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4cx5000 help

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

  1. nomadradio

    nomadradio Analog Retentive

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    The one other thing I would wonder about is the output-tank circuit.

    The tube has a lot of output capacitance, and this appears in parallel with the plate-tune capacitor. Puts a severe upper limit on the capacitance that it can add to the circuit and still resonate the coil.



    When the coil is trimmed for best output, that control will tend to be near, but not all the way to the minimum end of travel.

    If it goes all the way to the end without showing a peak, the coil needs to get spread out a bit. What usually works best is the max coil inductance that still permits you to peak the plate-tune near but not quite at the minimum-capacitance end.

    73
     
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  2. 543_Dallas

    543_Dallas Sr. Member

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    Hopefully he used the plate impedance formula to get him close then worked out the tank with an antenna analyzer. You can do the same with the input tune if you know the impedance. When I changed a gs35b amp to an 8877 the input tune was perfect and the plate and load settings were close before it was fired up for the first time.

    If you're working on the imput match by trial and error I would use an electrical half wavelength of coax between the amp input and swr meter. That way the coax isn't influencing the input swr reading as much. You may think you have a good input tune then it all goes to hell when the jumper length is changed. Once you get the input tune right you should have a low input swr with any coax length you use.
     
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  3. 543_Dallas

    543_Dallas Sr. Member

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    Coil q should also be considered. Too low will make the circuit wide banded and inefficient. Too high will make tuning touchy and high circulating current can damage light weight components....probably not going to be an issue with this amp. There's more to it than just having enough range with your capacitors.
     
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  4. BlowinSmoke

    BlowinSmoke Active Member

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    Nowhere in any spec sheet by ANY company who makes a 4CX5000 tube do they show that tube being used in grounded grid. It is NOT an option. You are on a fools mission. Go to a real triode meant for GG use. Don't waste your time any more
     
  5. BJ radionut

    BJ radionut Supporting Member and 6m addict

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    strapped to triode, 4CX5000 A and R tubes. By strapping them both to ground, you have a huge triode. Problem being, equalizing currents between screen and grid. It can, and is done, but isn't the "best" way.

    You can split the currents, Bill Orr, as others, have designs in his and other radio style handbooks.

    The 4CX5K is good for about 4 kilowatts of RF carrier, AM modulated.

    http://amfone.net/Amforum/index.php?topic=18369.10;wap2
    "I admit no experience GG-ing this tube family but I have been told by one guy who GG-d a 4CX5000 that great care has to be taken during operation that the loading is enough, or the grids will fry before the operator knows it. "
    Just another quote from info link above.


    For linear grounded grid (GG) operation driving the tube in triode config, you will need at good 1500w pep output amplifier for drive.

    Just info I found in regards to your query...I also looked at your set-up and may not be seeing everything from pictures, but was not seeing the bonding of the screen and/or grids to good ground...using the self-taping mounting screws for this purpose may present an issue.
    I question air flow being adequate also...that's a bunch of air space in the bottom of that chassis to pressurize...but having never used that type of config...only speculation (SWAG) on my part.
    GL
    All the Best
    Gary
     
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  6. nomadradio

    nomadradio Analog Retentive

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    All other things being equal, that 3000 Watt average reading on a Bird should be roughly 12000 Watt peaks.

    How much do you need?

    Not too shabby for a repurposed tube that was not optimized for this service.

    And if this tube is a "pull", you may be finding out why it got pulled.

    73
     
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  7. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    If it can actually provide that without flat-topping. When pushed to the limit of it's abilities it may not provide much more peak power than steady state carrier especially if the tube is soft.
     
  8. 543_Dallas

    543_Dallas Sr. Member

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    Yep. There was a time when I would run an amplifier so hard that the difference between pep and average was not very much. That was our version of alc. :LOL:
     
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  9. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Sounds like the pair of 6KD6's that I ran until the plates were the same colour as the filaments. :LOL:
     
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  10. BJ radionut

    BJ radionut Supporting Member and 6m addict

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    About like running RCA or Sylvania 811A's with 2700vdc B+...back in the days when you could buy them for 5 bucks...:love::ROFLMAO:
     
  11. DARKDUDE

    DARKDUDE Member

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    Perhaps he might try a zener on his cathode so it is easier to drive?
     
  12. Shockwave

    Shockwave Sr. Member

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    Holding 7,500 volts on the plate is good but what we really need is an idea what the plate current is doing with respect to drive and output power? If your output circuit is right, you should be seeing under 1 amp at the output power you have now. If it's higher that this, the zero bias is not suitable for this application or your plate cap may be too high in value. What is the minimum pf value of the plate tune cap?

    For AM you really don't need more than 250 ma. of idling current on this tube. At this point, current should still be increasing proportionally as more drive is applied. If it does not, this is a strong indication the tube is soft. Even if the input circuit is off, more drive should still draw more current and produce more output. Improvements here should only reduce drive requirements and input VSWR.

    Additionally, that resistor in series with the filament that Nomad pointed out is not very good at providing a stable voltage to the tube. The 13.8 volts from the vehicles electrical system is not nearly regulated well enough to start with before using the resistor. Fluctuations of 1 volt are very easy to see in a condition where tenths of a volt matter. If you drop just a half volt on the filament under load, that can kill your peak emissions totally.

    Once it does start making good output, you are likely to find that chimney is not suitable for the tube either. It is not forming any type of seal around the base of the tube and much of the air pressure may be escaping around the tube base.
     
    #27 Shockwave, Oct 22, 2019
    Last edited: Oct 22, 2019
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  13. Shockwave

    Shockwave Sr. Member

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    Because the cathode in this tube is a directly heated filament, powered by the vehicles negative ground electrical system, it makes floating the cathode below ground potential, difficult to do. This in itself should not reduce drive requirements but it can free up wasted plate dissipation and power supply current being used to heat the tube. That can cause output power to go up once corrected through improved efficiency.
     
    #28 Shockwave, Oct 22, 2019
    Last edited: Oct 22, 2019
  14. Shockwave

    Shockwave Sr. Member

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    I had an old homebrew 4 tube RCA 811A amplifier that actually ran 2,700 volts on the red plates. Worked well for many years until those RCA tubes had to be replaced with newer versions. Then, the only thing that got the amp back on the air was a set of 572B tubes.
     
  15. Shockwave

    Shockwave Sr. Member

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    I noticed there is no DC blocking choke across the output. That could end up being a safety problem and one should be installed. The RF bypass caps on the filament choke are also missing. That could cause significant RF to be on your DC supply voltage. Neither of these should be effecting the output power though.
     

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