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4cx5000 help

Discussion in 'Home Brew' started by Snake753, Aug 30, 2018.

  1. Snake753

    Snake753 New Member

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    I've got a 4cx5000a setup for mobile use in my truck with grounded screen and grid and can't get more than 3kw rms at 7500v with plenty of current to spare is there a different way I should go about this maybe?


     

  2. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Hmmmmmm.....grounded grid so it is low gain. What is your drive level going into it?
     
  3. Snake753

    Snake753 New Member

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    I went by eimac data on turning a tetrode into a high power triode and Ive drove it with 100w rms and I've drove it with 500rms and the most I can get is 3kw give or take on modulation this is all with a bird and mobile 3phase setup at 7500v
     
  4. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    If the amp was grid driven it would have MUCH higher gain and much more output even as a triode.
     
  5. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    Sounds like an interesting set up. Do you have any pictures?
     
  6. Snake753

    Snake753 New Member

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    20180818_203529.jpg
     
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  7. Snake753

    Snake753 New Member

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    20180729_135736.jpg
     
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  8. 543_Dallas

    543_Dallas Sr. Member

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    Is this setup all completely new to you or did the low power happen all of a sudden? It looks like you are playing with the input tuning in that last picture so Im guessing its a fresh build.

    Are you sure the tube isn't soft? The input swr will get higher when the toob starts getting soft. A gg triode gone soft will also run higher grid current than it normally would.

    Are you just looking at plate volts during operation? What about plate current, grid current and filament voltage. Make sure nothing is going crazy during operation.

    A tetrode in grounded grid triode configuration should have at least 10db gain. You should be able to hit 3kw with 300 watts or less.
     
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  9. nomadradio

    nomadradio Analog Retentive

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    Have you checked the impedance match to the tube's input while it's keyed?


    I see a couple of small coils laying around loose, as if they're meant for a PI-network to match the input side. The pic appears to have been taken while the input-match circuit was being zeroed in?

    Are you running this tube with zero bias? Looks like it. Might be worth finding out how much idle current this tube draws when keyed with no driver. If it's too high, that can be trouble.

    73
     
  10. Snake753

    Snake753 New Member

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    It is a new build the tube is rebuilt buthe I just checked it again I put 300bird in I get right about 3kw to 3500bird out at 7500v and the input circuit has changed alittle since that pic because Ivery been having trouble figuring out how to calculate the input circuit since I'm driving at the filiment leads not the grids if some one can guide me in the right direction that would be great also but with 300w in I've got 20w input reflect I haven't got it just right I've mostly guessed on the input tuning and played with it
     
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  11. Dmans

    Dmans Well-Known Member

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    Snake753,
    300w in with 20w reflect calculates to about a 1.7:1 SWR. Slow methodical, painstaking adjustment/component fabrication would get it lower but you would need to get the reflect down to 2 watts to have a 1.1 SWR.
    Doubtful that is your issue.



    73’s
    David
     
  12. 543_Dallas

    543_Dallas Sr. Member

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    That's cathode driven.

    The input needs some work. Get that right first.

    A high mu triode like a 3cx3000 would be better in this application. JMO
     
    #12 543_Dallas, Aug 30, 2018
    Last edited: Aug 30, 2018
  13. nomadradio

    nomadradio Analog Retentive

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    This tube's cathode draws 90 Amps at 5 Volts.

    The wire on the cathode choke looks a little wimpy for that kind of current.

    Have you measured the cathode (filament) voltage *AT* the tube socket? Excess voltage drop to the cathode not only holds back power, but you risk damage to the cathode. Running RF with the cathode at a temperature that's to low screws up the vital layer of thorium. Without that, the cathode becomes "soft", and saturates at a lower current than it's rated to pull.

    Best example of this is with the Black Cat JB2000 amplifier. Anyone who runs it from 120 Volts will see the filaments dim when you key the thing. Wears out a set of tubes in months of heavy use. The power just drops off, bit by bit until you can't even get half what it used to deliver.

    That model is vulnerable because it uses only a single transformer. Makes the filament voltage drop more than it does in a design with a separate filament transformer.

    Running it from 240 Volts removes this risk. And simply turning the power down to half would probably prevent the "120-Volt problem".

    Never have seen anyone try that solution.

    73
     
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  14. Snake753

    Snake753 New Member

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    This is a 4cx5000a runs at 7.5v at 75amps and I've tested and I'm sitting right at 7.5v and pulls 74amps on my amp meter and I've got test leads that connect right to the socket to test the voltage
     
  15. nomadradio

    nomadradio Analog Retentive

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    Sounds like you have that angle covered.

    Worth asking, though.

    73
     
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