THE MULLARD WORLD SERIES VALVES (iii) - PENTODE TYPE PL81

Early tellies were beastly things with wobbly pictures and if you took the back off to see what you could twiddle  there could be up to 7kV flying about.  Now although this may have been lethal for the twiddler, please spare a thought for the line output valve which had to be a pretty tough cookie indeed.  

As usual, the Mullard boffins came up with the perfect device, capable of working from a 170V HT supply whilst handling a peak anode current of 350mA and robustly standing flyback pulses of up to 7kV all in a miniature envelope they gave us the PL81.  Although primarily designed for television use, PL81 sales were bolstered as Mullard also specified the PL81 as eminently suitable for series pass element duties in voltage regulators.