[ARC5] No Doubt Dumb Tube Question

[Kenneth G. Gordon]

On 17 Jun 2013 at 12:12, Robert  Eleazer wrote:

> 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.

In my experience, rectifier tubes with indirectly-heated cathodes are far more 
unusual than vice-versa.
 
> But now I see in the Sylvania book that there are not only rectifier
> tubes like the 5Y3 that use just a filament

and 5U4 and 866 and 872 and 836 and 816 and 80 and 83...

> that use just a filament
> but also those like the
> 5V4GA and 5AR4

and 6X4 and....

> 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?

The voltage rating is the controlling parameter: rectifiers with indirectly 
heated cathodes have a much lower voltage rating than those with directly 
heated cathodes, AFIK.

I have always been unsure of why one would want to use a rectifier with an 
indirectly heated cathode at all, in fact.

Ken W7EKB