KPA-1500 efficiency with a Low-Power Setting

[Elecraft mailing list]

I would like to see the KPA-1500 have a "Medium Power" setting with optimized bias for that power level so the amp could be operated at 750-watts output and good efficiency.  While the KPA-1500 can operate FT and RTTY contests at 1,500 watts, I believe the amp operated at 750-watts with an optimized bias setting for that power level would generate less heat and less fan noise, both of which would be very desirable when full power isn't required.   

I'd sure appreciate Elecraft adding a half-power setting with optimized bias at that power level.  While the KPA-1500 can be run at half-power now, the efficiency isn't very good.
73,Dick- K9OM

In a message dated 2/10/2020 9:14:07 AM Eastern Standard Time, [hidden email] writes:

Andy,    Yes, you are right and this applies to both KPA 500 and KPA 1500.    Theconditions for maximum efficiency are not necessarily those for minimumdissipation or reduced stress.    I would like to know if running 2/3, 1/2or even 1/4 maximum power output increases reliability and MTBF.    It wouldbe a considerable surprise to me if this were not the case and it would begood to know the numbers.    Improvement in MTBF can be several fold forsome devices for a reduction in voltage, current or power of as little as10%.    Good design practice normally means that this is part of theengineering design process.  The proof of derating is looked for in designreviews.  Too many hams run their amplifiers flat out for very littlebenefit.  Signal cleanliness is still another issue; higher voltage butlower power seems to help here.
    Using FT8 or WSPR on dead band (160M in daytime or 12M), I sometimesrun 20 Watts plus.  This can be stressful on the K3 especially for WSPR.I crank the K3 down to a watt or so and drive the KPA 1500.    Yes, my totalenergy consumption is no doubt up but stress on either K3 or KPA is minimal.This however is not my concern, I would like to know if running the KPA atlower power increases reliability.  In EI we can run 1500 Watts forcontests  but outside of contests can only run 400W max.
                              73 Doug EI2CN

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[Ignacy]

The bias would do nothing for low power.
Low power can be implemented in two ways, with different taps at final
transformer (KX3), and with different supply voltage (Expert amps).  The
first one is hard at a KW level. The second one requires  power supply with
different voltages. If the power supply can deliver only one voltage, not
much can be done.

Ignacy, NO9E



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