[AMRadio] DC Load


Don Merz don at thedjbrothers.com
Wed Aug 2 17:30:01 EDT 2006


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 Electric
and 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 connections
so 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 circuit
using 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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