PFL200 Philips
Developed by Mullard to meet the needs of the latest dual-standard television receivers, the PFL200 is a video valve of unique construction. A novel feature in its design is the use of a 10-pin B10B valve base. This new base - the decal base - makes possible the inclusion of two completely separate pentode systems in one envelope. The pentodes in PFL200 are dissimilar and have been given complementary electrical characteristics which make possible the design of exceptionally economic full-performance television receivers. Particular care has been taken in the pinning and in the screening between the two sections of the valve to provide independence of operation.
Characteristics of the PFL200 - Output Section
The PFL200 'L section is designed to operate as a high-gain video output pentode capable of producing a large output voltage across a low-value anode load resistor. To achieve high gain, which also permits the application of negative feedback, a frame-grid control grid has been used, and this gives the valve a slope of 21mA/V at 30mA.
The note has been taken in the design of the 'L section of the increasing popularity of high-level contrast control circuits in which the contrast-control potentiometer is placed in the anode circuit of the video output valve. Such circuits provide constant drive to the synchronizing pulse separator and AGC. system regardless of the setting of the contrast control. The capacitance added to the anode when this technique is employed necessitates the use of a low-value anode load resistor. The PFL2O0 can provide adequate current to produce a composite video output voltage of 100 V peak-to-peak across an anode load as low as 2kΩ.
Care has also been taken in the valve design to permit screen-grid dissipation limits that allow for short-term overloading. This can arise when the UHF tuner in a dual-standard television receiver operating from a signal having negative modulation (BBC2) is switched so that neither video signal nor noise is present to provide bias for the grid of the video output valve.
Amplifier Section
The 'F section of the PFL200 is a voltage-amplifying medium-slope pentode designed mainly for application as a synchronizing pulse separator. For this application, the valve has been designed to provide adequate current at a low anode voltage, and care has been taken to reduce to a minimum the feedback capacitance from the anode of the 'F section to other electrodes in the double valve.