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Flex 6500 with Elecraft KPA500/KTA500 Combo - Amp Output Pulses Above 300 Watts

Skip Donnell
Skip Donnell Member
edited January 2019 in FLEX-6000 Signature Series
Received my KPA 500 / KAT500 combo for Christmas.  Got them hooked up and they were working like a charm and have for several days.   I am mostly on digital and will typically run around 300 watts out.  I am using a Flexradio 6500 and my output on it is around 22 watts.  Today, I pushed the output up to around 27 (which drove the amp over 310 watts).  At that time, the power meter started surging (sorry, best way that I can think to describe it).  It amp never went above 310, but it was like the power was being cycled on and off to the unit.  The output was pulsing high and low.  When I backed the Flex back down to 22 watts, and the output of the amp was 300 or less, this issue stopped.  I am assuming it is the protection system for the amp.  Still, I would like to know what the possible cause of this likely is so that I can troubleshoot and fix it.  Any advice that can be provided would be greatly appreciated!!



  • Doug Hall
    Doug Hall Member ✭✭
    edited January 2019
    Do you have the ALC line connected? It sounds like the Flex ALC is responsible for the power fluctuation. I have been using a KPA-500 for years with my Flex and never used ALC.
    Doug K4DSP

  • Rick - W5FCX
    Rick - W5FCX Member ✭✭
    edited January 2019
    Yeah, sounds like ALC. Mine would peak to 600 watts max but was a little overdriven - would run full out at 500 all day as I recall, no problems.
  • bobby
    bobby Member ✭✭
    edited January 2019
    22watts should give an output of 350 watts with the kpa500 if your modulation is correct so if you have poor modulation and increase your power you are creating instability and hurting the amp by surging due ti in sufficient drive. You wii most likely find with a rtty or fm modulation it will drive the amp normally. You dont need alc connected to the kpa500 at all.
  • Lawrence Gray
    Lawrence Gray Member
    edited January 2019
    If you are using ALC, I would disconnect it.  You do not need ALC with the KPA 500.  I have a 6500, KAT500 and KPA500.  I use it on digital and phone.  I just set the drive for each band/mode to provide the desired power.  I don't see any issue at all.   It just like having a 500 watt transceiver.

    Is it possible you have the KPA500 connected to a soft AC line?   You can check the HV on the KPA screen while transmitting with power.  Top left button on the KPA front panel.

    Larry, W1IZZ

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