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Old 29th Nov 2022, 5:39 pm   #15
G6Tanuki
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Join Date: Apr 2012
Location: Wiltshire, UK.
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Default Re: Combined power-pentode and double-diodes?

Some fascinating insight has emerged; I wonder if the popularity of the EBL21/UBL21 in Eastern European radios came about as some kind of post-WWII planned-economy thing to try and minimise the diversity-of-different-valves needed for production and maintenance?

I'd forgotten about the likes of the Pye Piper and similar; didn't K-B also do a small receiver that used reflexing so the audio signal went back through the IF amp? I always wondered how well that would work when the IF-amp was subject to AGC action... still, it was probably as good or better than the ubiquitous post-WWII "Premier" 3-valve TRF radios that filled the need for a bedside/kitchen radio at low cost.

The 6BV7 - that's a new one on me!

In the same context of power-valves being in the same bottle as diodes, there were a number of US versions that combined a half-wave rectifier with a small output-valve; the 12A7 for example, or the 25A7 or the later 70L7/117M7/117N7/117P7 which could produce a Watt or so of audio as well as enough rectified current from the 117V mains to allow the feeding of the heaters of a series-string of 1.4V battery valves. I remember at least one US battery/mains portable used this approach - on batteries the output-valve was a battery-filament one but on mains one half of the battery output-valve's centre-tapped filament was turned off and it became the driver for the combined output-pentode/rectifier, giving greater audio output.

Cunning trick: on mains, the series-connected battery-valve's filaments became the output-pentode's cathode-resistor so the LT supply was essentially 'free'!
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