[Synchrodyne]

Re Q1:

Evidently the ECL80 was designed from the start for a variety of applications. This is shown in the first two attachments, one being a page from Philips Book IIIC covering the initial range of TV valve and the other being the Wireless World release announcement.

Given that the noval base forced a choice between a common cathode with a separate pentode suppressor pinout and separate cathodes with an internal suppressor connection, one assumes that Philips saw the former as being more appropriate for the intended range of applications. But it was probably a marginal call.

Re Q2:

The usual reason for having a separate suppressor pinout for pentodes was to allow direct earthing of the suppressor and so minimize the risk of deleterious feedback that could cause bandpass response tilting and oscillation. The mechanism by which this could occur is well explained by Cocking in “Television Receiving Equipment”, pertinent excerpt therefrom as third attachment.

A second reason was that the suppressor grid could be used as second control grid, such as for agc, gating or mixing. For gating and mixing though, purpose-built dual-control pentodes, such as the 6AS6 and 6F33, were usually preferred.

RCA stressed that a separate suppressor grid pinout, allowing direct earthing, was essential where the pentode was operated with part of its cathode resistor unbypassed, this usually being done to minimize detuning and passband tilting due to the agc bias. Thus for its 6X8 TV frequency changer it chose a separate suppressor pinout with combined cathode. It could not have been a major issue though, as Tung-Sol chose internally connected suppressor with separate cathodes for its contemporary 6U8 TV frequency changer. Philips did the same as Tung-Sol for its later ECF80/PCF80. And although the PCF80 was initially described as a frequency changer valve (e.g. Mullard Valve Tubes & Circuits #15 & #16), it soon segued into a multipurpose valve (e.g. Mullard Valves, Tubes & Circuits #26 & #27), in which role it covered most, if not all of the applications that had been listed for the ECL80. That, I think, demonstrates that the pinout configuration chosen for the ECL80 was a marginal call, as the evidence is that it would have worked well enough the other way around.

That the ECL80’s common cathode could at times be a bit awkward was acknowledged by Mullard in design for a push-pull amplifier using a pair of these; see Valves, Tubes & Circuits #2 & #3.

On balance, one might say that Philips made the wrong call with its ECL80 pinout choice, and that separate cathodes would have been preferable. Many ECL80 circuits appear to have been arranged with the cathode directly earthed, meaning that an internally connected suppressor grid would also have been directly earthed. But then it could have been that many circuit designers saw direct earthing of the cathode as sine qua non for the ECL80, to prevent any cathode coupling between triode and pentode, and so designed accordingly, even though they might have done differently had there been separate cathodes.

Still, what look to be wrong calls in hindsight are probably less the result of missing due diligence than simply the way a designer or a design group assessed the trades-off in the light of what was known at the time.

Cheers,