5881's in a 6V6 amp?

Discussion in 'The Workbench' started by Wildeman, Sep 27, 2020.

  1. Wildeman

    Wildeman Well-Known Member

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    I have a nice old pair of 5881's and was wondering if i could try them in my Fender Excelsior?
     
  2. Matt_Krush

    Matt_Krush Well-Known Member

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    5881's are the equivalent to 6L6 tubes.
    You could put the 5881's in your amp, but it will probably be very cold bias as they are designed to be run at higher plate voltage than the 6V6.
     
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  3. Kinkless Tetrode

    Kinkless Tetrode Well-Known Member

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    The rule according to Aspen Pittman is that you can go up in tube grade but not down. That is: you can put 5881s in a 6v6 amp but in most cases you shouldn't put 6v6s in a 6L6 amp.

    A true 5881 is in between a 6V6 and a 6L6GC.
    6v6: 14 watts
    6L6 (1940s): 19 watts.
    5881: 23 watts
    KT66: 25 watts
    6L6GC: 30 watts
    7581A: 35 watts.
    So going to a 5881 in a 6V6 amp is isn't a big jump. It should work fine.

    The Sovtek tube often labeled a 5881 that became popular during the 90s, is a 30 watt tube. So it should be labeled a 6L6GC not a 5881. But when people think of a 5881 that is usually the tube they are thinking of. That is kind of a jump.
     
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  4. william vogel

    william vogel Well-Known Member

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    The power transformer probably can’t handle the additional heater current. The Excelsior isn’t a “high grade” amp. Meaning they probably didn’t engineer any wiggle room in the capacity of the components. It’s cathode biased, so even if the transformer could handle the current draw from the heaters, a bias resistor change would be needed to take advantage of the “better tubes”. Also the speaker impedance would need to be halved to put the 5881’s better into their proper operating conditions. If all this was met, 5881 sound better to me than 6v6 but it really won’t be an enormous difference.
     
  5. thetragichero

    thetragichero Well-Known Member

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    my excelsior (which at this point doesn't even resemble an excelsior since it has 5e3 transformers and preamp wired on tagboard, global negative feedback, reworked power board) runs 6l6gcs at around 25w
    i used the stock transformers in a fixed bias tube power amp that was designed around 6v6 but i ran 6l6g in it without issue. stock transformers are rather puny though
    i found that some mods to the preamp (paulc mod to phase inverter, changing preamp values to more fender-like along with running negative feedback from the power stage up to the preamp (have fun getting the wire up that packed conduit!) will give more clean headroom. just changing the power tube type without adjusting the cathode resistor ain't gonna do much
     

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