Hi Felipe,
> While making price quotations for the transformer for y 2 4cx1500b
> amp, the guys often asked me what its duty cycle would be.
I would say that for normal SSB and CW work, even 30% would be enough!
But if you want to use that kind of speech processor that pegs the
needle with background noise, then you need more, of course.
50% should be more than enough for any ham application, en CW and SSB.
But if you want to run RTTY contests, at full power, with long CQs and
nobody replying, you need more. And to transmit an RTTY bulletin lasting
half an hour, you need pretty close to 100%!
The other extreme is light SSB without heavy voice processing. For such
a use, 20% is enough!
> I dont know what exactly this changes in a transformer, but I imagine
> that that might have something to do with why some ham radio amps are
> so small and lightweight. hihi
Oh yes! Basically a transformer designed for intermittent service is
much smaller, lighter and less expensive. There is also a difference in
the copper-to-iron ratio. A transformer optimized for small duty cycles
will have a lot less copper, proportionally to its iron mass. This is
done because the iron works all the time at full loss, while the copper
produces loss that varies with the square of the output power. So a
transformer for intermittent service can do with a lot less copper.
It also affects the flux density level for which the transformer is
designed. Low duty cycle transformers are more efficient at a slightly
lower flux density than high duty cycle transformers.
> One of the quotes was rated at 3200v 2.5amp 140kilogram (the guy said
> they would use good qualitty core material, grain oriented)
I think he is trying to sell you a truck while you were asking just for
a bycicle! 140kg, with grain oriented steel, is heavily exaggerated for
that 8kW transformer. About one year ago I build two transformers
designed for 10kW, 100% duty cycle, and they came to 92kg each. And they
use a medium grade of non-grain-oriented steel, nothing fancy. But at
10kW they do work hot. I used class H insulation to be safe at that
working temperature.
Under normal conditions, these transformers are now being used at 4.2kW
CCS. At that low power, they get barely lukewarm. Maybe 15 degrees over
the ambient temperature.
For a legal limit tube-type class AB ham amplifier, I don't see any need
for a transformer weighing more than 30kg, if you want to run long time
in RTTY. For SSB use, even 20kg is plenty, as long as the design is well
optimized.
Manfred.
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