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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 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 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 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
  11. N5MOA

    N5MOA Subscriber QRZ Page

    I had a TS450SAT and a SB200, I still have/use the SB200 with my TS480.

    As noted, the SB200 will handle a mismatch pretty well. You don't want to "tune" your antenna with the amp out of line, unless you need a tuner for your antenna, in which case you need one after the amp. With your antenna, it would appear you only need one between the radio and amp.

    I'm guessing your radio is folding back due to increased input swr between the radio and amp. That's where you need to keep the radio happy, the amp should handle the mismatch to the antenna if you have it reasonably close. Mine will load just fine into a 3:1.

    Either re-tune the input slugs, or use a tuner (internal or external) between the radio and amp. I use my internal tuner.

    Turn your internal tuner off, decrease drive to the amp to 20 watts.

    Key your amp and tune for max output.

    Look at the swr meter in your radio. If the swr is high with the amp keyed, hit the internal tuner with the amp keyed so the radio can be happy and not fold back.

    Once you have the radio liking the load it sees into the amp, increase drive and tune per the manual. Tweak the tuner, with the amp keyed, again if needed.

    You probably already know this, but use cw or some constant carrier mode to tune, and the white part of the scale is 500ma when checking the plate current and 100ma when checking the grid current. Stay under 500ma plate and 100ma grid. Your tubes and the people listening will thank you.

    If fold back is indeed your problem, that should fix it.
  12. N7SGM

    N7SGM Ham Member QRZ Page

    Hey Tom,

    Thanks very much for your detailed reply. I will definetly try your procedure out. As you can tell, I am new to the use of an amp and HF in general so that is why I look to the learned for answers. Thanks again.

    de Bob
  13. WA7PRC

    WA7PRC Ham Member QRZ Page

    Hi Bob,

    One question that I haven't seen asked is, how much grid current can you get? You should be able to drive it to 100mA (the top of the white scale) at full output. If you have little or no grid current on 80m, you need to look at the input circuit... L5, C30, and the bandswitch. If you don't have it, I found the manual here.

    vy 73,
    Bryan WA7PRC
  14. N7SGM

    N7SGM Ham Member QRZ Page

    Hey Bryan,

    What you describe is the problem on 80M, that is I have very little in the way of grid current or plate current as well. That is why I was eluding to to fact that my overall SWR is high on this band. Despite what Steve, WB2WIK states, SWR is key when it comes to RF output at the antenna. If indeed SWR was not an issue then we would not need tuners period. Also Glen points out the fact that "foldback" occurs when the SWR is high and this also prevents max output. The Kenwood TS-450S/AT has protective circuits that won't allow much more than 10W out when SWR values are high in order to protect its finals. Therefore SWR is very important. Heathkit must of thought that SWR is important as well because they included it as part of the circuitry on the SB-200.

    I appreciate your comments Bryan and will further evalute the compents and circuits you mention. I just don't know if the culprit is the radio, SWR, or the amp it self. As stated by me earlier, I am new to HF (18 months) and newer with the amplifier. Heck I'm an Extra, I should know all this stuff huh?
    Just kidding of course. I'm trying to learn all I can and do realize I have a long way to go but that is what I like about ham radio. Thank you.

    de Bob
  15. K9STH

    K9STH Ham Member QRZ Page


    The match between the output of the transceiver and the input to the SB-200 has nothing to do with the "SWR" of the antenna. Get this concept that the SWR of the antenna is affecting the transceiver completely out of your head!

    The only thing that the r.f. from your transceiver "sees" is the cathode input to the 572B tubes in the amplifier, NOTHING else. The input coils to the 572B tubes, the tubes themselves, and finally the pi-network output of the amplifier are all between the transceiver and the antenna.

    The Heath SB-200 was designed for use with the Heath SB-400 and SB-401 transmitters and the SB-100, SB-101, SB-102, HW-100, and HW-101 transceivers. All of those units have the capability of matching loads other than a fixed 50 ohm load like your transceiver has. Of course, most of the "boat anchor" (tube-type) transmitters/transceivers have the ability to match a wider range of impedances and therefore work fine with the SB-200.

    Because the SB-200 was designed to be used with transmitters that do NOT have to have a fixed 50 ohm load the input impedance to the SB-200 is NOT 50 ohms. Depending on the band, the input may approach 50 ohms but there is no guarantee that the input is 50 ohms. Since your transceiver MUST see a fixed load that has an impedance of 50 ohms the transceiver incorporates a protection circuit to "fold back" (reduce) the power when the SWR that the unit "sees" is much more that 1:1. This circuit is to protect the final amplifier transistors from damage when the SWR increases.

    Since you are seeing very little grid current when operating on the 80 meter band this indicates that the transceiver is "folding back" and is not putting out much power. This, in turn, does not "drive" the 572B tubes in the SB-200 correctly and therefore there is very little grid current and very little power output.

    The percentage of bandwidth of the 80 meter band is the highest of the HF bands. This means that there is virtually no chance of having a low SWR with a fixed tuned circuit (like the SB-200 has in the input to the 572B tubes) across the entire band. As such, you have to have "something" between the transceiver and the amplifier to match the fixed 50 ohm output of the transceiver to the input impedance of the amplifier. The "on board" tuner may, or may not, be able to handle this mismatch to the input of the amplifier. If it cannot handle the the varying impedance of the input to the amplifier then you MUST use some sort of tuner between the output of the transceiver and the input to the amplifier. Unless you get a considerably better "match" between the transceiver and the amplifier the power output of the transceiver WILL continue to "fold back" which results in insufficient drive to the amplifier which, in turn, results in very low output from the amplifier.

