Quote:
Originally Posted by Tom Albrecht
There are some strange sets out there with transformers for filaments and voltage doublers for the B+. Zenith had some sets like this, which appear to be "safe" when you see the (relatively large) power transformer, but turn out to be hot chassis designs. A particularly dangerous design for the unsuspecting technician.
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Usually if you see selenium rectifiers, it's a hot chassis set. For some reason, they rarely used vacuum tube rectifiers in series string hot chassis sets. They did use vacuum tube diodes for damper diodes, likely because seleniums were too slow. But most series string sets had excessive heater voltage to burn in power resistors, so a vacuum tube rectifier could have made sense. Something like a 25V @450ma heater, for say 150ma plate current and a PIV around 400V. Yes, you'd have voltage drop to to B+, but seleniums do that too. At the cost of the tube and a socket, you'd save a power resistor and a selenium rectifier. Same amount of heat to ventilate. A "What if" kind of question...
For the filament transformer hot chassis sets, you could throw in a 5V secondary and use a 5U4 with both plates strapped together.