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Tube shields on EL84 output tubes?

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  • Tube shields on EL84 output tubes?

    Would it be a good idea or bad idea to put tube shields on EL84 output tubes? Thanks.

  • #2
    Originally posted by Axtman View Post
    Would it be a good idea or bad idea to put tube shields on EL84 output tubes? Thanks.
    No, as this would significantly increase bulb temperature and thus lower tube life.
    - Own Opinions Only -

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    • #3
      I've seen factory retainer spring loaded tube shields on 6aq5 output tubes. I have also seen IERC tube heat dissipating shields on tubes including 7189A, which is a 440v version of a EL84. Here is some temp reducing numbers.
      https://www.qsl.net/kh6grt/page4/shields/shields.htm

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      • #4
        I've had lab/test equipment where EL84s are screened with black painted aluminium cans and also certain tube amps where the reverb driver tube is an EL84 with a screening can.

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        • #5
          In general power tubes are not high gain preamp tubes, so shielding is not really needed.
          Education is what you're left with after you have forgotten what you have learned.

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          • #6
            I was speaking about typical tube shields, not special heatsinking types.

            A metal tube that isn't in good thermal contact with the bulb cannot act as a heatsink as the air between is a good thermal insulator.
            And a metallic inner surface will reflect infrared radiation back onto the electrodes, thus increasing their temperature.

            That's especially critical with EL84s as these are often operated at or above their dissipation and temperature limits in power amps.

            And - as Enzo said - power tubes in amps don't need shields, while preamp tubes might.

            (BTW, shields can only shield when grounded.)
            - Own Opinions Only -

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            • #7
              I agree, what's the point unless they're specially designed heat-sink types (and even then?) OTOH, spring retainers are a good idea, particularly if the tubes normally live "pins up."

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              • #8
                So, sounds like the heatsinking type would not be a bad idea for EL84's as they are typically run overly hot, and known to have longevity issues.
                Also wonder if the heatsinking types help reduce microphony. That would make them a double win for EL84's.
                "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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                • #9
                  The reason for me asking the question is that I am working on has four 9 pin tube sockets designed to take tube shields. There is a preamp tube, phase splitter tube and two output tubes. Of course when I got the amp all the shields were missing.

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                  • #10
                    Do not assume that since a tube socket has a base to accept a shield that it is supposed to have one. I use shield based sockets all the time without having shields in mind.
                    Education is what you're left with after you have forgotten what you have learned.

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