[AMRadio] Solid State 575 mercury vapor RectifierReplacementsneeded

sbjohnston at aol.com sbjohnston at aol.com
Sun Oct 4 16:17:58 EDT 2009

```Jim W5JO wrote:

>What kind of resistor do you use in this type of rectifier build to
>simulate the voltage drop?

I prefer cylindrical wirewound power resistors for this job (they seem
to be able to take more surge current than the sandy rectangular ones
in case of a fault downstream).

To calculate the value of resistor, use Ohms law R=E/I  and P=IxEwhere
R is in ohms, E is the desired drop in volts, and I is the current in
amps, and P in watts

For example, lets say your B+ is 50 volts too high.  The load draws
0.250 amps.
R= 50/.25 = 200 ohms.
P= 0.25x200=50 watts.

If the resistor ends up to be hard to find or expensive, it might be
easier to eliminate the reasons why the higher B+ is a problem.  And
more efficient too.   Upgrading some capacitors could be a reasonable
approach.

Steve WD8DAS

sbjohnston at aol.com
http://www.wd8das.net/
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-----Original Message-----
From: Jim Wilhite <w5jo at brightok.net>
To: Discussion of AM Radio in the Amateur Service
Sent: Sun, Oct 4, 2009 10:40 am
Subject: Re: [AMRadio] Solid State 575 mercury vapor
RectifierReplacementsneeded

Steve,

What kind of resistor do you use in this type of rectifier build to
simulate the voltage drop?

Jim/W5JO

----- Original Message -----

> I've made my own solid-state plug-in replacement rectifiers for a
> variety of tube types.  I look up the tube specs and select
> solid-state
> diodes that stack up appropriately for current and forward and reverse
> voltages. I usually end up using either 1N4007s or 1N5408s.  From
> on-lline sources like Digikey or Mouser or All Electronics the diodes
> are very inexpensive, ten cents to maybe 30 cents each.
>
> I put the diodes on a rectangular piece of perfboard sized to fit
> upright with its bottom end tucked within the sides of the base of the
> defunct tube it is replacing.  A small L bracket holds it in place.
>
> If you use diodes from the same "run" (which is easy if they are sold
> on the paper tapes used by manufacturers today) then I find no
> equalizing components are needed.  I add a few more diodes than the
> calculated voltage required for an extra measure of safety and have
> of lightning and power bumps to deal with.
>
> You can expect somewhat higher voltage from the new stack than the
> original tube rectifier - if that is a problem for the rig, then add a
> series power resistor to simulate the internal voltage drop of the
> earlier tube.  So far I've only needed to do that once when the new
> higher B+ exceeded the rating of the by-pass caps in the other
> sections
> of the rig.  Otherwise the rigs seemed to thrive on the higher
> voltage.
>
> Steve WD8DAS

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