[AMRadio] Solid State 575 mercury vapor RectifierReplacementsneeded


sbjohnston at aol.com sbjohnston at aol.com
Mon Oct 5 09:26:59 EDT 2009


Thanks to everyone for pointing out my brain lapse  -grin-  apparently 
I got confused in my calculations.

I wrote:

>To calculate the value of resistor, use Ohms law R=E/I and P=IxE where 
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.

The last line should be

P= 0.25x50=12.5 watts.

When I came up with 50 watts, I thought to myself at the time, "that 
seems like a big resistor, oh well... la-dee-dah..." and I went about 
my business instead of checking my work.  I should trust the Force when 
feel an error coming on like that.


Steve WD8DAS

sbjohnston at aol.com
http://www.wd8das.net/
---------------------------------------------------------
Radio is your best entertainment value.
---------------------------------------------------------


-----Original Message-----
From: sbjohnston at aol.com
To: amradio at mailman.qth.net
Sent: Sun, Oct 4, 2009 3:17 pm
Subject: Re: [AMRadio] Solid State 575 mercury vapor 
RectifierReplacementsneeded









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 surge20current 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/ 

--------------------------------------------------------- 

Radio is your best entertainment value. 

--------------------------------------------------------- 
 


-----Original Message----- 

From: Jim Wilhite <w5jo at brightok.net> 

To: Discussion of AM Radio in the Amateur Service
<amradio at mailman.qth.net> 

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/W
5JO 
 


----- 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 

> never had a failure, even in broadcast rigs on the air 24/7 with lots 

> 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 
aC2

> 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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