[Hallicrafters] Replacing rect tubes with diodes question

[jeremy-ca]

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.

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.

Carl
KM1H


----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at verizon.net>
To: "WØQFC" <erastber at tampabay.rr.com>
Cc: <Hallicrafters at mailman.qth.net>
Sent: Wednesday, February 13, 2008 12:41 PM
Subject: Re: [Hallicrafters] Replacing rect tubes with diodes question


On 13 Feb 2008 at 8:48, WØQFC wrote:

> I want to replace the # 80 in my S-40A with diodes so I can reduce
> heat by less draw on the tranny.

Can be a good idea if done correctly. You will reduce the
tranny heat by about 10 watts.

> Can I just use two diodes that will handle the current,

and voltage,

> or is there
> something else I should do ~ additional parts, etc?

You should include a resistor in series at the input of each
diode, or at the output of the two, to maintain the output
voltage at the correct voltage. Diodes have far less forward
voltage drop than a tube-type rectifier, and if you simply
replace the tube with a pair of diodes the output voltage will
be too high, especially since modern line voltages are higher
than when the receiver was built.

You MAY have to bypass each diode with a .001 mfd disk
ceramic since, in my, and others', experience the diodes can
sometimes cause RF noise in the receiver because of their
very "hard" switching characteristic.

> Also, I would like to replace the 6H6 for the same reason, but what
> diodes should I use?

You would have to use diodes with very high back
resistance. When I did what you want to do in a Scott SLR-
F, I used 1N34s, but there may be better diodes today. This
was in the 1960s.

> I know that these items are available but in the interest of saving
> money, I would like to do it myself. . . .

Easily done. Have at it.

Ken Gordon W7EKB
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