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Driving KPA500 with 2W for digital work?

Rob Fissel
Rob Fissel Member
edited May 2020 in Third-Party Software
So I've had a KPA500 up and running for a few weeks now. I do a fair amount of digital work, and was thinking to myself...

Instead of bypassing the amp, and using the Flex to run, say, PSK @ 30 watts, why not excite the amp with 2W to get about 30W out? Frequently, I'll get into rag chews where the rig is keyed down for upwards of 4-5 minuets, and the rig will start to get pretty warm. I figure, why not let the amp do the heavy lifting, and save a little life on the Flex's transistors. 

Is this ok? I have read that IMD starts to suffer at low drive levels, but I haven't found much out there regarding this phenomenon with solid state amps. 



  • Norm - W7CK
    Norm - W7CK Member ✭✭
    edited February 2017
    I do it all the time.  I have never had a problem with distortion or anything like that due to low drive levels.  Although I've run my drive level down to 2 watts or so, most of the time I am driving it will a bit more than that.  I usually run about 50 watts out of my amp when running digital.  This is because of the poor antenna I'm using.  I guess you could call Elecraft if you have doubts about driving with only 2 watts.

  • k3Tim
    k3Tim Member ✭✭
    edited May 2020
    Whereas PSK is a constant envelope mode, amp nonlinearity shouldn't be a concern, correct? IMD is concerns are for SSB. You should be able to use a class C amp for PSK. I believe this is correct but someone knows for certain, PSE jump in
  • Doug Hall
    Doug Hall Member ✭✭
    edited July 2015
    No, PSK31 actually isn't a constant envelope mode, although it sounds like one. Unlike RTTY it does not have a crest factor of 1. Data is encoded by shifting the phase of the signal 180 degrees (for a zero bit) or not shifting it (for a one bit) and subsequently the signal is filtered to smooth rise and fall times. This eliminates discontinuities (clicks) and minimizes occupied bandwidth. A linear amplifier is necessary to get the benefit of this filtering.
    Doug K4DSP

  • Norm - W7CK
    Norm - W7CK Member ✭✭
    edited February 2017
    I put mine up on 20m PSK this evening and ran about 2 watts into it without any problems at all. 
  • Rob Fissel
    Rob Fissel Member
    edited July 2015
    Got a quick response from Elecraft, for anyone that is interested:
    You can drive the KPA with 2 watts as you suggest. We have customers driving the KPA with their KX3's and it works fine. You will not have a IMD issue driving the amplifier with a low drive power.
  • Dave - W6OVP
    Dave - W6OVP Member ✭✭
    edited June 2017
    Part of the magic of the digital modes is they force us to use low power because our transmitters literally cannot "take the heat". This limitation has created a new low power paradise which makes it possible for QRP (for various reasons) hams to compete, and the virtues of digital to be demonstrated.

    I shudder to watch the power slowly creeping up from a few watts to more, and dread the day when amplifiers that can run "1500W, 100% duty cycle, no time limitation, key down forever" find a home here. Not that I would personally mind having a little more power on JT65 of course (cough cough) ... <g>.
  • Rob Fissel
    Rob Fissel Member
    edited July 2015
    I'm with you, Dave.

    FWIW, the Alpha amps are actually already rated at 100% key down full duty continuous @ 1.5kW. Imagine a JT9 signal at a full gallon and a half. 

    For the most part, I run PSK anywhere from 15-30 watts. I figure I might as well offload that work onto a beefier PA. 

    JT modes I rarely ever go above 10W, and that's after a few attempts at lower power to work another station. When calling CQ, I almost always run Joe Taylor modes at 500mW, WSPR included. 

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