On Jul 15, 2006, at 6:50 AM, Joe Isabella wrote:
> Since you mentioned that, why does the front of the SB-220 say "2KW
> LINEAR AMPLIFIER" if it was designed as a 1kW input amp?
>
> I've often wondered that...
The measured PEP output of a SB-220 is c. 1500W max with voice SSB on
40m, so the "2kW" would have to be peak input.
>
> Joe, N3JI
>
>
> ----- Original Message ----
> From: Tom W8JI <w8ji@w8ji.com>
> To: amps@contesting.com
> Sent: Friday, July 14, 2006 11:01:33 PM
> Subject: [Amps] Transformers
>
>
>> The Heathkit 220 and a few other models had around 15%
>> regulation which is right on the edge of
>> saturation (they ran a high flux density to save on weight
>> and cost).
>
> The SB220 transformer was designed for a kilowatt INPUT DC,
> which was the legal input when it was designed. This was
> about 450 mA at about 2200 volts. It was a good conservative
> design for that power level. At rated power on a good power
> line it sags less than 10%.
>
> The problem is most people seem to think it was designed for
> 1000-1300 watts CW output, and they expect a 1000 watt INPUT
> power design to be stellar at two or three times the rated
> CW dc input.
>
>> They don't saturate, but under full
>> load they're cutting it pretty close.
>
> A regular power transformer certainly does not work the way
> that statement might lead us to believe.
>
> Maximum flux density, which means closest operation to
> saturation, occurs with NO load. As load is increased flux
> levels do not increase. Increased current, because of
> resistive losses in the primary circuit, actually causes the
> transformer's flux level to decrease. When a transformer is
> designed the highest primary voltage under no load is used
> to set flux density at a safe level.
>
> The actual mechanism inside a transformer is the secondary
> develops a counter-MMF. This opposing flux would reduce flux
> density, but primary current increases in order to try and
> maintain the **same** flux density. When mains and primary
> resistances carry more current from increased load, the
> primary voltage drops slightly. This REDUCES flux density,
> moving the transformer further from saturation. Not closer
> to it.
>
> If the transformer had a separate secondary winding with
> constant current you would actually see the voltage on it
> decrease with any increase in load on the secondary with
> variable load. This voltage decrease is because flux density
> decreased.
>
> 73 Tom
>
>
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R L MEASURES, AG6K. 805-386-3734
r@somis.org
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