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Old 28th Apr 2021, 4:08 am   #13
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,778
Default Re: Philips 663A combined radio/television 1948

Originally Posted by Sideband View Post
ECH35 as a TV frequency changer The mind boggles! How good was that at 45Mhz? I thought they struggled at 20Mhz shortwave but at low VHF......?
In the immediate post-WWII period, some of the standard AM frequency changer valves were claimed to be satisfactory for use up to 100 MHz, and so suitable for use in FM receivers. This group included the 6BE6 (X77) and X79 (6AE6). Both the ECH42 and ECH81 were used in this way, although a better FM application for the ECH81 was with the triode as a self-oscillating frequency changer, and the heptode as IF amplifier (pentode-strapped). In advertising, if not in the data sheets, the ECH81 was described as being suitable for use up to 100 MHz. The X81 and X79 were both used as frequency changers in a BBC FM receiver design; Lowther used essentially the same circuit in its first FM tuner, but with a 6AH8. Nonetheless, the realizable performance on FM with these valves was not very good, and in the case of the 6BE6, reputedly rather poor. Thus they were soon eclipsed by other solutions, typically the 6AT6 (ECC81) frequency changer preceded by an RF amplifier in the USA, and the ECC85 RF amplifier and frequency changer in Europe, the latter evidently an “economy” solution.

In that context, it is not so surprising that there would have been one or two applications from this valve group in Band I-only TV receivers, on the basis that “good for 100 MHz” implied “good for around 50 MHz”. Evidently Philips saw the ECH35 as being suitable.

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