[ARC5] No Doubt Dumb Tube Question

[Robert Eleazer]

I am reading some in an early 50's USAF training manual, Radar Circuit Analysis, because it gives a good description of vacuum tube circuit basics.  What I saw there led me to dig out my Sylvania tube manual and look up some data.

Last year I was rebuilding the ham-built power supply of a BC-348 and was surprised to find that the transformer HV went straight to the filament of the rectifier tube.  I had never thought about it until I had to wire a rectifier tube socket, but had assumed that such tubes (e.g., 5Y3) used indirectly heated cathodes.

But now I see in the Sylvania book that there are not only rectifier tubes like the 5Y3 that use just a filament but also those like the 5V4GA and 5AR4 that use indirectly heated cathodes.  Furthermore, such tubes seem to still have the filament hooked to the cathode internally.

So why do some rectifier tubes have indirectly heated cathodes and others do not?

Thanks,

Wayne