23-04-2017, 02:49 AM
The Eddystone 640 was more highly specified than the later 670 and 840 series. The 640 was of the (1 x RF, 2 x IF) configuration, with a 1.6 MHz IF, whereas the 670 and 840 series were of the (1 x RF, 1 x IF) configuration, with a 450 kHz IF. So with three-gang front ends throughout, the 640 would have had better image rejection at the higher HF frequencies. Arguably the higher IF required the use of two IF stages to meet selectivity and possibly gain requirements, but even so, two stages at 1.6 MHz were probably better than one stage at 450 kHz. The top-of-the-line (at the time) 680 model was of the (2 x RF, 2 x IF) type, with 450 kHz, this layout being common for higher performance communications receivers of the time.
Evidently with the 640 Eddystone obtained the desired AGC performance with agc bias applied to the RF, mixer and 1st IF stages, but not to the 2nd IF stage. Having no (or at least significantly reduced) agc bias on the final IF stage was considered advantageous, and helped with the reduction of modulation rise distortion.
The 670 was designed as a marine cabin broadcast receiver, so was required to be able to operate on DC as well as AC supplies, down to 110 volts. Thus U-series valves were required. Probably the voltage and cost constraints limited it to a (1 x RF, 1 x IF) configuration, although that was quite respectable by domestic receiver standards. The 840 series was more-or-less a communications receiver version of the same concept. A point of interest is that the final 670C and 840C variants, released in the early 1960s, retained the U-series Rimlock valves of the originals, probably long after Philips/Mullard had ceased offering them for new designs.
Strictly domestic receivers with serious shortwave broadcast reception intent seldom went past the (1 x RF, 1 X IF) layout. For example, the Bush EBS44, Murphy TA160 and Pye PE80 were all of this type. But the Ekco contender in this class, the A182, I think had two IF stages. At the very upper end one found the (1 x RF, 2 x IF) format as for example in the Dynatron T69 and T99, Chapman S6BS and some of the Armstrong models. Here there was more than enough gain for broadcast programme listening purposes. As evidence, some of those, such as the Dynatron (from the T99A) and the Chapman S6BS (later variants at least) had preset IF gain controls that allowed the gain to be reduced where required. Perhaps that’s why a 2nd RF stage was very rare in domestic receivers. Where four gangs were used for better RF selectivity, as in the Dynatron T69D and T99, the fourth was used to form a bandpass, either input or interstage. That also avoided undue narrowing of the RF passband. On the other hand, for communications purposes, where reading of signals that were much weaker than the signals than were usable for entertainment purposes was required, and very narrow passbands were acceptable, the extra RF stage was advantageous.
Cheers,
Steve
Evidently with the 640 Eddystone obtained the desired AGC performance with agc bias applied to the RF, mixer and 1st IF stages, but not to the 2nd IF stage. Having no (or at least significantly reduced) agc bias on the final IF stage was considered advantageous, and helped with the reduction of modulation rise distortion.
The 670 was designed as a marine cabin broadcast receiver, so was required to be able to operate on DC as well as AC supplies, down to 110 volts. Thus U-series valves were required. Probably the voltage and cost constraints limited it to a (1 x RF, 1 x IF) configuration, although that was quite respectable by domestic receiver standards. The 840 series was more-or-less a communications receiver version of the same concept. A point of interest is that the final 670C and 840C variants, released in the early 1960s, retained the U-series Rimlock valves of the originals, probably long after Philips/Mullard had ceased offering them for new designs.
Strictly domestic receivers with serious shortwave broadcast reception intent seldom went past the (1 x RF, 1 X IF) layout. For example, the Bush EBS44, Murphy TA160 and Pye PE80 were all of this type. But the Ekco contender in this class, the A182, I think had two IF stages. At the very upper end one found the (1 x RF, 2 x IF) format as for example in the Dynatron T69 and T99, Chapman S6BS and some of the Armstrong models. Here there was more than enough gain for broadcast programme listening purposes. As evidence, some of those, such as the Dynatron (from the T99A) and the Chapman S6BS (later variants at least) had preset IF gain controls that allowed the gain to be reduced where required. Perhaps that’s why a 2nd RF stage was very rare in domestic receivers. Where four gangs were used for better RF selectivity, as in the Dynatron T69D and T99, the fourth was used to form a bandpass, either input or interstage. That also avoided undue narrowing of the RF passband. On the other hand, for communications purposes, where reading of signals that were much weaker than the signals than were usable for entertainment purposes was required, and very narrow passbands were acceptable, the extra RF stage was advantageous.
Cheers,
Steve
