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    MK III bias recommendation with Electro Harmonix KT88's

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    gflash

    Posts : 3
    Join date : 2014-01-19

    MK III bias recommendation with Electro Harmonix KT88's

    Post by gflash on Sat Nov 22, 2014 11:30 am

    I just replaced the 20 year old Sovtek 6550 WA's in my rebuilt SOS (sort of stock) MK III's with a set of Electro Harmonix KT88's. By sort of stock I mean the circuit is stock, but the first two capacitor sections in the power supply are handled by discrete caps mounted on a board under the chassis and the metal quad cap handles the second two capacitor stages. I put some pictures in the photo section. My B+ voltage is 548V (measured at eyelet #4 on the driver board), too close to the rating of the quad cap for comfort. The line voltage at my house is 123VAC.

    Anyway, the Dynaco manual recommends a bias current of 70mA per tube. But I understand this was based on a B+ of 475V and vintage 6550 or KT88 tubes. Right now I am running the EH KT88's at 45mA/tube, which seems to be about right given the high B+ voltage. Everything seems to sound fine. I am more interested in tube life than .1% less distortion.

    I tried searching the web for recommendations, but they are all over the map. What do other people on this forum use?

    Thanks.

    Guest
    Guest

    Re: MK III bias recommendation with Electro Harmonix KT88's

    Post by Guest on Sat Nov 22, 2014 1:23 pm

    a safe BIAS setting for the KT88 is 45-50mV per tube or 90-100mV if the BIAS of two tubes is tied together as it was in the old circuit.

    deepee99

    Posts : 1337
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: MK III bias recommendation with Electro Harmonix KT88's

    Post by deepee99 on Sat Nov 22, 2014 1:57 pm

    123 volts is pretty hot for a tube amp, so I wouldn't go above 50. I run my KT-120s at 45/tube and hear no audible difference than when they were set at 60.

    gflash

    Posts : 3
    Join date : 2014-01-19

    Re: MK III bias recommendation with Electro Harmonix KT88's

    Post by gflash on Sat Nov 22, 2014 2:01 pm

    Yes, the bias circuit is stock so 45mA*2*11.2 Ohms = 1V. (Or pretty close)

    Thanks.

    sKiZo

    Posts : 1312
    Join date : 2013-04-01
    Location : Michigan USA

    Re: MK III bias recommendation with Electro Harmonix KT88's

    Post by sKiZo on Sat Nov 22, 2014 4:40 pm

    Not only the tubes ... everything has to work harder at that VAC. That's why I built me mah lil bucker ... allows me to load the amp properly at more suitable line power levels.



    I run my KT88's at 55mV and KT120's at 60mV ... no problems, and a bit more punch.

    I like punch ...


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