10m SB200

Discussion in 'Amateur Radio Amplifiers' started by KF5NAJ, Jan 25, 2017.

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  1. KF5NAJ

    KF5NAJ Ham Member QRZ Page

    I have a SB200 with new tubes that seems to be working pretty good, about 500w on all bands except 10m.

    The amp tuning on 10m is very critical or touchy, if I slightly move the load control the output power will jump 40 plus watts and the swr jump also.
    The other bands tune ok with a slight increase/decrease of power with normal adjustments
    I have checked all the coax and feeding into a dummy load
    I can get about 480w on 10m with very careful tuning
    *Is the 10m band normally a little harder to tune?

    I have also observed that sometimes peak power will be greater when tuned into my antenna (5BTV) then the dummy load, what does this indicate?

    Thank you
  2. W1QJ

    W1QJ Ham Member QRZ Page

    The SB-200 IS VERY touchy on 10 meters. A slight hair movement on the plate or load and you loose power. the "sweet spot" is very very critical. This is nothing new with the SB-200, they are all like that. The input match you will see is most likely pretty bad. Often times your radio swr meter will read a very high input swr yet, your radio won't fold back and you can still get good power output. Also you may see the input swr go wildly as you adjust the load. Often times the input circuit needs some TLC a difference in capacitance or some turns removed from the coil. It all depends. The input circuit isn't good for solid state radios. SO what you are seeing is very typical of this amp. As far as the power output you see into dummy load vs antenna, if your antenna isn't 50 ohms resistive like the dummy load you won't read the same power going to it. The reading will be false one way or the other.
    AF6LJ and K4RKY like this.
  3. AI3V

    AI3V Ham Member QRZ Page

    Pretty much every wattmeter ever made is really a voltmeter that is calibrated in watts into a 50 ohm non reactive load.

    So if you have any vswr your meter will be wrong.

    There is another issue, there are always at least 2 different impedances for any given vswr.

    They will give very different readings on a voltmeter calibrated in watts.

    Here is a link for a manual for a ten tec corsair, there is a chapter in the manual called "the 2 types of swr" I think you will enjoy reading it.


  4. AI3V

    AI3V Ham Member QRZ Page

    One other thing.

    Most rigs use a rf voltmeter calibrated in watts to set the radios output power via alc.

    The ten tec manual will describe how the two types of vswr effect the final transistor collector current and power output.

    Its basic ohms law math and a real eye opener.

    Iow, not only does your wattmeter accuracy suffer from vswr, the vswr causes the actuall power out to be much different than you think it is,and different than the rated power.

  5. KF5NAJ

    KF5NAJ Ham Member QRZ Page

    Thank you for the replies, I was a little concerned about 10m but since the issues is typical to SB200 I will be extra careful tuning 10m
    I never realized that there was so much to vswr, a lot to learn.

    Thank you again
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    If you're mechanically handy (and this is purely mechanical work) you can add 10:1 reduction drives to both PLATE and TUNE capacitors on the amplifier.

    I've never done that with an SB-200 but have done it with an SB-220 and I think it shouldn't be any more difficult with the 200 unless there's something really in the way that I don't recall.

    They look like this:


    and give you a 1/4" shaft for the knob with a coupling for another 1/4" shaft for the capacitors. They take up a bit of panel space but if nothing is interfering can be installed in an hour or two (some disassembly and reassembly required!), and making "tuning" 10:1 slower than direct-drive.

    I see guys selling these on eBay for about $7-8 each. They work.
    KC8VWM likes this.
  7. KF5NAJ

    KF5NAJ Ham Member QRZ Page

    They are pretty neat, I will look into it

    Thank you for the effort

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