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Thread: EZ81, 6CA4, EL84 Question.

  1. #1
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    EZ81, 6CA4, EL84 Question.

    1. 6CA4/EZ81 Tube. Can I use this tube with Four EL84 instead of a one GZ34.

    2. Can I use Two.

    Strange question I know,but I have wanted to build a matchless or AC30 Style Amp with EZ81 as the rectifier tube.

    PT is 350 - 350 VAC.

  2. #2
    Supporting Member Alex R's Avatar
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    I know it won't work with one, and will leave the question of running two to someone who knows... but what I would warn against is running a rectifier on the same heater circuit as the rest of the tubes (which may be what is tempting you). These rectifiers are prone to short plate-to-cathode (and heater) when they fail, which has a lot of potential for damage throughout the amp.

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    Thanks, for the reply, I have experienced that fault with an older EL84 Tube amp with two EL84's and EZ81.

  4. #4
    Noodle of Reality Steve Conner's Avatar
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    I don't see any reason why running two EZ81s in parallel wouldn't work.

    But yes, EZ81s have the cathode that's supposed to be isolated from the heater, hence that nasty failure mode where the insulation breaks down. If you want to use them, it's best to run them off a separate heater winding that you can connect to the rectifier cathodes.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    Quote Originally Posted by Steve Conner View Post
    I don't see any reason why running two EZ81s in parallel wouldn't work.

    But yes, EZ81s have the cathode that's supposed to be isolated from the heater, hence that nasty failure mode where the insulation breaks down. If you want to use them, it's best to run them off a separate heater winding that you can connect to the rectifier cathodes.
    Steve,
    Sorry that I never got back to this. Any suggestions on how it should be wired or a suggested circuit?

  6. #6
    Senior Member ThermionicScott's Avatar
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    You'd need either a PT with two separate 6.3V windings, or an additional filament transformer that just heats the rectifier tube. Failing that, I think the best policy is to stay within the max cathode-to-heater voltage ratings, and to make sure the filament voltage is not too high (additional heat makes the filament insulation more conductive). Using an artificial center tap on the heater winding as opposed to simply grounding the CT would be a good idea as well.

    - Scott

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    Another choice to consider is the Weber Coppercap. They don't require heater taps and allow greater flexibility in choosing voltage sag and the first filter cap value. I find them very handy for cathode biased amps to adjust the B+.

    Weber Copper Cap Rectifiers

  8. #8
    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    Just use solid state diodes instead of two EZ81s... assuming the 350-0-350 PT has a high voltage center tap like it should, put a 5 watt, 47 ohm to 100 ohm resistor in the center tap lead to ground for a little bit of sag, simulating the effect of a tube rectifier. Not a perfect solution but VERY inexpensive.
    Bruce

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    www.missionamps.com
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