SB200 refurb power drops off after exciter goes beyond 45 W

Discussion in 'Amateur Radio Amplifiers' started by VE3MJD, Feb 22, 2021.

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  1. VE3MJD

    VE3MJD XML Subscriber QRZ Page

    Hi.
    Did a refurb of an SB200 that I acquired. looks like I can get about 450w out with 45 w exciter but if I go beyond 45 w power drops off quickly. Transceiver to amp and load has low swr........

    Not sure why the power drop. Grid max is 30 mA and plate is approx 300 mA at 450w

    Any advice would be greatly appreciated. Caps and resistors replaced on grid bias circuit....
     
  2. W1QJ

    W1QJ Ham Member QRZ Page

    Normally when you increase drive power the amp will make more power. The amp should never put out less power with increase drive, it should always go higher. So, if you increase the drive and power then falls off that means something is breaking down. Most likely a feed line problem or an antenna system problem. That means everything and anything that passes RF beginning at the tube anode to the tip of your antenna. Something is obviously breaking down. The way to troubleshoot this is to connect your dummy load DIRECTLY to the amp and try into the dummy load. If all is good then suspect the antenna system and everything involved after the amp. If the problem happens into a dummy load then it’s something inside the amp.
     
    WG7X likes this.
  3. N8VIL

    N8VIL Ham Member QRZ Page

    If you have the ALC from the amplifier connected to your radio, disconnect it and see what happens.
     
  4. VE3MJD

    VE3MJD XML Subscriber QRZ Page

    Unfortunately I don't have a dummy load large enough yet although it is on order but delayed by Covid....What I have found in the interm is that if I run SSB and whistle, I can get expected output but if tuning using cw, the symptoms happen so more continuous power with cw causes the issue. I don't work cw, only SSB so can run as is for now. Grid and Plate currents are fine. As you say, I'll wait for the dummy load before assuming it's in the amp. ALC is disconnected. Thanks for your input.
     
  5. WQ4G

    WQ4G Ham Member QRZ Page

    Are you sure the exciter is not folding back the drive power for some reason?

    WQ4G
     
  6. VE3MJD

    VE3MJD XML Subscriber QRZ Page

    Thanks for your replies everyone. Just discovered that I had an in-line lightning arrestor rated for only 200 watts. I had forgoten about this as I had never originally planned to go high power. Took the arrestor out of the equation and now can get 500 watts cw with 55 watts exciter and around 600 watts with 75 watts exciter.
     
    WQ4G, KA0HCP, N8VIL and 2 others like this.
  7. W1QJ

    W1QJ Ham Member QRZ Page

    Something is obviously breaking down. The way to troubleshoot this is to connect your dummy load DIRECTLY to the amp and try into the dummy load. AS I SAID ABOVE AND YOU FOUND OUT WHAT WAS BREAKING DOWN. glad it is fixed
     
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    There ya go!
    [​IMG]
     

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