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    KT88s in the VTA ST-70 (again)


    Posts : 228
    Join date : 2013-04-02

    KT88s in the VTA ST-70 (again)

    Post by Dave_in_Va on Mon Aug 26, 2013 11:32 am

    I did a search and I think I've got the correct info but figured I'd better double check.

    My amp is a new VTA ST-70 and it's loaded with Valve Art EL34's biased at about 40 mA. From what I can gather, I can run KT88's by re-biasing at 50mA. Correct?

    I have a quad of Sovtek Kt88's from about 1999 with low hours. These are in a Vox AC100 clone and they are kind of a hassle to get to (half hour job). Do you think it's worth the time and trouble to swap in the KT88's?

    * It looks like I can swap the EL34's into the's a pic..


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    Bob Latino

    Posts : 2644
    Join date : 2008-11-26
    Location : Massachusetts

    Re: KT88s in the VTA ST-70 (again)

    Post by Bob Latino on Mon Aug 26, 2013 3:08 pm

    Yes - You can use any KT88 tube in a VTA ST-70 and bias them at 50 milliamps (.50 VDC bias voltage). On earlier VTA ST-70's sometimes you have to lower the main bias resistor from 10K to 7.5K (1/2 watt) so that KT88's will bias more towards the center of the bias pots rotation. On any CCS version of the VTA ST-70 amp this would not be necessary, since all CCS VTA ST-70's already have a 7.5K bias resistor as a main bias resistor (R39).


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