FA-PA50 linear amplifier with Elecraft rigs

  • Good afternoon to all the DL QRP crew


    I am sorry for the bandwidth and also for the language. Unfortunately my German is not good.

    I am thinking about the possibility of assembing the FA-PA50 kit to boost the sigs of my QRP rigs basically to join the SSB net and be able to talk to my ham friends, in a reliable way, in 40 and 80m.

    Anybody in the group has experience interfacing the amplifier to Elecraft rigs?. I understand that the band change will be manual and only an interface cable to implement the PTT will be needed. In my K2 I already added the T-R keying circuit of N0SS (SK) and should workm fine. I understand that in the KX3 I should configure the ACC2 port through the menu.

    Any comments/suggestions and advice will be welcome.


    Thanks in advance and 72/73 de Vicenç, EA3ADV

  • Hi Vincent,

    welcome to this forum.

    Sorry I cant help you but there has been a thread here already about this question about two years ago :


    K2 mit FAPA50 betreiben


    Its in German language but perhaps you can use Google translator and the included link in second posting is helpful for you.


    If you have problems with Google Translation I can try to translate it.


    73 / 72 and have a nice evening!

    Peter DL3NAA

    DL3NAA
    Name: Peter
    QTH: Kehl (JN38VN)
    DOK B14, HSC 1023, VHSC 186
    QRP von 80 Meter bis 10 Meter CW


    Life is too short for QRP!

    Satis longa vita - Das Leben ist lange genug! (Seneca)

  • Google translator

    I'll prefer https://www.deepl.com/translator

    Much much better than google. :thumbsup:


    Peter, probiers mal aus, ist klasse, das Teil. Der Tipp kam übrigens hier aus dem Forum. Bin seitdem Fan von deepl. Kannst viel verändern und passende / bessere Begriffe im Text einbauen. Zudem Made in Germany. Steht im Impressung und die Pressestimmen kann ich nur bestätigen. Deepl benutzt zwar einige Tools von google, ist aber bei weitem nicht so "gesprächig".

  • Hi Vincent,

    as written in the linked thread I used the K2 together with the FA-PA50.


    Therefore I used a modified version of the circuit described in the links:

    http://www.mwrs.org.au/2014/04…amplifier-keying-circuit/

    https://ftp.elecraft.com/K2/Mo…er%20Keying%20Circuit.pdf (new link) - I think the one that you have built in your K2.


    Just if you are interested: My modification is to use a voltage divider (out of two 1k Ohm Resistors) to use it with the connector of the KIO2 PC-Interface (that serves only 12V instead of 6V), because did not want to build this inside my K2. You can see my version in the attached pictures.

    The version without the modification should work perfectly fine.


    The setup is quite simple: You must only configure the PA to be low-active as described in the manual and connect the PTT-jack of the transciever with the one from the PA. Then the interface should work without problems, like it did in my case.


    I wish you much fun and success with your project.

    73 de Olaf, DL6OH

  • I understand that in the KX3 I should configure the ACC2 port through the menu.

    Hi Vincent,

    thats a misunderstanding. For PTT there is nothing to configure in the KX3 Menu! Just take the signal on the ring (plus ground) which pulls the external device PTT Line to ground. For correct use, the PA must be configured to low active as Olaf, DL6OK wrote in his posting.


    Here is what the KX3 manual says:


    Keyline Out and GPIO (ACC2)
    The 2.5-mm stereo ACC2 jack provides a keyline
    output (ring contact) and a general-purpose 3-volt
    logic signal (tip contact).
    The keyline output goes low during transmit, and
    can be used for transmit/receive switching of linear
    amplifiers and transverters. For keyline voltage and
    current limits, see Specifications.
    The general-purpose signal, GPIO, can be set up for
    various equipment control functions. For example,
    it can send band-change information to Elecraft
    XV-series transverters. See the ACC2 IO menu
    entry for a full list of uses for this signal, as well as
    hardware interface requirements.

    73/2 de Peter, DL2FI
    Proud member of Second Class Operators Club SOC and Flying Pig Zapper #OOO (Certificated Kit Destroyer)

  • Hi Vicenç,


    The FA-50 has a switch on the front panel to insert an attenuater.

    The KX3 is capable of much more output power than it takes to blow the final transistors in the amp.

    I know from experience :-(


    You must be sure the attenuater is switched in ALL THE TIME.

    My transistors were replaced, and the switch hard-wired so that the attenuater cannot be switched out.


    73 - Rick, DJ0IP

  • Good morning


    Many thanks to all those who answered to my post. Finally I managed to buy here in EU a Hardrock-50 unbuilt kit toguether with the ATU kit for a good price and without all the customs pain and the extra cost of taxes an VAT.

    Although I am not sure if I am going to use it too much (I am 100% QRP and CW) the main reason to have those extra 10dB was to be able to join the EA-QRP SSB net on 80m and chat with my friends with more or less reliable signals.


    Anyway, once again thanks to all who answered my post and have a great day.


    72/73 de Vicenç, EA3ADV

  • Vicenç,


    One more important point: CAUTION, DO NOT OVERDRIVE THE AMP


    The Hardrock-50 is spec'd to be driven with 5 Watts.

    Your Transceiver can put out more than 10 Watts. This amount of power will probably kill the transistors in your amp.


    I strongly suggest you install a 3dB pad inside of the amp to prevent it from being over-driven by the radio.

    I will attach the schematic of 3dB pad used inside of the FA-50 amplifier.

    The resistors are 2 Watt resistors.


    Believe me, the transistors die much faster than you think.

    I speak with painful experience!


    73 - Rick, DJ0IP


  • Maybe. Still it does not hurt to place a pad in the amp, unless you have a second QRP transceiver which only has a few watts output power.

    Replacement transistors are not cheap!


    73 - Rick