1. In celebration of the launch of the new redesigned site, we just gave away a FREE antenna! Click Here to See Who Won!

homebrew 811A amplifier project

Discussion in 'Amplifiers' started by loosecannon, Jun 7, 2009.

  1. loosecannon

    loosecannon break on through

    Joined:
    Mar 9, 2006
    Messages:
    1,968
    Likes Received:
    175
    hi all,

    i am researching the feasibility of building one of these over the summer.

    i have a set of plans for building a 2 driving 4 6LF6 tube amp, but obviously these tubes are far too obsolete and expensive to consider using.
    yes, EL-509's would work, but im thinking of using a real transmitting tube that will be around for a while to come.

    yes, i also know that i could make a single tube amp, but i think ill be happy using the 811a's.

    i want to do a 1 driving 4 grounded grid configuration, and i want to know how this will work.

    i have an old kenrich 2 driving 5 20LF6 amp that i would like to scrounge as many parts from as i can.

    is this possible?
    is the plate voltage required for the 811a's too much for the components in the kenrich amp such as the tune and load caps?

    the plans i have are basically to build a palomar 350z amp.

    so can it be done?



    basically, build a 1 driving 4 sweep tube amp but use 811a's?

    any insight is appreciated.
    LC
     
    #1
  2. Slam Dunk

    Slam Dunk Member

    Joined:
    Jul 11, 2006
    Messages:
    79
    Likes Received:
    0
    Yes, it could be done but, think along different lines and come up with a better amplifier.

    The 811a tube was originally designed for audio use and it really wasn't for use at RF frequencies but, what do I know, the manufacturers use them all the time (but you don't see them operating above 30 MHz).

    The 572b tube is the beefed-up RF version of the 811a and, it fits in the same tube socket and has the same voltage and current requirements. If, you were to build that amp as a single stage model with 4 X 572b with the right B+ transformer you could see 1200 watts PEP or more.

    There is not much you can get from the Kenrich amp chassis, plus, you are wanting to build a reliable amp, you are better off buying new or surplus parts for your project.

    Get all of your parts and then lay them out on you 19" chassis; under the bottom, separate the input section and output sections with metal walls. Buy yourself some air variables (out of a Heath-kit or buy vacuum variables on eBay).

    PM me if you get beyond the contemplation stage.

    .
     
    #2
  3. n8fgb

    n8fgb Active Member

    Joined:
    May 18, 2008
    Messages:
    399
    Likes Received:
    44
    Why even use glass. Use Russian GI7B's instead. Will last you forever, if you build it right, and don't abuse it.
    Rich
     
    #3
  4. Captain Kilowatt

    Captain Kilowatt Professional Amateur Staff Member

    Joined:
    Apr 6, 2005
    Messages:
    11,519
    Likes Received:
    2,902

    Actually the 811A is a very good RF tube providing it is operated within it's limits. Do not exceed 65 watts plate dissapation and do not exceed 1500 volts on the plate.Max plate current is 175 mA per tube.

    While the 572B is basically a beefed up version of the 811A the voltage and current reuuirements are not the same other than the filaments.The 572B likes to be run near their max plate voltage of 2200 volts and will withstand a plate current of 275 mA per tube.They will handle 160 watts of plate dissapation however. I believe Svetlana specs their 572B for 2750 volts but I would not do that regardless what they say. That is a little extreme.The Svetlana 572B is a differant animal and is built for the audio market, not the RF market. The element spacings are wider which makes them able to handle a higher voltage but makes them more suited to audio freqs than RF.
     
    #4
  5. linearone

    linearone King of NY

    Joined:
    Apr 2, 2005
    Messages:
    614
    Likes Received:
    4
    did this ever get completed? I'm still interested.
     
    #5
  6. it would be interesting to see how inexpensively someone could build a 25-30MHz box with ab biasing and filtering just using a single one of the 811a tubes . it seems like it could be done for less than the 2x2879 x-force base amp , but with glass bragging rights . plus a 811a is cheaper and easier to replace (when necessary) than a pair of 2879s .

    or make it run on 12 volts so folks that already have a 40 amp or larger power supply can save even more bucks .
     
    #6
  7. n0zna

    n0zna W9WDX Amateur Radio Club Member

    Joined:
    Nov 3, 2008
    Messages:
    1,253
    Likes Received:
    140
    A frend made one with three 811a tubes and parts from hamfest,he spent less than onee hundred dollors,he uses it everywhere,3.900 to 29mhz...3 to 100 wt drive...its great on 11 mtrs also...he is in Missouri..about a mile from me..73s de JW
     
    #7
  8. Kamikaze

    Kamikaze Active Member

    Joined:
    Dec 27, 2010
    Messages:
    318
    Likes Received:
    7
    Eimac 4-65As are around and might play OK at 800-1000 volts. They can be triode strapped.

    Might be worth a try.
     
    #8
  9. n0zna

    n0zna W9WDX Amateur Radio Club Member

    Joined:
    Nov 3, 2008
    Messages:
    1,253
    Likes Received:
    140
    cx250 is cool also
     
    #9
  10. Crusher

    Crusher Active Member

    Joined:
    May 12, 2007
    Messages:
    539
    Likes Received:
    58
    A ham operator I often talk to suggested using 1 811A to drive 6 811A's for full output. Run it at 1kV on plate and u could cruise all day.
     
    #10
  11. wavrider

    wavrider W9WDX Amateur Radio Club Member

    Joined:
    Jun 2, 2009
    Messages:
    2,157
    Likes Received:
    324
    Some of the old Gonsets used four 811-A, with a heck of a PS in them old amps.

    Back in the day they believed in CCS PS, worked AM real well.
     
    #11
  12. n0zna

    n0zna W9WDX Amateur Radio Club Member

    Joined:
    Nov 3, 2008
    Messages:
    1,253
    Likes Received:
    140
    it depends on what you are running,if its a low wt radio,have a driver tube...if its a good wt radio rci,hr2510 or ft101 or so no driver tune...811 tubes or 572 or cx250 rock,73s from the cold county of Missouri...
     
    #12

Share This Page