A few years ago I built something pretty similar to K4EAA's design,
which can be found (with great pictures) at:
http://www.k4eaa.com/dummy.html. Has worked FB for measuring low power
stuff: mostly the radios before the amp. I have a bigger palstar model
for amp work, but have often thought of building another homebrew can as
well, and upping for 2kW or so. I'm sure with some value tweaking it
could be done easily. The LP version cost me under $40.
Mike N1TA
-------- Original Message --------
Subject: [Amps] Dummy Load Blues
From: "Jerry Kaidor" <jerry@tr2.com>
Date: Thu, June 14, 2012 7:54 pm
To: amps@contesting.com
Hello,
To mess with amps, a good dummy load is a must. I went to my storage,
and pulled out a Cantenna of unknown provenance that I had purchased
about
30 years ago at a flea market. Not knowing what was in it, I assume it's
poisonous PCB transformer oil. I had kept the dummy load in a plastic
bucket
with a hole for the coax, sealed with coax seal. Now I figured I'd
dispose of the old oil, clean it out, and put in fresh mineral oil. What
a mess!
OK, the old oil is gone; it'll be headed to the local hazardous waste
dropoff. The guts are all washed out in hot water and detergent. The
heathkit can is toast - rusty and leaking. The silver
straps around each end of the resistor are broken. So I can reconstitute
this thing with new silver straps ( luckily I have some silver sheet
laying around ). And the can - can be replaced with an ordinary one from
the paint factory. Only trouble with that is - 1 gallon paint cans are
now *plastic*, not metal. I guess it'll still work, just the heat
transfer won't be quite as good.
I also scored a 250W load cheap on Ebay. Seller said it came out of a
cell phone installation. I popped it open - the resistor is the size
of a postage stamp! I was messing with one of my SB2-LA's, used that
load - figured a momentary toot from the amp wouldn't hurt it. I was
wrong. Apparently these little resistors are not only heat limited,
but *voltage* limited. I hit transmit, there was a small *pop* and a
puff of smoke. Hole burned right through it.
There are similar load resistors on sale on Ebay - 800W for IIRC $40.
But that's just the resistor, you have to bond it to a heatsink. I
have
the heatsink from the defunct 250W load - I could machine off a big
enough
flat spot for the 800W resistor.... failing that, I have a really big
heat
sink in my junk.
- Jerry Kaidor, KF6VB
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