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    ST-120 / EL-34 and or 7591 ?

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    scottlowe

    Posts : 38
    Join date : 2014-04-05

    ST-120 / EL-34 and or 7591 ?

    Post by scottlowe on Sat Aug 02, 2014 3:26 pm

    can I use EL-34 or 7591 tubes in a VTA ST-120? IF so what is a good bias range ? I now use KT-120's and run them at .585, and I have some 6550's I like to run at .535.thank you .I bet this could win stupidest question of the week if it wasn't for the fact that the stupidest question is: the one you don't ask.

    Bob Latino
    Admin

    Posts : 2377
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-120 / EL-34 and or 7591 ?

    Post by Bob Latino on Sat Aug 02, 2014 4:36 pm

    I don't recommend the use of EL34 tubes in a VTA ST-120. Today's EL34's would probably have a short life in an ST-120 which runs B+ voltages in the range of 500 VDC. EL34's are better for ST-70 type amps which run a B+ of about 440 VDC. Another thing .. The bias system of the VTA ST-120 is set up for 6550, KT88 or KT120 output tubes. If you try to bias an EL34 in the ST-120, it will bias way over to one side of the bias pot's range OR you may not be able to bias an EL34 at the recommended bias setting of .400 VDC.

    Bob

    sKiZo

    Posts : 1285
    Join date : 2013-04-01
    Location : Michigan USA

    Re: ST-120 / EL-34 and or 7591 ?

    Post by sKiZo on Sat Aug 02, 2014 9:08 pm

    Wouldn't help tube life due to the excessive B+, but it's my understanding you could "swing" the bias range on the controls with an R39 resistor substitution? EL34's are cheap enough and have a unique flavor that it might make sense to give it a try ...



    tubes4hifi
    Admin

    Posts : 1261
    Join date : 2008-11-30

    Re: ST-120 / EL-34 and or 7591 ?

    Post by tubes4hifi on Sun Aug 03, 2014 10:51 pm

    7591 tubes have a totally different pinout and can't be sub'ed for a 6550, EL34, or KT88

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