A new Toy

On Thursday I received my field test version of the KPA500, Serial Number #15
It’s the soon to be released 500W solid state linear amplifier from Elecraft covering the HF and 6m bands


This picture shows the KPA500 in the place where My ACOM1000 Normally goes, The ACOM just fits in this space.

This is my first experience with a solid state amplifier and I must say it’s been smooth sailing so far.  it’s certainly very nice to have an amplifier that has no 3 minute warm up period. Just switch on and it’s ready to go. I have had a few hundred QSO’s on it already and have not run into any issues so far

The amp is the same size as the K3 and uses an internal linear Power supply so it’s quite a bit heaver than the K3, weighing in at around 13.5Kg (including shipping materials, see comments)

The amplifier is rated to develop 500W into loads that are 2:1 SWR or better, band switching is automatic, it simply senses the  RF and switches band as needed, this happens nearly instantaneously, before you have even completed the first ‘dit’ it has switched in the appropriate bandpass filters . The metering is very effective and compares reasonably well with my LP100 external power meter.

The amplifier produces the Irish legal limit of 400W out for around 20W drive

Hooking it up with the K3 requires  RF connections, Power (120 or 230V) and a PTT connection. No need to hook up ALC, if you overdrive the amp it will automatically switch in an attenuator and flash the fault LED  to protect it’s self, if you persist in over driving the amp will take it’s self off line.On the rear there are also an ALC input and RS232 for firmware updates. The metering is extensive, the LCD display can show various parameters such as the Heat-sink temperature, ‘HV’ voltage, PA current, SWR, Power and defaults to displaying the current operating band.

transmit/receive switching is with PIN diodes it’s full QSK and silent, The internal fan comes on in 4 stages and is reasonably quiet, the slowest speed does not engage until the amp’s heatsink warms up to 45 degrees C so for light duty operation and in standby or receive the amp is utterly silent.

Into a dummy load the KPA500 will generate up to 650W before it brings in the attenuator, a look at the spectral purity of the output on my SDR-14 software defined receiver shows a very clean output signal.

The KPA500 has sophisticated microprocessor controlled protection systems built in and is designed with very ample safety margins. Mechanically it is very well built and sturdy. Internally there are Transformer taps covering 100-120V and 200 to 240V.  This would be an ideal amplifier for Dxpeditions and IOTA activations.

It looks like my ACOM1000 may soon be for sale

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8 Responses to A new Toy

  1. Dale says:

    Nice report. I’ve been following the KPA500 with much interest. The most problem I see with only a 500 watt amp at the $2000 US price point is if it cuts back with higher SWR and if the mains line voltage drops resulting in lower power out. Soon with these wrenches in the gears the power out becomes not much better for the money spent. Your report sort of states other wise. At 2:1 swr the rated out would still be at least 500 watts? Maximum out you seen was over 600 watts, I wonder what conditions allowed that, such as perfect 1:1 antenna match and perfect mains voltage?

    Anyway, enjoyed the report and I’ll keep watch on this amp, it’s very interesting and would match up beautifully with my mighty K3.

    tnx 73
    Dale, K9VUJ

    • ei6iz says:

      My line voltage goes from 246V Daytime to 226V Nighttime, the KPA500 still makes it’s rated output output and then some with the reduced line voltage, it can still reach the trip point of 650W at the reduced line voltage. With the reduced line voltage it needs a couple of more watts of drive but it still has enough headroom to produce a very clean 500W.
      I am using the 240V tap here.

      The amp is rated to full power into 2:1 or better and it comfortably delivers this, my 30m antenna is 2:1 as is my 160m Inverted L above around 1860 and my 40M antenna outside the EU CW Sub-band.
      The mostfets used are rated for 300W (minimum) each into 3:1 or better so it’s well within it’s ratings to do this
      you can see the datasheet for the VFR2933 on-line at http://www.microsemi.com/datasheets/vrf2933_a.pdf

      the KPA500 uses a pair of these.

      The last thing to always remember is that the ‘usefulness’ of power is best measured in decibels, the difference of 50W with reference to 500W is only 0.4dB so very little is gained by running an amp over it’s specified design rating, I doubt anyone will be able to detect a 0.4dB extra except perhaps with accurate measuring equipment.
      However going from 50W to 100W is 3db (~1/2 S point and worth having)
      Going from 100W to 500W is 6.9dB
      Going from 100W to 650W is 8.1dB
      Going from 100W to 1Kw is 10dB
      Going from 100W to 1.5Kw is 11.7 dB

      Elecraft Spec it to 500W, the protection kicks in at 650W but why run it habitually above the design spec?
      The advantages this amp has is that band changes are instantaneous and automatic and there is no 3 minute wait for the Tube to heat up.
      It’s also Small, relatively light and can be run from a modest generator
      If you want (a lot?) more power then there are lots of other options, For me with our Irish power limit of 400W (outside of contests) the KPA500 is a perfect fit.

      Brendan EI6IZ

  2. Tom says:

    Hi Brendan. I linked to your page based on your call-sign, which I got from the K3 reflector. I read your recent post about why you bought a K3 and enjoyed it – was well written and thoughtful.

    I was excited to see you received a KPA500 a few weeks ago and hope it’s still working well for you. I do plan to purchase one this year and figure once the initial first batch or two are produced, I’ll be ordering one myself.

    Elecraft has proved to be a very interesting company and the K3 of course, a great radio. I have only used support once and it was excellent. The modularity, performance and continued firmware enhancements are impressive to say the least w/so much forethought being put into the radios design to allow for these to occur.

    Anyway, good luck w/the rig and amp.

    Tom / W1TXT

  3. N6TV says:

    Hi Brendan,

    Are you sure that your KPA500 weighs 13.5 Kg? I carefully measured mine (without a power cord) using a digital luggage scale, I got 26 lbs (11.8 Kg).

    Bob, N6TV

    • ei6iz says:

      HI Bob

      I haven’t access to a luggage scale 😉 So the 13.5Kg figure is based on the Shipping weight that Elecraft used. I Guess your figure is more accurate.
      I can say that the KPA500, including packaging materials, shipping container and US mains cable is under 13.5Kg .
      Elecraft Shipped it in a sturdy box with 2 high density Foam ends designed to protect the KPA500.
      Brendan EI6IZ

  4. Bill says:

    Does the KPA 500 fault to a needed manual reset if swr limit is exceeded or does it roll back until swr issue is fixed within a preset time limit?

  5. Doug VE3VS says:

    Hi Brendan,
    I think that your finger missed a key as you typed the following comment. Did you mean to say the difference between 650W and 500 W is only 0.4db…….

    from Feb 23, 2011…..
    “The last thing to always remember is that the ‘usefulness’ of power is best measured in decibels, the difference of 50W with reference to 500W is only 0.4dB so very little is gained by running an amp over it’s specified design rating, I doubt anyone will be able to detect a 0.4dB extra except perhaps with accurate measuring equipment.”

  6. Petr says:


    as Brendan wrote:
    “the difference of 50W with reference to 500W is only 0.4dB”
    … which means increasing power from 500W to 550W will make 0.4dB which is correct.
    Of course because of log scale at 600W ref level the increasing from 600W to 650W will make only 0.34dB etc.
    If we are talking about 500 to 650W increasing then it is 150W higher from 500W ref level which will present +1.14dB.

    Hope it helps,
    73 – Petr, OK1RP

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