I'm building a 'retro' transmitter for high quality amplitude
modulation. The RF part is to be a Johnson 500 (single 4-400A), and
the power supplies, modulator and control relays are homebrew. To
pay homage to tradition and have the bouncing violet glow when I say
1-2-3, I started with 866A mercury vapor rectifiers. But due to space
limitations on the big chassis and the size of the ancient iron I
have acquired, now I am using silicon diodes. What was interesting in
all this is that after reading old handbooks (both ARRL and Editors
and Engineers - pre Bill Orr and after) you can observe the trend
away from L input filters and center tapped secondaries with 2
rectifiers >>>> to Cap. input, 4 diode full wave brides, or doublers
for SSB amps. With the change to silicon came higher peak current
ability (than tube rectifiers) and the change in filter
configurations. I hear some of the folks here using as much as 100 uF
of capacitance, unheard of 30-40 years ago. Back then it was big
iron, swinging chokes, smoothing chokes, 4-8 uf of cap, low peak
currents, low noise, and fairly good regulation with load changes (as
long as a resonance wasn't hit during modulation). Resonant input
chokes were good here also.
I have heard that some of those old CT secondary transformers can't
run FWB at full tilt without risking punch through near the center of
I thought that iron was going to be hard to come by. Two local
surplus junkyards had more than I could hope for, anything from 1500
to 3500 VDC, single and three phase, 0.2 to 1 Amp DC. To top it off,
I now have THREE plate modulation transformers, all variable tapped,
from 300 to 600 W capability. All this for about a hundred dollars. A
pair of 813 beam power tubes will be modulators, running AB1.
This is the fun stuff in ham radio that I had nearly forgotten about.
Thanks to this mailing list, I have been inspired to do it.
If anyone needs plate iron, I got the place. Its a little rusty though.
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