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    Question regarding high bias voltage

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    wolftrouble

    Posts : 4
    Join date : 2013-05-01

    Question regarding high bias voltage

    Post by wolftrouble on Wed May 01, 2013 8:36 am

    Hopefully someone can help out with this!

    I recently (a month or two back) finished an ST-120 build with great success. At the time I was using them with 8ohm speakers, and adjusted the bias appropriately.

    The other day my new speakers were delivered, but they're 4 ohm, so I cracked the case open again to switch to the 4 ohm takeoffs from the output transformers. It went without incident (I think...), but when I went to adjust the bias I found that the left channel was showing a ridiculously high voltage (30v) from the takeoff pin. The other 3 were, as expected, right around the .550v-ish mark.

    Interestingly, there's nothing audibly wrong with the sound - When playing some test tracks everything sounds fine, as far as I can tell.

    I don't know for certain that this was reading 30v before I switched to the 4ohm takeoffs, so it's possible it's unrelated, but it's the most recent change I've made, so I figure it's worth bringing up.

    Any idea what could be wrong?

    Zimmer64

    Posts : 112
    Join date : 2013-01-29
    Age : 52
    Location : Switzerland

    Re: Question regarding high bias voltage

    Post by Zimmer64 on Wed May 01, 2013 10:38 am

    Try to switch that tube around and check if the issue follows the tube. If it does not, check the resistor values on the tube socket. If the issue follows the tube then the tube is damaged.

    Michael

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-27
    Location : Massachusetts

    Re: Question regarding high bias voltage

    Post by Bob Latino on Wed May 01, 2013 1:34 pm

    Hi,

    Michael has given good advice ... Check first to whether the tube is at fault by moving a different tube to that tube socket. If another tube does the same thing in that socket then check to see if the 10 ohm resistor from pin #8 to chassis ground on that tube socket still measures 10 ohms. If that resistor does not measure 10 ohms then you need a new resistor there.

    Bob

    wolftrouble

    Posts : 4
    Join date : 2013-05-01

    Re: Question regarding high bias voltage

    Post by wolftrouble on Wed May 01, 2013 6:27 pm

    Thanks guys. I had a moment this morning before I left for work to test the tube swap and it did NOT follow the tube, so it looks like the (likely) culprit is the resistor. I'll test it out tonight to verify. Thanks again!

    wolftrouble

    Posts : 4
    Join date : 2013-05-01

    Re: Question regarding high bias voltage

    Post by wolftrouble on Thu May 02, 2013 4:48 am

    And confirmed that pin 8 to ground is about 12M ohm. That's not right!

    Conveniently I have a number of places nearby I can pick up a new one, so I'll try replacing it tomorrow and see how it goes. Thanks for the advice.

    wolftrouble

    Posts : 4
    Join date : 2013-05-01

    Re: Question regarding high bias voltage

    Post by wolftrouble on Fri May 03, 2013 7:15 pm

    Just to close this discussion out: Opened up the case last night and found that the bias resistor on that socket had cracked/chipped (I might have damaged it installing it), and after replacing it everything's working great. Thanks for the help guys!

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