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

    I would agree they have come a long way in thirty eight years.
    There are better transformer designs these days, Hypersil comes to mind.
    I do want new iron anyway. The Heathkit iron does command a decent price, a replacement transformer will run me something on the order of $450.00. Considering the cost of a new amplifier vs rebuilding the 220, I think I will be money ahead offsetting my labor with the fun factor of doing the work.
    True
    makes sense, you jogged my memory, I haven't thought of that since college.
    Agreed...
    As for how much the high voltage changes under normal operation, I'll have to look next time, seems to me right now it is about 400V that is with about six or eight volts drop in the primary side also.
    Since the voltage divider for the plate voltage meter was also defective until the rebuild I cannot tell you how much the voltage use to change from no load to full load.
    It is really a shame, the only good tubes left are used in grounded applications.
    There are no decent tetrodes in production anymore....
    Bipolar transistors have to be operated in very underrated conditions to maintain gain linearity.
    FETs are coming along, but require some really costly protection circuits to make them usable for amateur service.

    I have 4CX250s sockets and chimneys, the downside to using them is the IMD goes up fast when you draw grid current, so they are only really good for class AB1 operation that is fine if all you want is five or six hundred watts from a pair....

    Maybe things will be better in the near future....
     
  2. WA7PRC

    WA7PRC Ham Member QRZ Page

    The rectifiers in the SB-220 are Heathkit pn 57-27. According to the cross-reference info I have, they're equivalent to 1N2071 (750mA, 600 PIV).
     
  3. AF6LJ

    AF6LJ Ham Member QRZ Page

    Just a bit underrated in the current department.
     
  4. WA7PRC

    WA7PRC Ham Member QRZ Page

    Yes. I replaced mine with pn R170 (2.5A, 1KV) rectifiers ca 1975.

    The HVPS for my 160m GS35b amplifier will use stacks of 6A10 rectifiers (6A, 1KV). I found fifty for $17 shipped (33 cents each).
     
  5. AF6LJ

    AF6LJ Ham Member QRZ Page

    That is a very good price.
     
  6. WA6MHZ

    WA6MHZ Subscriber QRZ Page

    There was a SB-200 on EBAY converted to 4CX250Bs. Didn't show how they did it, just said it didn't work.

    The problem with the 4CX250Bs is they need LOTSA Forced air, and a regulated screen supply. Also need a timing circuit to keep the HV off the tubes until the filaments are properly heated.

    Wish I could find out how he did it. I built a 4CX250B amp, but it didn't work right and got way way too HOT! Need a SERIOUS Blower to move the air through those tubes!!!!

    For the SB-220, U still need a regulated screen supply! I don't see where and how they did it!
     
  7. AF6LJ

    AF6LJ Ham Member QRZ Page

    One of these days we should get together, I have some ideas regarding your 4CX250B six meter amp.
     
  8. N2EY

    N2EY Premium Subscriber QRZ Page

    There's always the tradeoff between fixing up and modifying something old, building from scratch new, and buying. All depends on the final goal and the available resources.

    The line voltage change may hide the improvement of the new caps.

    Did you mean "grounded grid applications"? If so, yes, and even those are getting harder to find.

    How about the Russian radar tubes like the Gi-7B family? How good are they for IMD and similar? Seems to me that for something like an SB-206 conversion they would be ideal. Or many other applications. Besides being inexpensive they are readily available and seem pretty rugged. You have to make or buy the sockets but that's a one-time thing. Much less work and headaches than a screen supply and forced-air sockets for 4CX family tubes.
     
  9. WB3BEL

    WB3BEL Ham Member QRZ Page

    I don't know why people keep saying you need socket for GI7b. It is just not needed.

    In grounded grid application you put the tube into a hole in the chassis and fasten it with clips.

    See this G4DCV method for some idea. This is a GS31b but the same technique works fine for the GI7b.

    http://www.g4dcv.co.uk/GS31/pages/gs1-2_jpg.htm

    He uses PVC elbow for cathode side airflow. You can also use small 12V DC fan from PC in the input compartment and a blower in the anode side.

    Also lots of examples of bolt it to the chassis construction here:

    http://www.qsl.net/y/yu1aw/Misc/6m eng.htm
     
  10. N2EY

    N2EY Premium Subscriber QRZ Page

    You need some sort of mechanical mount for the tube(s). As you note, such a mount can be as simple as a hole in the chassis, a clamp on the grid ring, and some clips for the heater and plate. Simple - but it still requires fabrication.
     
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