Heathkit sb-200 how to convert to 811a tubes

Discussion in 'Amateur Radio Amplifiers' started by K5BOO, Sep 4, 2014.

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  1. K5BOO

    K5BOO Ham Member QRZ Page

    Now that the 572b tubes are not availble does that make the sb-200 amps junk? I have two of them and up untill now i thought they were great, in fact i put one of them togather in 1973 and am still using it although its down to 400 watts top output but that is not bad with the original centron tubes in it. Now i find out that out friends in china are not going to make the 572b tubes any longer. My question, is this true?

    If this is true i bet it has been brought up several times, my appoligies, i just don't know how to get around in the forum's very good yet.

    Thank you, keith k5b00
     
  2. W1QJ

    W1QJ Ham Member QRZ Page

    You should be able to get 572b tubes.
     
  3. K5BOO

    K5BOO Ham Member QRZ Page

    No one seems to have them, i have called ameritron, rf parts and other leading tube companys and they have all said there are none avaible and do not expect china to make any more for some time at the best.

    I know the ameritron can be changed to 811a's by changing two wires, ameritron has it on their site but do not know if the sb-200 can be converted to other tubes or not, without a complete rebuild. If anyone knows how to do it please let me know.

    Thank you for the reply
     
  4. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Hi BOO,
    If you can hold out until spring, RF Parts is gearing up to have some 572's made that should be OK. While this idea is vaporware now, hopefully it will come to pass that we can get decent, new 572's. 811's are a bit fragile in that the plates don't have much thermal mass so you can scorch them while tuning up an SB-200, who's HT is a bit high for 811's.

    If you can't wait until spring '15, I bet you can find a pair of used tubes (on eVilBay, swap fests, etc.) that would hold you over.

    400 watts, eh? What's the HT supply voltage reading? 1973? How are the PS caps? A new set of those may bring your power back up. I've got 40 year old Cetrons in a well used amp that really came back to life with new caps!
     
  5. AF6LJ

    AF6LJ Ham Member QRZ Page

    The short answer to the main question is; you can't convert an SB-200 to run 811s the power supply generates too much voltage.
     
  6. N2EY

    N2EY Premium Subscriber QRZ Page

    In theory, it can be done.

    In practice, you don't want to do it. Too much work, and the resulting amp will run half the power, at best.

    572Bs can still be found; they're just not easy to find, nor cheap - particularly if they are good Cetron or similar brand.

    The only worthwhile tube conversion of an SB-200 that I can think of is to convert it to use Russian Gi7Bt ceramic triodes. That's a lot of work too, but those tubes are available, and not expensive.
     
  7. AF6LJ

    AF6LJ Ham Member QRZ Page

    For one thing you wouldn't have a Heath SB-200 anymore.
    The power supply is a voltage doubler supply. changing it to a bridge rectifier would yield only 1100V unloaded and just over 800V fully loaded. The result would be considerably less than half power.
    Might as well homebrew an amp from scratch.
    [
    572Bs can still be found; they're just not easy to find, nor cheap - particularly if they are good Cetron or similar brand.

    The only worthwhile tube conversion of an SB-200 that I can think of is to convert it to use Russian Gi7Bt ceramic triodes. That's a lot of work too, but those tubes are available, and not expensive.[/QUOTE]

    Best to wait for the proper tube or homebrew an amp with a more obtainable tube such as a 3-500Z.
     
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I see 572Bs for sale every day. They're for sale by private parties, mostly, and "NOS" (new old stock) which should mean although they're old, they've never been used and are in original cartons.

    Here's an ad from QTH.com just posted yesterday:

    New Old Stock Cetron/Dentron 572b Tubes [​IMG] I have a matched set of 2 Cetron 572B/T160L tubes made by Cetron for Dentron. with Famous Blue lettering. "Made in the USA".
    These are NEW OLD STOCK. I have the original Boxes and packaging.
    These tubes have super thick glass, Very heavy, also very beefy inside.. you wont believe the difference, there so heavy duty.
    These will out perform any 572B tube that is sold today..
    They have been in storage, stored in the upright position so no Filament sags,
    There Brand new old stock. These are the last 2 i have. They will last a lifetime if you tune your amp correctly. $250 shipped USA only.
    Don't waist your money on the junk tubes that you will have to replace,and take a chance on ruining your expensive amp. These are the ones you want.. The Tubes will be packed securely and Insured.
    thanks, 73 w8nvc GOD BLESS,
    Listing #1151936 - Submitted on 09/03/14 by Callsign W8NVC - IP: adsl-75-10-140-24.dsl.bcvloh.sbcglobal.net Click Here to Email -- Click Here to View Picture -- Send this Ad to a Friend
     
  9. AF6LJ

    AF6LJ Ham Member QRZ Page

    Cetron made millions of those things and for years sold them for eighteen bucks each.
    It is a good tube, well designed and people bought spares thinking they might need them.
    Unless you really abuse them they will last as long as the hams using them, maybe longer.
     
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Cetron has a rich history of electron tube making, dating back to the earliest years of the industry.

    And then in 1981, Richardson acquired them...:(

    But way before that, Cetron was big; they acquired Bloomfield Electronics and Tung-Sol during my lifetime (maybe early 70s?) and acquired part of RCA (tube production line) in the late 70s. They were doing great (I thought) but when Richardson wants an acquisition we all know what happens. Those acquisitions have not been good for amateur radio, although I'm sure they've been good for somebody.
     
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