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Re: [Amps] SB-220 Output Network

Subject: Re: [Amps] SB-220 Output Network
From: Stan Gammons via Amps <>
Reply-to: Stan Gammons <>
Date: Fri, 13 Aug 2021 23:09:04 +0000
List-post: <>
Yeah, I have a Drake L-4B I use for high duty cycle modes. I don't push
it over max output with it set on low plate voltage, CW mode. That's
usually 500 to 600 watts. I have two 120mm x 120mm fans on the power supply.



On 8/13/21 10:02 AM, Rob Atkinson wrote:
>> 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
> SB220s.
> Rob
> K5UJ
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