SB-200 and IC-746 Pro - Tuning up - confused.

Discussion in 'Amateur Radio Amplifiers' started by KB2FCV, May 16, 2011.

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  1. KB2FCV

    KB2FCV Ham Member QRZ Page

    Hello folks,

    I recently added an amp to the station. It's been a good 15+ years since I've had an amp. The amp is an SB-200 with all the Harbach mods (including the soft-key). I am using this with my Icom IC-746 Pro.

    I am trying to get this amp tuned up and it's a bit challenging. I am tuning it into a heathkit cantenna.

    I am trying to dip the plate, peak the grid but what is happening is the circuit in the amp is causing the rig to see fluctuating impedance which in turn the rig cuts it's power back so it's hard to gauge what is going on.

    I'm guessing I want to tune it so the rig sees a good swr and I want to keep the plate current low and go for max power output? Initially I am applying about 15-20 watts and I bring it up to about 50 out my icom.

    I'm trying to follow the tuneup sequence in the manual, but it gets tricky with the fluctuating impedance the rig is seeing. Any suggestions of how I can get a good tuneup procedure with the two?

    Thanks,
     
  2. KB2FCV

    KB2FCV Ham Member QRZ Page

    Yep, I'm following the steps in the manual to tune it up. So my radio should see 50 ohms impedance from the amp (I guess that would be the input tuning network?) at all times during tune up? If so, then I definitely have a problem.
     
  3. W1QJ

    W1QJ Ham Member QRZ Page

    Make sure your "auto tune" feature is turned off in the menu!!!!! The Icom 746 is the worst radio for swr foldback. Try using another radio if you have one to see if the swr varies as you tune up. I have a 746 too and recently used it with 2 Sb-200 amps I was working on. The amps exhibited the same problem with the 746 as you describe. I have used many other SB-200 amps with other radios and didn't notice that problem. I have found that the 746 is problematic in this regard especially with older amps. Using the same 746 with bigger Ameritron amps I don't because I think the Q of the input circuits are a bit higher on them then the SB-200 and therefore satisfies the radio better. So, I would not go crazy until you try it with another radio and see if another radio likes the amp. You could go on a fool's mission if you try to suit the amp to the radio. Tom Boza had this all worked out on his website but it is down now. He went through and redesigned the input circuits on the SB-200 for a higher Q for SS radios. I found no problems with the SB-200's using other radios with less sensitive swr foldback circuits. All my Yeasu radios are fine. I specifically bought an Icom 746 for this very reason. I use it to design and test input circuits because I know if it satisfies the 746 it will any radio. As far as loading the SB-200, try for the moment not to dwell on the problem and do this. Set the meter in rel pwr and apply 20 watts of drive, tune for maximum output, then go to 70 watts and tune for maximum output, then go to the full 100 watts output and tune for maximum output. If you can't do this because the radio folds back, do try another radio if you have one. make adjustments quickly and just tune for maximim output. Then after that switch the meter to grid and check the grid current. if it is in the safety white zone and the plate current is good too leave it. If the grid current is out of the white zone then lower the drive to get it in the safe white zone. Do not retune. Now the amp is loaded heavily enough to handle power peaks. With the last SB-200 I worked on I tried adjusting the coils for the lowest swr but could not get it flat, although 40 meters was flat. So, if you have another radio, please, try it with the SB-200 and see if you have the same problem. if you don't have another radio and you want to go through matching the amp to the radio, you would have to do the same thing Tom Boza did. Though I think his site is down I may know someone who has all the data on the redesigned input circuits he worked out. let me know if you want to go that route and I will see if I can get that data for you.
     
  4. W9GB

    W9GB Ham Member QRZ Page

    Tom had a section on his web site (earlier) for his work with the Heathkit SB200 input.
    He removed it a couple of years ago -- likely due to many Heathkit SB200 amplifier owners calling for FREE technical advice for their broken amp.

    Tom's web page is working (I like his 2010 "new ham shack")
    http://www.ne7x.com

    PA0FRI has a section on his SB200 web page that covers this same topic, with less detail than Tom provided.
    http://www.xs4all.nl/~pa0fri/Lineairs/SB200/sb200eng.htm

    w9gb
     
  5. KD0CAC

    KD0CAC Ham Member QRZ Page

    The 1st thing that occurred to me was " I am tuning it into a heathkit cantenna. " , shouldn't you be tuning into the antenna system that your using .
    The 2nd already mentioned , make sure the auto tuner in the rig is turned off .
     
  6. W8JI

    W8JI Ham Member QRZ Page

    Here's how it works.... :)

    It is NORMAL for the input impedance of any grounded grid amplifier to vary as the output is tuned. This is because the input system is in series with the output system. This causes significant feedback between output and input.

    In the SB200 this is aggravated by lack of neutralization, and by the very poor grid bypassing system. The SB200 only has a single 200 pF grid bypass capacitor on each tube. This makes the amp less stable, increases IMD, and increases in-phase feedback from output to input for some conditions of tuning.

