Hello the List:
I wanted to share with all the approach I took for the linear amp I put on
the air last spring. I used a 5 KW pole transformer for the plate supply.
This was a unit that was designed for a 14,400 VAC input, and used for
residential power. It had two 7,200 primary windings and the standard
120/240 VAC single phase output.
I first pulled it out of the can and let it drip dry for about two weeks. I
used some of the oil in my Heathkit Cantenna, replacing the low temperature
mineral oil that was present. It was a great way to improve the cantenna
heat capacity, and use some of the high grade oil in the pole transformer.
I was able to determine that the high voltage side in the pole transformer
was the top winding. I found two separate windings, each being 7,200 VAC
wound side by side on this core. I then cut off windings, layer by layer
until I reached a output voltage of 1950 VAC, with the now declared
"secondary" windings in parallel, instead of a the original series
configuration. The transformer came with a switch that allowed the end user
to switch between a 7,200 VAC or a 14, 400 VAC primary configuration -- I
just paralleled the two windings. Prior to this effort, I did use a
micrometer and checked the wire size, and determined I could pull .730 amps
on a continuous basis, with only the wire size taken into consideration.
I used an autotransformer and simply applied 10 VAC to the new primary
(120/240 VAC side) and used basic math to determine the resulting new
secondary output, as each layer was removed. It took an entire Saturday
afternoon, so I never cut the lawn that day or picked up the dog poop. I
did end up very close to 1950 VAC output with the 240 VAC input.
The HV supply was built up using a buck/boost transformer driven by an
auto-transformer. I used a full wave bridge rectifier using 1N5408 3 amp
diodes and a 168 mfd filter capacitor bank at 4000 VDC. I can vary the HV
from around 2600 VDC to 3400 VDC and I get no heat from this transformer
running about 1300 watts output using a pair of 3-400Z triodes. I had a
cooling fan in the power supply, but I no longer run it , due to lack of
heat and a low level of interest in the noise generated.
This transformer was made in the time frame of 1988-1992, after the PCB
situation had been made so visible. When purchased it was declared as PCB
free. So, for my $30.00 purchase price on the original pole pig, I ended up
with a plate supply that changes about 100 VDC from no load to full load on
SSB.
Chuck
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