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using broad banded bricks for amateur radio

Discussion in 'Amateur Radio Amplifiers' started by KR4IS, Sep 6, 2017.

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  1. KR4IS

    KR4IS Ham Member QRZ Page

    In the 80s I was a qrpp guy using a tentec Argonaut 509. Then I wanted more power and I had a 150 watt brick I used for cb...with a resonate ant I would work 40m cw at about 80w. Didn't drive it hard...So fast forward and want to use my mobile 10m system on 40m..the system is a 10/11 meter radio into a 2 transister driving a four transister linear amp. I would replace the hr2510 radio with a knwd 2000 ,,other than not over driving and being sure there is perfect 52 ohm load on amp what should one worry about.
  2. KF5FEI

    KF5FEI Ham Member QRZ Page

    Most CB amps were designed and built using circuits outlined in the output device data sheet or app notes -- and did not include any filtering. You'll need to use filters or the amp will emit spurs all over the place. Fortunately, some of the QRP amp kit providers also have filter kits, so all you need to do is select the band you want, order the kit, and build it.
    AF4RK likes this.
  3. N2EY

    N2EY Ham Member QRZ Page

    What is "a resonate antenna"?
    AF4RK likes this.
  4. N7EKU

    N7EKU Ham Member QRZ Page


    Broadband push-pull transistor amplifiers typically are only down 13dB on their 3rd harmonic. It has nothing to do with over-driving them, it's just the fact that transistors produce harmonics due to their internal capacitances and varying gain.

    That means when you were transmitting 80W on 40M, you were also transmitting around 4W on the 15M band. There were probably some guys wondering why you didn't answer back!

    As to your question. First, those CB brick amplifiers are not linear because they don't have a decent biasing system; this means use should be restricted to FM, CW, and some frequency modulated digital modes. The bias system can be upgraded through a homebrew circuit if you know what you are doing.

    Second as mentioned above, you would need a low pass filter on the output for each band that you want to use (some filters can cover more than one band though). This is to prevent transmitting your harmonics on other bands and other parts of the radio spectrum (e.g. police, fire, and emergency services). Communications Concepts website is a good place to get these different filters.

    Ten-Tec actually made a matching broadband linear amplifier for the 509. You can still find them on eBay and sometime on ham classifieds sites.


    WA7PRC likes this.
  5. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    There are "legit" SS amps in this power range, even RM Italy has one that's FCC certificated. It does have a bandswitch and lots of low pass filters.

    Ten Tec still has one in their catalog, but I have no clue if they're actually shipping. Elecraft does, too, and they are shipping.
    WA7PRC likes this.
  6. AI3V

    AI3V Ham Member QRZ Page

    CW, and any other mode that has a amplitude variation require a linear amplifier.

  7. N7EKU

    N7EKU Ham Member QRZ Page

  8. AI3V

    AI3V Ham Member QRZ Page

    It's the rise and fall times of the CW waveform that is the issue.

    A class "c" amp will shorten, or eliminate them and give rise to sidebands (key clicks).

    The whole misconception comes from the bs painted on the front panel of illegal cb amps, trying to suggest they are legitimate amateur CW amps.


    I suppose it's also a bit of semantics, amateur Morse is a type of on/off keying, vs a true continuous wave
  9. K7JEM

    K7JEM Ham Member QRZ Page

    There are lots of CW rigs that use class C amplifiers as their finals. The major thing to beware of is the rise/fall times and how they pass through the amplifier. Once these are accounted for, there would be no problem using a class C amplifier, in fact for a good operating CW transmitter adding a class C amplifier shouldn't change things that much.

    If you start with key clicks, then they could be made worse, that would be the main concern.
    N2EY likes this.
  10. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    The CW rigs using a class C final stage were also "keyed" at the final stage, usually. At least my old tube-type Class C CW rigs always were. Cheap stuff used cathode keying, better stuff used grid-block keying, but the keyed stage was actually the final amplifier.

    That's different.

    Are there any solid-state CW rigs today that use Class C PAs? There might be, but I can't think of any offhand.

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