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Old 8th Dec 2019, 5:42 pm   #24
David G4EBT
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Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 4,201
Default Re: Philetta BD 244U

Lots of smaller radios use an autotransformer and the heat dissipated is minimal.

The Unitra Figaro for example, has quite a small autotransformer but it's a 'minimalist' circuit with just three valves. Unlike your Philetta, it has parallel 6.3V heaters, and the heater winding is tapped at 1.4V for the DM70 magic eye. The Pye R33 is another dinky little set which uses an autotransformer. The cabinet looks like it's Bakelite but in fact it's injection moulded plastic so would be susceptible to excess heat. (The Pye R33 uses a series heater chain like the Philetta).

Not sure where an autotransformer would fit inside a Philetta though - it's a tribute to the designers that they squeezed so much radio into such a dainty cabinet as it is. Well done on replacing the resistors and caps - it's just a nightmare beneath the chassis! Both the Figaro Special and Pye R33 used a metal rectifier to save space and current. I guess one rather radical option with the Philetta which might yield space internally for an autotransformer would be to remove the rectifier valve and fit a solid stage bridge, but maybe that would be a step too far.
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