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Old 27th Nov 2017, 2:27 am   #17
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,943
Default Re: Introduction PCC189 in Mullard tuners?

There’s also an interesting aspect in respect of valve development. As previously noted, the PCC84, whose triodes were of the sharp cutoff type, could be used – as a cascode amplifier - in either the sharp cutoff mode, with divider bias for the upper triode grid, or in the remote cutoff mode, with sliding bias for the upper triode grid. The PCC88, also with sharp cutoff triodes, was the frame-grid successor to the PCC84, but according to the Philips datasheet, was to be used only with divider bias, and therefore in the sharp cutoff mode. So, one may infer that at this juncture, Philips had decided that the sharp cutoff and remote cutoff requirements would be covered by two separate valves, and not by different circuit configurations for a single valve as had been the case in the PCC84 era. The PCC89 had remote cutoff triodes, and could be used with either divider or sliding bias, offering a range of grid bases that could be described as running the gamut from semi-remote to full remote cutoff.

In 1957-58, Mullard issued a couple of papers about its high-gain sync-cancelled AGC system. The first was: “A.G.C. Circuits for Positive Modulation Television Receivers”, published in Mullard Technical Communications No.27 of 1957 December. The second was: “Automatic Gain Control Circuits in Television Receivers for Negative Modulation Systems”, published as an IRE paper in 1958 May.

The positive modulation circuit was developed around a receiver that used a PCC84 RF amplifier, evidently in remote cutoff mode, PCF80 oscillator-mixer, EF85 1st IF stage, EF80 2nd IF stage, with AGC, and EF80 3rd IF stage, without AGC. Presumably this represented Mullard’s preferred valve line-up of the time. The paper also included the AGC divider network calculations for the case that an all-EF80 IF strip were used, but no mention was made of the alternative use of the PCC88 as RF amplifier. I’d guess that the EF85 1st IF amplifier covered the case where it handled both sound and vision, with sound takeoff after the that stage. The all-EF80 IF strip would have been applicable in the case where the sound takeoff was right after the tuner.

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The negative modulation circuit on the other hand was based upon a PCC88 RF amplifier and an all-EF80 IF strip. I’d guess that this was the kind of circuit that Mullard would recommend to the UK setmakers for their export receivers. And if it were supplying VHF tuners to any of these makers, they would have been fitted with the PCC88.

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This is, I think, indirect evidence that Mullard saw the PCC88 as being less suitable for positive/AM system receivers, although it was fine for negative/FM receivers. That was a view that was apparently not shared by Philips itself, though, as it used the PCC88 RF amplifier in VHF tuners for Belgian multi-system and French 819-line receivers. One may wonder if the PCC89 was developed mostly to keep Mullard happy, and if the bulk of PCC89 and PCC189 production was for the UK market.

Given that the (Mullard) PCC89 was announced in the second half of 1957, I’d say that Mullard did its AGC development work before then.


Cheers,
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