It was always my understanding with the SB-220 and other older amps that
the reason for lowering the plate voltage on the 3-500's or whatever tube,
was to comply with FCC regulations. Years ago the rule was 1KW DC input CW
and 2KW PEP input SSB hence the change in plate voltage. Now that this rule
has changed I was thinking. It is my observation that the 3-500 tube
performs much better with 3000 or more plate voltage, tube makes nice power
with lower grid current for the same power at a lower plate voltage. Hence,
my idea.
I routinely use 7 1N5408 reversed diodes to replace blown zeners.
Expanding on that idea I was thinking of removing the power transformer
primary
windings from the CW/SSB switch and wire nutting the wires together that
produce the higher plate voltage, then making up a small perf board with
about (have to experiment) 11 diodes and using the CW/SSB switch to short out
4
of the diodes on CW to lower the idle current and have normal SSB idle
current when switched to SSB. That has been the modern day approach to this
on the newer amps since the FCC ruling has changed. Many hams I know went
to running CW with an SB-220 and other older amps in the SSB mode anyway.
Thoughts? 73 lou
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