SB-220 Find

Discussion in 'Amateur Radio Amplifiers' started by AF6LJ, Dec 19, 2012.

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  1. AF6LJ

    AF6LJ Ham Member QRZ Page

    that is my thinking.
    I might be able to fix the line voltage problem...
    I want to build a grid driven amplifier, something with a decent tetrode.
    The good tubes that were made for grid driven service seem to be all gone.
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    If you don't mind really high voltage and a big tube, a 4-1000A is very available and pretty cheap.

    My 6m amplifier (homebrewed in May 1984) uses one, and it was surplus when I got it in '84...still works fine in 2013.:eek: Cost me $100 for a "pull."

    I have a spare, never needed to use it.

    Socket's cheap, tube has a lot of gain on HF and even on 6m. 20W drive = at least 1200W output power (that's with 4000V on it). Grid and screen bias supplies aren't difficult because in AB1 they draw about zero current. My 6m amp will run 1200W output showing "no" grid current at all, and 1500W showing maybe 1-2 mA. Screen voltage is 600V, and current can go slightly positive or negative (several mA) but when "properly" loaded, it is also zero.
     
  3. AF6LJ

    AF6LJ Ham Member QRZ Page

    That would be perfect, if I am not mistaken a filament transformer built for 2 3-500Zs will work just fine.
    I may have to look into that. I don't have a problem with higher voltages.
     
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Nah, the 4-1000A filament voltage is 7.5V at 21A.

    3-500Z is 5.0V at 14.6A.

    But it's a rugged tube.
     
  5. N2EY

    N2EY Premium Subscriber QRZ Page

    WB2WIK's 4-1000A suggestion for a 6 meter amp sounds good to me if you want the grid-driven-tetrode path. IIRC, 4-1000A were used in FM broadcast transmitters, so 50 MHz is reasonable.

    One trick I have seen for 6 meter amps is to use link coupling and shorted-turn tuning instead of a variable cap. Using series feed, you eliminate the plate RF choke and blocking cap, resulting in a very simple and clean amp. Of course you have to be careful with insulation and such, but if you are comfortable with 4000 volt B+ you already know what to do.

    The transmitting tube situation won't get better because there simply isn't enough market for the transmitting tubes we hams want.
     
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I already did this with a 4-1000A back in '84 and it became a feature construction article that was published: http://archive.org/stream/73-magazine-1985-07/07_July_1985#page/n51/mode/2up

    I used a standard pi-net with a vacuum variable for tuning (~3-30 pF), that made it feasible with a reasonable plate tank Q. It's stable as a rock and pretty efficient for AB1 (~50%).
     
  7. AF6LJ

    AF6LJ Ham Member QRZ Page

    OOPs...

    Steve's design is nice, I am not sure that I wouldn't just build a monoband amp for seventy five. or maybe a dual bander 75/160. I currently don't have a lot of interest in six meters. The two meter KW that Steve built out of the handbook (I have that article also) might be where those 4CX250Bs I have go, and it for the time being wouldn't have to be liner. It wouldn't get much use but it would be a fun exercise. :)
     
  8. KD0CAC

    KD0CAC Ham Member QRZ Page

    I need to start a thread about a small budget 220 build /mod , to find a cheaper way to do something with one that is available .
    It would be nice to learn more , but you guys / gals are putting big-bucks into these :)
     
  9. AF6LJ

    AF6LJ Ham Member QRZ Page

    Cost wise the caps were the most expensive part of my rebuild so far, about $130.00 for eight caps.
    Except for the caps I bought more of what I needed to rebuild the power supply board.
    As far as that power supply PCB goes, all you really need to replace are the carbon resistors.
    That is only five resistors and of those five the 4.7Megaohm resisters are the only ones that require any special attention, they need to be of a type that handles a higher voltage across them.
    Look further up in the thread and the Mouser part number is there.

    The easy route for rebuilding the capacitor bank is through Harbach electronics, http://harbachelectronics.com/main/page_products_sb220__sb221.html
    You can get everything you need to update the amplifier there. The reason why I didn't go with the Harbach filter cap replacement kit is explained in the thread. There is nothing wrong with what they offer, I wanted to take a different path.
     
  10. KD0CAC

    KD0CAC Ham Member QRZ Page

    Thanks Sue , I need to get hands on this amp being offered to see that the major parts like transformer are OK .
    To me it seems that needing an amp for 160m would be better , but then living in a RV and metro areas , it would be harder to get 160m antenna going than an amp .
    Just planing ahead for the future antenna farm :)
     
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