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[Amps] SB-220 Output Network

To: "" <>
Subject: [Amps] SB-220 Output Network
From: Rob Atkinson <>
Date: Fri, 13 Aug 2021 10:02:51 -0500
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>Hopefully some of you here may remember my adventures with this SB-220.

>I'm the one that ran RTTY at full power in a RTTY Contest and literally melted 
>down the HV transformer.

>And in the process toasted a LOT of other stuff with it.

>So a complete re-build and change of the HV section was needed.....

SB220s are designed for light duty cycles.  SSB and CW.   It is
probably obvious to you by now that they were not made for full strap
at 100% d.c. RTTY for any length of transmit time.

You don't need higher secondary voltage on the plate supply
transformer; you need a transformer that is rated for higher current,
probably at least 1 amp or 1.2 amps so you don't have much temperature
rise.  You may need higher RF current capable fixed value capacitors
that augment the load air variable on the pi output network.  You need
much better tube cooling, i.e. higher CFM.  Better yet, tubes in
chimneys.  The bandswitch may be marginal; ditto for the output
coil(s) and plate choke.

The upshot is that you're trying to turn a light duty ham RF amplifier
into a CCS grade amp; something made by Harris, or Scientific.  I'd
look for a different amplifier made for this.  Something like an old
Henry console, maybe a Drake L4B which could be improved a bit, or one
of the early Dick Erhorn Alphas.  Brad Thompsons that ran 3-1000s may
be okay.

Also, some RTTY operators go with old Heathkits like the Warrior.
It's a 500 watt amp on RTTY, but that sucker is built to sit there and
put out 500 watts all day.  A Warrior weighs about as much as three

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