Many operate the SB-220 on RTTY and digi modes where the CW position is a
key to transformer survivability.
The same for 12/17M where the tank circuits are far from optimum and
bandswitch arcing can occur at 1200W.
Some owners are a bit slow in tuning up and the CW position reduces the
plates from brilliant orange to a darker version.
Carl
KM1H
----- Original Message -----
From: <Gudguyham@aol.com>
To: <amps@contesting.com>
Sent: Saturday, August 29, 2009 7:09 AM
Subject: [Amps] SB-220 bias question
> 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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