, yet to be corroborated, the varicap derivative of two-mosfet 341 front end was used in the Sudgen R21/R51 tuner. This was first shown at Sonex 1971 (as reported in WW 1971 April), with the first HFYB listing being 1972. This was not a very early use of varicap tuning in a domestic product. For example, AREF (Denmark) had offered varicap-tuned FM front ends from late 1967 (as reported in WW 1967 December). However, as early varicap applications were reputed to adversely affect signal-handling and spurious response performance, D&W (and its customers) may have chosen to wait for some progression along the learning curve. On the other hand, from the start, dual-gate mosfets offered benefits as compared with small-signal bipolar devices.
Re D&W’s use of bandpass input tuning with a single-tuned interstage in its 341 series, support for this approach may be found in a 1966 RCA paper “A Comparison of Solid-State and Electron-Tube Devices for TV-Receiver RF and IF Stages”. Therein it was said, in respect of the poorer cross-modulation characteristics of bipolar transistors as compared with valves:
“Further improvement in cross-modulation can be achieved, at the cost of additional sacrifice in noise figure, by use of additional pre-selection prior to the transistor input. For this reason, it is good design practice to use a double-tuned input circuit in transistor rf stages. As shown in Table IV, a noise figure of 3 dB is typical for bipolar transistors. Therefore, a degradation of as much as 1.5 dB will still provide a device comparable to tubes.”
Although RCA was discussing VHF TV front ends, I think that as Band II is between Bands I and III, interpolation to the FM case is reasonable. Normal practice for valved TV front ends was a single-tuned input with a double-tuned bandpass interstage, and that was retained when dual-gate mosfet RF amplifiers were introduced (initially by RCA in 1968).
Cheers,