[ARC5] No Doubt Dumb Tube Question

[WA5CAB at cs.com]

AFAIK, all of the 5.0 VAC heater 4-pin and octal base rectifiers have 
directly heated cathodes.  The reason (or one reason) is voltage rating.  It's 
easier to get an efficient cathode with a high voltage rating by putting the 
requirement on the transformer and not on the tube.  Several of the tubes 
have PIV ratings above 1400 volts.

Robert Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480

In a message dated 06/17/2013 11:12:37 AM Central Daylight Time, 
releazer at earthlink.net writes: 
> 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  
>