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

    Today, the Smoldering HULK of a SB-200 came in from Ebay. I paid DEARLY for it, almost $200, which is considerably more than I paid for a perfect working SB-200 that I got at a swapmeet. That one went for only $125 and is my backup HF kilowatt.
    But this one is just for playing and experimenting with.

    [​IMG]

    The good news is that more of the amp is there than expected. It had a line cord, the original Relay, the Power switch and many other parts. Of course, the final circuit has been ZAPPED, no tubes and the power supply PCB has been BRUTALIZED! That's OK, I am replacing all that!

    As mentioned in the thread on Vaccuum relays, I will use the Jennings RFID-26S relay along with the Mitrek relay as suggested by Jim W7RY.

    The good news is the Transformer is GOOD!!! That was my greatest worry on this!

    The Meter has been wasted, but I got it to work enough to measure it's parameters. It seems to be a 200uA movement with .28VDC across it at full scale, so that comes out to be a 1400 ohm impedance. I have a Heathkit HM-102 wattmeter that uses a 200 uA meter and is the same size, so will see if I can just trade out the meter plates and use that one. I have a surplus of HM-102s so I can sacrafice one.

    Time to order all the replacement parts from Mouser! And start building! I have a pair of Cetron 572Bs n the house SOMEWHERE, but couldn't find them this morning. I hid them someplace SECURE where they wouldn't get broken. Of course I have FORGOTTEN where that was, so alotta looking to do.

    But the project is underway.

    yes, I KNOW SB-200s won't work on 6, but alotta people have done it successfully, so it CAN be done! And I will do it!
     
  2. W1QJ

    W1QJ Ham Member QRZ Page

    Ok, who is going to make the popcorn? First mistake is the HM 102 SWR meter is not an exact replacement and it's not 200ua nor 1400 ohms. Keep us posted.
     
  3. AF6LJ

    AF6LJ Ham Member QRZ Page

    The good news Pat is with your expertise you can have a working amplifier inside of two months.
    (assumes working in your spare time).
    One good thing about that Mitreck T/R relay, you won't have to dig up an input connector.
    It's good to hear you have a working transformer, by the way, I would stash that loading cap.
    Those are well built and may come handy in another project.
     
  4. W1QJ

    W1QJ Ham Member QRZ Page

    Stash the loading cap? He should use the cap for his conversion.
     
  5. WA6MHZ

    WA6MHZ Subscriber QRZ Page

    I have a special tuning cap for the plate cap, and will use the load cap for the load cap.
    I have a conversion plan on the HM-102, where you put a resistor in series and one in parallel with it. K6JCA did this to a SB-220 meter they needed for replacement
    http://k6jca.blogspot.com/2009/08/sb-220-meter-repair.html

    The SB-220 plate meter is 200 uA at 1400 ohms, so it is the same movement as in the SB-200.
    I have ordered the 2.74K and 1.8K 1% precision resistors for this change, and will use the original meter plate. From what I hear, though, I might have to machine it abit to get it to fit. Shame to damage a perfectly good working HM-102, but I have about 10 of them, so I guess one can be butchered. Then, if I ever see another HM-102 meter, I can replace it.
     
  6. WA6MHZ

    WA6MHZ Subscriber QRZ Page

    All new electrolytics, bleeders, diodes, meter resistors, glitch resistor, and misc caps have been ordered as well. This is going to be a SHARP 6M Kilowatt, all thanks to the many Hams who have helped out and documented their tales here on the internet.

    The front panel is kinda gouged, and scratched up, so I might just repaint it black and relabel it. No need for a Heathkit Restoration here. Also, I plan to run a plastic 1/4 inch shaft through the hole originally for the bandswitch to the rear panel where it will tune a cap for the grid input circuit. Looking for values for that and the associated coil.
     
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The loading cap can? How?

    I always moved the TUNE cap over to the LOAD position, and used that one on six meters.:eek:
     
  8. N2EY

    N2EY Premium Subscriber QRZ Page

    The relevant question is: Why?

    I can see using the SB-200 cabinet and power supply with a couple of those Russian triodes (GI-7B?) Or a pair of 4X150As or some such.

    But 572Bs are flaky on upper HF because the grid and filament leads are so long. Why waste them on 6 meters?

    Maybe it made sense in the old days when you could get all the brand-new Cetron 572B/T-160Ls you wanted for not much money. But it's not 1972 any more. It just seems wrong to waste a good pair of rare bottles in an application they are simply not made for. Particularly when there are better tubes for the task available. Same for the whole vacuum relay/QSK thing.

    It's a matter of good engineering. Using tubes meant for the application will make the project easier, faster, and result in more useful power output.

    73 es GL de Jim, N2EY
     
  9. WA6MHZ

    WA6MHZ Subscriber QRZ Page

    I don't have the Tune cap. Not included in the deal. I was going to disconnect one bank on the loading cap to make the capacity less.

    I still can put in the GI-7Bs, not too late to do that. But I already have the 572Bs. I would have to score a set of sockets for the GI-7Bs and the tubes themselves, probably at a considerable cost.
    GI-7Bs and their equivalents are going for about $35 each, and the sockets are very expensive, around $80 each. They will require a new Bias board and Filament transformer too. So several more hundred bucks to upgrade it to the GI-7Bs.
     
  10. AF6LJ

    AF6LJ Ham Member QRZ Page

    That loading cap is rather large as I remember it's 425PF give or take per section.
     
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