26-04-2017, 01:17 AM
(25-04-2017, 07:23 PM)Geordie McBoyne Wrote: DC stabilising of the pentode amplifier is performed in a similar manner as the 3-3 amplifier.
A difference though is that in the Ferguson case, the pentode screen grid bypass capacitor is taken back to the (unbypassed) pentode cathode, not to ground. But as the screen grid feed is taken from the unbypassed cathode of the triode, it would also be modulated with AF as well as carrying the desired positive DC voltage. This AF would be antiphase to that on the pentode cathode, as seen on the other side of the screen bypass capacitor, so presumably the arrangement allows the desired DC feedback to the pentode screen grid without also providing a material AF feedback path. In the Mullard 3-3- case such gymnastics were unnecessary, as the EF86 screen feed came from the bypassed EL84 cathode.
The use of the PCF80 rather than the ECF80 is hard to rationalize, given that the ECF80 was a common and abundant valve type, and had a 100-volt heater-to-cathode rating. Possibly there was an element of chronology, in that in the 1950s Philips was inclined to introduce P-type TV valves ahead of their E-counterparts. But the ECF80 was certainly used in some of the first crop of UK FM receivers, so it would have been available to designers during 1954 at least.
Cheers,
Steve
