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    ST-120 / EL-34 and or 7591 ?



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    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

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    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.



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    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 ...


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

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