The SB-200 on 6: A MONUMENTAL Project!

Discussion in 'Amateur Radio Amplifiers' started by WA6MHZ, Oct 2, 2012.

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  1. WA6MHZ

    WA6MHZ Subscriber QRZ Page

    The DC paths have been verified correct. The HV works great. I have bid on another wasted HM-102 meter to score another meter movement. So the ONLY reason, other than a dirty antenna relay, that is doesn't work is the probability the tubes are bad. IF they ARE bad, then, rather than buy 2 new 572Bs, I might opt for the GI-7Bs. And, as I have said before, I HAD a SB-200 with 572Bson 6 back in 1975 that worked GREAT! So the project isn't over, I am just catching my wind while getting the 222 Mhz Repeater for our Radio club ready to go to be installed and doing some WORK stuff. There is no time limit on this project. If it doesn't progress for a few days, notta problem. I have NOT given up, just had the wind knocked out of me.
     
  2. N2EY

    N2EY Premium Subscriber QRZ Page

    Maybe.

    Have the 572Bs that are suspected of being bad been tested in a working amp?

    Have the input and output pi-networks been tested by means other than grid-dipping? (The input pi-net could have a shorted or way-out-of-tolerance capacitor and still test OK by grid-dipping).

    Etc.

    IMHO the GI-7Bs are a better tube for the job. And by using a separate heater transformer, the load on the main power transformer is reduced by about 50 VA AND the main power transformer can be switched separately.

    73 de Jim, N2EY
     
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    GI-7Bs aren't "instant-on," though.

    While waiting for my 4CX250B 2m amp or 8877 6m amp to "warm up," I miss "instant-on" stuff.

    Guess it depends how much time you have.:eek:
     
  4. N2EY

    N2EY Premium Subscriber QRZ Page

    Of course.

    But if you build the amp so that the heaters have their own transformer, you can keep the tubes warmed up without running the HV all the time.

    btw, the GI7B has one side of the heater tied to the cathode. This means that in a GG application, you ground that side of the winding, not the center-tap. Since they run on 12 volts, a simple half-wave rectifier can give 12 volts DC for controlling relays, etc.

    73 de Jim, N2EY
     
  5. AF6LJ

    AF6LJ Ham Member QRZ Page

    I think for the fun of it I would borrow the tubes from your working SB-200 just to get it working.
     
  6. WB3BEL

    WB3BEL Ham Member QRZ Page

    Huh? In GG the grid would be best DC and AC/RF grounded and the cathode bias superimposed on the cathode along with the heater voltage. It's simplest to use AC transformer. No CT needed.

    Also I would not leave the heater on all the time. If you want instant on use something like 3-500Z.

    By the way I don't think that the DC paths have been verified like N2EY suggested. All you are saying Pat is that you have measured the B+ and it is good and that the HV meter shows it. I am not sure you verified it was going through the RF choke and suppressors and was actually at the anode. I don't think you have verified the cathode bias. I don't think you have gotten the right idling current. I would check those first thing. Maybe if the idling current is as expected you could skip some of the intermediate steps. But since it's not. I would not worry yet about the RF match. Fix the DC then fix the Plate and Cathode match second.

    You don't necessarily need a replacement meter for the amp "Product" to do this. If you have a Simpson 260 or some other meter/s you can use temporarily to make measurements with it can also work fine. Making a HV divider from a string of 100K or 1Mohm resistors is not that hard. As always be careful working with HV.
     
  7. W1QJ

    W1QJ Ham Member QRZ Page

    Jim, I say that in jest. You know the typical throw it in the dumpster scenario? What I really mean is to give it up and if the objective is to concentrate on doing a 6 meter conversion on an SB-200 (which could be a feat on it's own) to make sure you start with a working SB-200 which you can begin the conversion on right from the get go. Now there are just too many things that he messed with to lay a finger on what is going on. Like Lynn said, start with all the DC working properly! I'm no novice to 6 meter conversions of HF amps. I've done all sorts. Parting that unit out should be the next step and looking for a decent working model to start with for the conversion.
     
  8. WA6MHZ

    WA6MHZ Subscriber QRZ Page

    Today (saturday) I dug out the Backup SB-200 from the house debris and brought it in. I used my Kenwood TS-830S as a drive transmitter, and loaded up the virgin SB-200 to full power. On "TUNE" position, I got about 400W output with the Cetron stock tubes.

    Next, I removed those and installed the Suspect RF Parts tubes.

    [​IMG]

    Those were loaded up too, but they seem kinda soft. I get about 250W output from that pair.

    The original Cetrons were re-installed in the SB-200 and it was carefully retired to standby service.

    While I had it open, I compared the chassis, back panel and circuitry for any differences.

    [​IMG]

    No significant differences were found, other than the Glitch resistor and fuse installed in the 6M unit Note both have homebrew Soft Starts and Soft keys.

    I reinstalled the RF Parts 572Bs in the 6M unit and put it back in the "MAYBE I WILL GET AROUND TO IT AGAIN SOMEDAY" pile.

    So, the RF Parts ARE Functional, but pretty soft. Not sure what to do next. The DC paths have been CONFIRMED to be 100% correct. Maybe I will find some more 572Bs and get a deal on them.

    I SHOULD have converted the SB-230 I scored at Dayton for 6M instead, but people on here advised me AGAINST that as a working SB-230 is worth alot of money. SB-200s are a dime a dozen. Well, I SURE have alot more in this one than that!!! Think I am now towards $300 into this project, with no progress or success.
     
  9. AF6LJ

    AF6LJ Ham Member QRZ Page

    I was thinking abut this a few days ago as I was about to drift off to sleep....
    I am wondering what shape the grid resistors and bypass caps are in.....
    Me thinks.........
    It might be a good idea to just replace them with new.
     
  10. W1BR

    W1BR Ham Member QRZ Page

    One thing I noticed in Pat's earlier photos concerning the new 6 meter tuned input pi-network. I couldn't see a DC blocking capacitor between the pi-net and the amplifier input (cathodes.) That might adversely affect the cathode biasing.

    Pete
     
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