The PFL200 combines a screened amplifier pentode and a frame grid video output pentode in the one envelope. It was designed for television use. The mutual conductance of the output pentode at 21 is very high as is required for wide bandwidth amplifiers.
The valve was used for video output in colour television receivers. Three would be used per receiver, one per gun. The second pentode would be used in the colour difference circuitry. The presence of the internal screen points to the video usage. When used in monochrome receivers the valve was used for video output and the low power pentode as a sync pulse separator.
The B10B base was a late introduction, near the end of the use of valves in consumer products. By putting two pentodes in a single envelope the price would have been lower for set manufacturers in overall assembly costs.
This valve carries the printed logo and name of Thorn but the valve has the PFL200 etched into the glass and thus it is likely that this was made overseas for Thorn televisions. If made in the UK by AEI it would probably carry the Mazda brand.
The small screened voltage amplifier pentode.
On the right is the output pentode. At full size you can see the suppressor grid in the smaller section and with care the screen grid can also be viewed.
The anode of the power pentode.
The thin glass tube envelope is 20 mm in diameter and, excluding the B10B base pins, is 71 mm tall.
References: Datasheet & 1040. Type PFL200 was first introduced in 1964.