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    ST-120 Bias issue

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    sackchamp56

    Posts : 2
    Join date : 2013-03-16

    ST-120 Bias issue

    Post by sackchamp56 on Sat Mar 23, 2013 10:56 pm

    I recently built an ST-120 kit and everything seems to have gone pretty well. I was able to get everything up and running without a hitch. On initial power up, i got all four bias readingsto read very close to the suggested .550 VDC. I listened to it for several hours and decided to recheck the bias readings. The first 3 were still very close to spec, but the fourth reading is now showing extremely high readings 37.4 VDC. Evertyhing still works fine. I opened it up and looked at everything and all of the connections are still looking good and I verified that everything is wired identically for all four. If the resistor that connects from that tube socket to the ground lug failed would it cause that problem? I went ahead and ordered a resistor to replace it, but I was wondering if anyone had any ideas in the meantime.

    stewdan

    Posts : 161
    Join date : 2010-03-07
    Age : 78
    Location : Houston Texas

    ST-120 Bias

    Post by stewdan on Sat Mar 23, 2013 11:26 pm

    I had one 10 ohm resistor fail on my VTA ST-120 unit and I could not bias that tube. (The bias read something like 45. volts).

    In my case when I measured the resistor's value between chassis ground and the end of the resistor where it attaches to the tube socket I got 0 Ohms.

    When I replaced that resistor with a new one, I was able to set the bias to the value I desired, which was 0.44 volts. Yes I know, on the low side. But, better to be low than sorry!!

    The amplifier sounds real good.


    sackchamp56

    Posts : 2
    Join date : 2013-03-16

    Re: ST-120 Bias issue

    Post by sackchamp56 on Sat Mar 23, 2013 11:53 pm

    Thanks, that sounds like what i'm seeing. I'll give it a shot.

    I agree the amps sounds incredible, couldn't be happier.

    guentherj2

    Posts : 50
    Join date : 2013-06-22
    Location : Bellingham, WA

    Re: ST-120 Bias issue

    Post by guentherj2 on Sun Nov 03, 2013 4:57 pm

    I have the already assembled ST120 (only a few months old) and am experiencing a similar problem. It has happened twice. First the left rear KT88 bias resistor blew and damaged that KT88 (or a bad KT88 damaged the 10 ohm resistor???). I replaced the bias resistor and put in four NOS Svetlana KT88s and ran it for maybe 50 hours - POP - the right rear bias resistor went bad and now I have another damaged KT88.

    My question is the chicken or the egg??? Did a bad tube result in damaging the bias resistor or is something damaging the bias resistor and then damaging the tubes? I don't think I had a bad KT88, I think some underlying problem is damaging the bias resistors - but what???

    Any and all suggestions appreciated.

    sKiZo

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

    Re: ST-120 Bias issue

    Post by sKiZo on Sun Nov 03, 2013 5:14 pm

    Good place to start would be to refer to the initial start up procedures that came with the kit instructions ... with only the driver tubes installed, check the pin voltages on the power tube sockets with a VOM and make sure they match.

    peterh

    Posts : 680
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: ST-120 Bias issue

    Post by peterh on Sun Nov 03, 2013 5:35 pm

    guentherj2 wrote:I have the already assembled ST120 (only a few months old) and am experiencing a similar problem. It has happened twice. First the left rear KT88 bias resistor blew and damaged that KT88 (or a bad KT88 damaged the 10 ohm resistor???). I replaced the bias resistor and put in four NOS Svetlana KT88s and ran it for maybe 50 hours - POP - the right rear bias resistor went bad and now I have another damaged KT88.

    My question is the chicken or the egg??? Did a bad tube result in damaging the bias resistor or is something damaging the bias resistor and then damaging the tubes? I don't think I had a bad KT88, I think some underlying problem is damaging the bias resistors - but what???

    Any and all suggestions appreciated.
    it started with a tube flashing. Or a severe overload ( electrical surge ?)

    But if the tube(s) are shorted after the incident i's say that it started with the tube.
    Try another brand.

    guentherj2

    Posts : 50
    Join date : 2013-06-22
    Location : Bellingham, WA

    Re: ST-120 Bias issue

    Post by guentherj2 on Sun Nov 03, 2013 7:24 pm

    As stated, mine is the pre-assembled so my paperwork does not describe the same initial startup procedures as the kit - apperently. That said, I will check the KT88 pin voltages with only those tubes installed after I get a new resistor in place. I did get as far as checking the rectifier tube pin voltages before the resistor popped the second time and those were all within spec. Maybe the rectifier diode mod saved me a rectifier tube?

    No tube flashing was noticed.

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