    Heath included an SWR bridge in the SB-200 as a "nice little bonus". Although Heath specifications say no more than a 2:1 SWR the amplifier can definitely "handle" a considerably higher SWR. Of course, for maximum performance it is always a good idea to keep the SWR of the antenna system as close to 1:1 as possible.

    Since there is no "fold back" circuitry in the SB-200 the power output is NOT going to be reduced by the SWR in the antenna. The reduction in power IS caused by the "fold back" circuitry in the transceiver which is being activated by the high SWR that occurs in the INPUT circuitry of the amplifier. Again, there is NOTHING that the higher SWR in the antenna system can do to cause the "fold back" in the transceiver when the transceiver is connected to the input of the amplifier and the amplifier is activated. Now when the amplifier is not activated then the transceiver is connected directly to the antenna and then the SWR of the antenna system will affect the transceiver. But, when the amplifier is activated there is nothing the SWR of the antenna system can do to affect the transceiver.

    The SWR bridge in the SB-200 will indicate when the amplifier is not activated. This is because the SWR bridge is connected directly to the output (antenna) connector on the SB-200 and the internal relay in the amplifier switches the SWR bridge between the input to the amplifier and the output of the 572B tubes.

    To sum everything up, you have to get a better "match" between your transceiver and the input to the amplifier and this can require some sort of "antenna tuner" between the transceiver's output and the amplifier's input.

    Glen, K9STH
  16. N7SGM

    N7SGM Ham Member QRZ Page

    Glen - K9STH,

    Your explanation and time are much appreciated. While there remains some debate, I agree with the point you made regarding the use of a tuner between the XCVR output and the amp's input.

    Thanks again Glen,
    Best 73
    Bob - N7SGM
  17. N7SGM

    N7SGM Ham Member QRZ Page


    If anyone is particularly following this thread I found another similar one on Eham which I find most interesting.

    The author has discovered the weak link for him was cheap 572B valves. In this sense, I guess you do get what you pay for.

    He also discusses the replacement of C27, 500pF, 2KV with a 1000pF cap but dismisses it later over the tube replacement.

    I am in the process of changing out many of the components inside the SB-200 as suggested by Mr. Robert Norgard, KL7FM and another SB-200 Mod website I found:
    I do not know the authors name on this last reference but I would like to know who this ham is. His mods make sense and his instructions are laid out very well.

    Many other sites are available with SB-200 mods. Simply Google Heathkit SB-200 Mods and see a whole host of available sites.

    If I can give a plug here I used Mouser because of parts availability, inexpensive shipping charges, and the chance to simply order as few items or as many as I cared too. Many other places were wanting to overcharge for shipping or not allow me to order just a few parts. Mouser accepts orders from individuals as well as industrial customers. Good business practice!!

    OK, OK, enough is enough, I understand and have beat this thread up all it can stand. Interesting stuff, this hobby we have. I find some of it very abstract but I like it anyway.

    de Bob
  18. WA0ARM

    WA0ARM Ham Member QRZ Page

    Another SB200 Question?

    I have enjoyed reading this thread and thank you all for contributing. I have an additional question to ask this brain trust. I too recently overhauled a SB200 that my father originally built in 1965. I too used Robert Norgard's KL7FM instructions to upgrade the amp. I also installed Rich Measures AG6k parasitic suppression kit and a new set of Taylor tubes from RF Parts. Put some Mouser parts in too.
    I am driving the amplifier with a Yaesu FT-857D. The amp feeds into a MFJ989D tuner as the antenna is a G5RV. I have carefully tuned the amp into a cantenna dummy load and it performs well on 80, 40, and 20. I am seeing some reflected power on 15 and 10 and will tune the coils for that a little later. My question is about excitation power vs. amp output. Presently when I have the exciter indicating an input of 10 watts. I read a "peak" output of 400-600 watts on the tuner meter on the various bands. The output I am getting matches what you all say the amp should put out but I am baffled by the low input? I routinely tune up the antenna with the amp off. I then turn on the amp and tune into the dummy load. Finally I tune(both the amp and tuner) through the amp and the tuner to the antenna.

    I am making great contacts at the 10watt drive level and was able to work South Africa on 40 Saturday night in the middle of a pile up. I know the high output with low input may be an indication of parasitic oscillation. My 100watt barefoot output is measured correctly by the MFJ meter. This is the only test I have done to verify that meter's accuracy. Are there other explanations other than output meter accuracy?

    Thanks again all!


    Bill WA0ARM
    Last edited: Dec 7, 2009
  19. N4CR

    N4CR XML Subscriber QRZ Page

    The MFJ 989x series of tuners does not have a true peak reading wattmeter, so all bets are off it you are depending on it to be accurate.

    If you look at the schematic for it, you will see a capacitor across the meter when it is switched to peak hold. While it reads a little higher on peaks than without the cap, the rise time is very limited and it really doesn't catch the real peaks.

    So, you can either watch the peak voltage with a scope and do the calculations or you can get a real peak reading wattmeter. The one you have does not show you the real picture.

    I like the LP-100A wattmeter because it offers so many more things than just peak reading. There are also some inexpensive kits you can retrofit any SWR bridge with that will convert it to true peak reading.

    Until you do something like this, you really can't measure what's going on.
  20. WA0ARM

    WA0ARM Ham Member QRZ Page

    Crown Fix


    Thank you very much for that tid bit. I suspected the meter but did not know why. So this is one of those Crown Fixes. I just have to entice one of my ham buddies, who owns a real wattmeter, over to my place with a promise of Crown Royal. That will fix it!

    Thanks again!

    73, Bill WA0ARM
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