Heathkit SB-200 Power Out ??

Discussion in 'General Technical Questions and Answers' started by N7SGM, Nov 17, 2009.

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  1. N7SGM

    N7SGM Ham Member QRZ Page


    Just curious as to why my amp will put out the max power on 40 meters? On other bands it is less. 80 meters is the worst <50W.

    Thank you.
    de Bob
  2. W9GB

    W9GB Ham Member QRZ Page

    Are these RF power measurements to a dummy load, or your actual antenna?

    If with your antenna, then what are you using for an antenna on 40 and 80 meters?
    I will assume that the SB-200 band switch contacts have been properly cleaned with DeOxit.

  3. K9STH

    K9STH Ham Member Staff Member QRZ Page


    You should see between 600 and 650 watts on 80 and 40 meters and close to that on 20 meters. On 15 meters the output should drop to around 500 watts to 550 watts. On 10 meters between 450 watts and 500 watts.

    If you are running more output then you are running the 572B tubes too hard.

    You need to check 2 fixed capacitors that are used only on the 80 meter band. There is a 100 pf at 5 kV that is in parallel with the plate tuning capacitor and a 500 pf at 2 kV in parallel with the antenna loading capacitor. If either of these capacitors is bad you will not get full output on 80 meters.

    Also, you need to make sure that your exciter is not "folding back" on 80 meters. The SB-200 was designed to be used with exciters using a wide-range pi-network output and therefore the input impedance to the SB-200 can be considerably different from 50 ohms. This "mismatch" will cause most "modern" (solid-state) transmitters/transceivers to "fold back" (not put out as much power). With less power on the input the linear will not put out as much power.

    Glen, K9STH
  4. VE3FMC

    VE3FMC XML Subscriber QRZ Page

    Ditto what Glen said about the exciter folding back it's output power.

    I have an SB-200 and if I run it behind my Icom 718 I need to run a tuner between the amp and the radio so I can tune the swr input between the amp and radio. If I don't do that the amp will not load up to full output.

    It may be that the input SWR is good on 40 meters but high on the other bands. So you will see full output on 40 because the rig is not dropping it's output down.

    Try running a tuner in between to tune down the SWR between the rig and amp and see what happens.

    I was told by Bill VE3NH who put in the Harbach mods on the amp I own (Bought it from the owner who had these mods installed by Bill) that he could not tune out the high input SWR between the SB-200 and his Icom 756 Pro III. But when he ran the auto tuner on the rig then the amp loaded right up.

    I normally run the amp with a TS-520 so the swr input is not an issue into the radio. What I do is load the rig, then the amp then reload the rig back into the amp to get the full output from the amp. Works fine for me.
  5. N7SGM

    N7SGM Ham Member QRZ Page

    SB-200 Output ??

    Thanks for the quick responses. I should have given more info to help you answer my question. The radio is a Kenwood TS-450S/AT @ 100W. I use the onboard tuner for my Hustler 6BTV, vertical, ground mounted antenna. I also have 20 radials in the ground with various lengths.

    The Heathkit SB-200 was purchased this past summer with no known issues. The tubes are 572B's, Chinese manufacture, with unknown hours on them.
    I installed two Harbach mods; soft key and soft start, myself without any known problems.

    During my tuning procedure I first tune the antenna and then adjust the amp.
    I have been careful to not exceed mid scale when adjusting the plate current with transceiver drive.

    80 meters is the worst band and mainly due to the antenna which is not noted for its 80 meter coverage and is not a big issue for me. I'm sure the SWR values are causing the weak output. Foldback, as noted by Glen, is certain due to poor antenna tuning SWR !!

    40 meters - 500W
    20 meters - 400-450W
    15 meters - 300W
    10 meters - ???

    I'm not complaining, just not sure of what to expect. I do not use a dummy load of any kind. Values are taken via a brief CW transmission to establish grid and plate current measurements. I use a SWR/Watt meter to measure the output wattage. I do need a good outboard tuner, that's for sure. The Kenwwod radio has a good onboard tuner but I realize it has limits. Perhaps a good tuner will be the best thing for this issue.

    Thanks again for the valued comments !!!

    de Bob
    Last edited: Nov 18, 2009
  6. K9STH

    K9STH Ham Member Staff Member QRZ Page

    The antenna is not going to cause that kind of problem when the SB-200 is in use. There is no "fold back" circuit in the SB-200 and, due to the fact that the SB-200 is between the exciter and the antenna, the "SWR" that the SB-200 "sees" is not going to have any effect on the exciter.

    If you are using a wattmeter on the output of the SB-200 you should see considerably more output on the meter if the SWR is high. This is because you have to subtract the reflected power reading from the forward power reading to get the "true" power output.

    The SB-200 can handle a lot higher SWR than the Heath "guaranteed" specifications (Heath states maximum SWR of 2:1). I used an SB-200 from the late 1960s until the mid-1970s and definitely ran it into higher SWR of 2:1. I do have another SB-200 on its way right now to finish the re-creation of my 3rd primary station from 1967 until 1973.

    Glen, K9STH
  7. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Your antenna tuner in the 450S isn't tuning the antenna at all -- in any way -- when you add the amp.

    You shouldn't do that.

    If you use the antenna tuner in the 450S, use it with the amplifier in line and operational. It will help match the 450S to the input circuit of the SB-200.

    Using it to "tune the antenna" does nothing for the amplifier.
  8. N7SGM

    N7SGM Ham Member QRZ Page

    Glen, K9STH

    Thanks for your reply. I do like the SB-200 and had a good time this past weekend working many stations worldwide so it does a good job. I enjoy the nostalgia as well.

    Would you agree that a tuner is needed or would you not go to the bother?

    Thanks again Glen.

    73, Bob
  9. N7WR

    N7WR Subscriber QRZ Page

    I agree with WIK. Do not use the tuner on the 450. It does nothing for the antenna and amp. If anything it is tuning the coax between the rig and amp and that's it. Suggest you check the antenna (each band) using an antenna analyzer. If the antenna checks Ok then you need to look at the amp. My $$$ is that the problem is likely with the antenna
  10. K9STH

    K9STH Ham Member Staff Member QRZ Page


    Even if there is a problem with the antenna the SB-200 is not going to drop all the way to only 50 watts output. That is going to happen if one, or both, of the capacitors that I mentioned before are bad or if the exciter output is "folding back" from a severe mismatch to the input of the SB-200.

    I would "bet" on the "folding back" of the exciter. However, it is possible for the capacitors to be bad.

    Glen, K9STH
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