Quote:
Originally Posted by Skywave
So, what I've learnt will do for now.
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Noted. However, as I had written the following before seeing that comment, I thought that I may as well post it.
For valved communications receivers with four-gang front ends, two single-tuned RF stages seems to have been the modal choice. Possibly circuit noise at the upper end of the HF band was a factor here. I think that we may “read back” from comments that Marconi made about its Hydrus solid-state compact ISB receiver in 1968, bearing in mind that this was before the paradigm shift in HF receiver front end design marked by the Racal RA1772.
“To reduce the effects of crossmodulation and intermodulation, highly selective circuits should be introduced between the antenna and the first active device in the receiver, consistent with a noise figure which may be usefully employed at the upper frequency limit.”
“The degree of pre-selection, and hence the number of tuned circuits which may be inserted between the antenna and the signal-frequency amplifier, is a compromise between sensitivity and selectivity. It is generally accepted that a noise figure of 6 dB to 8 dB may be usefully employed at the upper-frequency limit of the h.f band ( 30 MHz). Provided that the noise figure of the active devices used in the signal-frequency amplifier can be made sufficiently low, say 2 dB, then greater scope is offered to the designer in providing antenna pre-selection. During the past two years, field effect transistors (f.e.t's) have entered the scene and are now available at economic prices. These devices offer attractive advantages over the more conventional bipolar transistors, resulting in improved signal-handling capabilities at lower noise figures. Depending upon circuit configuration, improvements in crossmodulation performance of up to 20 dB have been measured. Coincident with better signal handling, device noise figures of 2 dB or less are realizable.
“In the Hydrus equipment it has been possible to introduce a band-pass coupled circuit in the antenna input, which produces 30 dB to 40 dB attenuation at 10% off tune. This circuit feeds the signal-frequency amplifier, which consists of two junction f.e.t's connected in cascode. Noise figures of between 4 dB and 7 dB have been achieved. The cascode arrangement is preferred as it gives good isolation between the input and output of the amplifier and it also provides a convenient terminal for the application of a.g.c.”
The implication is that the jfet cascode was quiet enough that it provided a noise margin against the desideratum, which then allowed the use of the apparently noisier (at upper HF) bandpass input circuit. In the valve era, with pentode RF amplifiers, and even with high-slope pentodes in the first RF amplifier, that margin was not available. At lower HF and MF, device noise was less of an issue.
Some confirmation of that position may be had by looking back at some of the Eddystone receivers. For the 680X, 730 and early 880 Eddystone had used the conventional 4-gang arrangement with two single-tuned pentode RF amplifiers. But for the 830, it used an ECC189 cascode as a single-stage RF amplifier preceded by a bandpass tuned circuit. Given that the double-triode cascode was likely quieter than a pentode in the 20 to 30 MHz range, it could have been that Eddystone was taking advantage of the noise margin that allowed the use of the bandpass input. Later, with the 940 and later editions of the 880, it used the cascode as 1st RF amplifier in a conventional 4-gang arrangement with single-tuned input, presumably to obtain better quieting curves at upper HF.
Eddystone had also used a 4-gang, single RF amplifier (12BA6) with bandpass input for its 909A marine receiver, whose tuning frequency range went only to 4.7 MHz. This choice seems to align with the notion that it was more at the upper end of the HF band that device and tuned circuit noise became an issue.
Returning to domestic-type receivers, Murphy was on record (post-WWII) as saying that a bandpass input was the best choice at LF and MF, this apparently independently of whether or not an RF stage was used. It did use a bandpass input, without an RF amplifier, on its A186 and A188C (both LW & MW only, no SW) receivers of the early 1950s. These had variable selectivity, with a wide IF bandwidth that I’d guess to be in the range 16 to 20 kHz or so. The use of a bandpass input might have avoided the already noted bandwidth restriction that could occur at the lower end of the MW band with single tuned circuits. In some wideband receivers/tuners (e.g. Quad AMII) the MW RF circuit Q was lowered (by switching in series resistance) in the wideband mode. Another technique, when there was an RF stage and so two single tuned circuits, was to slightly stagger tune them at the low end of the MW band.
Murphy’s TA160 export receiver was of the 3-gang with RF amplifier type. But on MW, two of the gangs were used for a bandpass input, with an aperiodic interstage. On the wide-range SW bands, a conventional approach with single-tuned input and single-tuned interstage was used. The single-tuned input would have been better from a noise viewpoint at upper HF (although not at MF and lower HF). The arrangement tends to confirm that Murphy saw the bandpass input as being superior on MW, even if it required an aperiodic interstage when used with an RF amplifier. But there was more to it. A high-slope, 6F1 RF amplifier was used in place of the customary remote cutoff type, such as a 6F15. On a standalone basis, that was not so unusual, as there were other domestic all-band receivers that also used high-slope pentode RF stages in order to obtain lower noise at the upper end of the HF band. Evidently Murphy saw this choice of RF valve as increasing the risk of cross-modulation with strong signals on MF, and opted for the bandpass input as a way of minimizing this. Murphy also had another reason for using the 6F1. On the bandspread SW bands, it used an image-rejection circuit in the interstage, this apparently reducing the RF stage gain somewhat, and requiring the high-slope valve to provide enough gain overall at upper HF. So the input tuned circuit and valve choices were somewhat interdependent.
Dynatron also used a four-gang, single RF stage front end on some of its receivers, namely the later T69 iterations (T69C and T69D, I think) and the T99. The earlier T69A and T69B had been three-gang. As best I can work out, the additional tuning gang supported a bandpass circuit placed in the interstage, not at the input. If so, this placement was logical, given that the SW coverage went up to 30 MHz and the RF amplifier was a pentode. (I suppose though that there is the possibility that the band switching might have been arranged to put the bandpass at the input on MF, but at the interstage on HF.) The later T139 reverted to three-gang, but I suspect that may have been something of a forced choice, as I think it used the standard Weyrad bandspread front end. Even so, Dynatron added an aperiodic-input grounded grid RF pre-stage on the bandspread and upper HF bands. This would have provided constant input conditions for the following tuned circuits. Thus not having to cater for the variable source impedance of random connected aerials, and with less concern about noise introduction, they might have been “tightened up” somewhat – speculation on my part, though.
Cheers,