[Hallicrafters] Replacing rect tubes with diodes question

[Kenneth G. Gordon]

On 13 Feb 2008 at 13:03, jeremy-ca wrote:

> Gotta disagree with you Ken, a 20-30V increase will hurt nothing
> except in an unrestored unit on the edge of failure OR a radio already
> at the edge by design.
> 
> The most significant problem with many Hallicrafters transformers is
> that when run at 125V they are already partially into saturation and
> generating excessive heat. Get the line voltage down to 110-115V as
> the first step.

I agree with you on this last, Carl, but doesn't this suggestion, taken at 
face value, sort of contradict the statement just above this one, at 
least up to "OR..."?

Most people I know seem to have far more trouble with bucking 
transformers than with resistors and diodes, maybe because the 
bucking transformer can't normally be mounted in an old octal socket. 
:-)

However, that being said, I also MUCH prefer the bucking transformer.

See this URL for some details:

http://www.mines.uidaho.edu/~glowbugs/receivers.htm

9) Line Voltage Adjustment

> You can test the saturation with a Variac and see where the secondary
> increase stops being linear. My SX-71 for example started to limit at
> 123V and the SX-62A at 124V which is my normal line here. Both had
> been recapped and the tube rectifiers in place; I was very surprised
> at how hot the xfmrs ran and decided to do some testing. Leave it to
> Halli to save pennies on the iron.

Yes, and swapping to diodes with series resistors lowers the 
temperature of the TRANNY, although, as you correctly point out, the 
overall heat dissipated by the entire radio doesn't change at all, since 
the resistors dissipate the heat the tranny and/or rectifier tube would 
have.

Personally, I think, not having tested it, that the main source of 
reduction in tranny power dissipation is in the elimination of the 
rectifier FILAMENT CURRENT requirements.

By using series-resistors with diodes, you would be preserving that 
gain by keeping the B+ current drain at the "normal", or pre-diode, 
levels, rather than by letting it increase without the resistors, thereby, 
at least partially, negating any gain you might have gotten by 
eliminating the rectifier filament current drain.

In National receivers, wherein the power transformers were not under-
rated as they were in many Hallicrafters receivers, direct replacement 
of rectifier tubes with diodes would require far less concern over 
transformer dissipation, wouldn't it? :-)

I think we agree on the basics: we just disagree a bit on the details.

Besides, if I were really an engineer, I'd be the "belt and suspenders" 
type. 

Ken Gordon W7EKB