[AMRadio] DC Load


KB2WIG at twcny.rr.com KB2WIG at twcny.rr.com
Wed Aug 2 18:19:49 EDT 2006


Somewhar in my files, I have a simular circuit...If u like, I'll 
rummage through and see what I can find..... I think it utilized a 
100th as the variable resistor....   klc

----- Original Message -----
From: Don Merz <don at thedjbrothers.com>
Date: Wednesday, August 2, 2006 5:30 pm
Subject: [AMRadio] DC Load
To: amradio at mailman.qth.net

> A local ham SK built himself a variable DC load which I recently 
> found in
> the N3BM estate stuff. The ham was W3QNI. QNI worked for Western 
> Electricand was a master builder. He must have died 10 or 15 years 
> ago. I have one
> of his transmitters and it is almost a work of art. This "Variable 
> DC Load"
> is no different.
> 
> So this is an aluminum chassis with 2 transformers, 2 tube sockets 
> and 2
> meters mounted on it. Controls are simple--one on/off toggle and 
> one pot
> labeled ADJ LOAD--MIN--MAX. There are two binding posts on the 
> front apron
> labeled positive and negative.. The Western Electric meters are 
> mounted on a
> phenolic panel that sticks up above the controls. The left hand 
> meter is
> scaled 0-30 VOLTS DC X100. The right hand meter is 0-300MA. A 
> label across
> the top of the panel says "VARIABLE DC LOAD  2500V 300MA MAX". The 
> two tube
> sockets are empty. They are the large old EFJ ceramic sockets for 
> twist-lock
> tubes like 203, 211, 805, etc. There are no grid and plate cap 
> connectionsso the sockets must have been intended for single-ended 
> tubes. The socket
> connections are wired in parallel--plate to plate, grid to grid 
> and fil to
> fil.
> 
> One transformer has 150V/50ma and 6.3V 1.5A secondaries. The 6.3V 
> just runs
> the pilot lamp. The 150V is wired, with a silicon diode, 
> electrolytic cap
> and two big ceramic resistors into the circuit with the load 
> adjustment pot.
> The other transformer is unmarked, but it provides filament 
> voltage to the
> tube sockets (in parallel) and the center tap is connected into 
> the load
> adjustment circuit. Filament voltage at the socket terminals 
> measures 10VAC.
> 
> I went through my files and found a photocopy of a very similar 
> circuitusing 6BG6 tubes in parallel. But I guess I'm not sure what 
> tubes this one
> was intended to use--likely something that is expensive these 
> days. I'd like
> to put this thing to it's intended use. But I don't want to put 
> $100 worth
> of tubes into it. What are the options? Any ideas appreciated.
> 
> 73, Don Merz, N3RHT
> 
> 
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