    So in the SB200 is it very normal to have significant input SWR change with tuning.

    Personally, I would change the grid bypass caps to .01uF disc caps with very short leads or rewire bias so I could just ground the grids. Having the grids float on capacitors sets you up for radio damage if a tube arcs.

    My suggestion is, if you are unable to handle rewiring the SB200 to a conventional directly grounded grid, to at least change the 200 pF bypass caps to a .01 disc with short leads. That will eliminate about half of the SWR change on 80 and 40 as you tune the output.

    When you tune, be sure to just tune for maximum output. Be sure any automatic tuner on the radio is off. Be sure to NOT use the ALC line. Be sure the input coax is good. You may also want to use a much longer than necessary input cable between the radio and the amp. The additional cable losses will tend to swamp out SWR changes and decrease instability on upper HF.

    73 Tom
     
  7. KB2FCV

    KB2FCV Ham Member QRZ Page

    HFRF,
    The amp has the Harbach Soft-Key mod in it. The rig also has the Harbach replacement power supply module, the soft-start, the replacement fan, and the replacement relay. With the soft-key it brings the voltage down from 120v to about .7 vdc @ 1.5mA. That is an add-on kit that allows modern rigs to key the amp. I run the keying line to the rca jack on the back of my icom that is intended for closing a relay when the radio is keyed. I'll work on testing the SWR.

    It does sound like what W8JI is exactly what is happening as when I tune the amp the SWR on the radio will vary anywhere from 1.1:1 to over 5:1.

    I don't use the auto tuner in my rig.

    I did try tuning for maximum output as W1QJ and W8JI suggested and when I got to max output the SWR looked reasonable (about 1.7:1) and the Grid and Plate readings were within safe limits. I'll defintely try it with another radio. I have a yaesu FT-950 sitting here that I can try out.
     
  8. W8JI

    W8JI Ham Member QRZ Page

    No, that is far too much change to be the problem I described. Typically an SB 200 will vary about .3 or so units when tuned in middle HF. It varies more on 80 (up to a .5 point change or more), and a bit less on ten meters. In other words SWR on the input might change from 1.5 :1 down to 1.2 :1 as you tune it, or any other change somewhat like that. It is a more significant change than other amps with properly grounded grids have, but not nearly as bad as you report seeing.

    If you are seeing a change from around 1:1 up to 5:1 you have a major problem. Some of the causes:

    1.) You are reading the meter wrong, or the meter in the radio is wrong

    2.) The amplifier is oscillating

    3.) A grid resistor or capacitor is open. Check the 33 ohm grid resistors and also get rid of the silly 200pF. Just change them out to .01uF discs and ground them directly to the chassis with a lug keeping leads short. The 200 pF are just stupid, because they are 200 ohms reactance on 80 meters!!! The reactance almost equals the total grid impedance on 80, which is terrible engineering.

    4.) Make sure you do not have an open ground on the shields of the cables by the antenna relay. Check the mechanical integrity of the shield grounds by the relay, and at the end of the one cable by the tuned input wafer and the other cable near the tank circuit. If the ground by the relay is bad in particular, you will have big input SWR changes with tuning on higher bands.

    73 Tom
     
  9. KM1H

    KM1H Ham Member QRZ Page

    When going with the .01 caps the already marginal 10-15M neutralization becomes worse. Those 200pf caps did work to some degree.

    Its possible to obtain 10M efficiency and full neutralization as good as 20M but it has to be approached as a system. The customers SB-200 I finished yesterday runs 750W out on all 5 bands and is as stable as a rock with a set of NOS Svetlana 572B's which many will claim wont work in that amp.

    A GG amp has poor reverse isolation to start with so there may be considerable reaction to the exciter depending upon the rig, interconnect coax length and the amps input circuit.

    When tuning up the internal ATU should be off and the maximum output obtained from the amp. Then use the ATU to maximize the drive power if the rig has folded back a bit realizing that a substantial QSY may require doing it all over again.

    Carl
     
  10. W8JI

    W8JI Ham Member QRZ Page

    Nonsense. I went all through this stuff with Heathkit when we were talking about revising all the amps, and the measurements we made were repeatable.

    Those 200 pF caps are 200 ohms on 80 meters. That's just silly in series with a 200 ohm control grid to cathode impedance, because it shifts phase 90 degrees.

    If they are replaced with .01 discs with short ground leads to lugs near the grid pins, all that silliness clears up.

    That 200 pF cap was a Bill Orr idea. He had some fascination about what he called "super cathode drive". Those caps were never intended to "neutralize", they were added to give that silly negative feedback idea that Bill swiped from Collins. If you run the S21 or S12 tests on the SB200, you'll see those caps make everything worse except on one range up around 60-120 MHz. A shorter grid lead though a bigger cap and good parasitic suppressor cure that.

    Those caps should go.

    73 Tom
     
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