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Old 29th Apr 2021, 12:32 am   #16
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,778
Default Re: Philips 663A combined radio/television 1948

I imagine that the noise floor was quite high. I think though that the use of the ECH35 (and like valves) at VHF was predicated on their being preceded by a suitably high gain, low noise RF amplifier, such as an EF50 or an EF42, which would diminish the effect of the mixer noise. To some extent, that approach, noisier-than-ideal mixer preceded by high-gain, low-noise RF amplifier soon returned with TV front ends. Something like the ECC81 (6AT6) would have been a very good low-noise frequency changer for TV use at Band III, and would not have made undue demands on the RF amplifier. And in fact GE developed the 6AT6 for TV as well as FM (and FM-AM) frequency changer use. But the upward shift in TV IFs, which put the IF channel very close to the lowest Band I channels, created a potential regeneration problem that was most easily solved by using a pentode mixer. But that created a potential noise problem at Band III, where without any other constraints a triode mixer would be the logical choice. That problem was solved by the development of a very high gain, very low noise RF amplifier, namely the cascode. RCA had started that development programme in the late 1940s for other reasons, but it moved from being a desirable target to a very necessary target, and in the event arrived at the same time as the TV triode-pentode frequency changer.

At HF too, a good RF amplifier would have been highly desirable with the ECH35 and its ilk, both for front end selectivity and noise reasons. And it was not uncommon in the later 1940s to use a high-slope pentode for the RF amplifier. As far as I know, the early version of the Weyrad bandspread front end was based upon an EF50 RF amplifier and an ECH35 frequency changer.


Cheers,
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