I have had my SB-220 for about 10 years. It was meticulously built by a
real old-timer. One of its true beauties is the relatively forgiving
tuning - by putting index card behind the two capacitor knobs and
marking them with the settings for each band, I can change bands and go
in about 3 seconds, without peaking it up until I have a little break in
the fun. That also helps save me from getting into the state where my
antenna is so mismatched that arcing occurs.
By the way, my SB-220 came with a full complement of Measures mods in
the plate circuit. I took them out, put in KM1H's suppressors, gained
100 watts output on 10M, and the amplifier has been unconditionally
stable, including on 12 and 17 meters. When operating RTTY in contests,
I have simply turned down the drive (my exciter needs to be run at 1/2
CW input anyway, I believe) and it cooks happily along at 500 watts
key-down, with the switch never out of SSB position.
Your mileage may certainly vary.
73, Pete N4ZR
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On 8/29/2009 9:00 AM, Carl wrote:
> 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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>>
>
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