[Hallicrafters] Replacing rect tubes with diodes question

[jeremy-ca]

----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at verizon.net>
To: "jeremy-ca" <km1h at jeremy.mv.com>
Cc: <Hallicrafters at mailman.qth.net>
Sent: Wednesday, February 13, 2008 2:30 PM
Subject: Re: [Hallicrafters] Replacing rect tubes with diodes question


> 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..."?

Im not following you Ken or vice versa; Im not sure which (:


> 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.
> :-)


Ive noticed that the concept of a bucking transformer seems unfathomable to 
some, never mind where to put it!


> 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


Dont need to, seen it before. It basically copied from a commercial product 
made by Perma Power (still in business) in the 50's for the same purpose. I 
picked up a 300W unit at a flea market maybe 40 years ago and its still in 
regular use with one of my SX-62A's.



>
> 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.


In that case the only heat saved in the xfmr is the rectifier filament. The 
xfmr has the same B+ load with or without the resistor. I thought hams were 
supposed to know Ohms Law?


> 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.


10W in the case of a S-40 dont mean squat if the xfmr is already into 
saturation which is the prime heat generator.




> 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.


The gain and extra current drain is more perceived than real. Its not a 
linear progression since cathode bias increases also. Its some xxx number 
less.



>
> 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? :-)


The big boxes were also on the edge, NC-183, 183D, HRO-50, 60, etc.  The 
earlier models had a decent overhead. My HRO, NC-101X, NC-200, NC-240D run 
rather cool even at 125V.  Halli wasnt alone in penny pinching.


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


More like semantics than actual techinal.


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

I dont need those but I still have my slide rule and pocket protector.

Carl
KM1H


> Ken Gordon W7EKB
> ______________________________________________________________
>