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    st70 bias problem

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    dynacoman

    Posts : 13
    Join date : 2011-09-03

    st70 bias problem

    Post by dynacoman on Tue Feb 21, 2012 12:37 am

    I'm in need of some direction with my bias problem. I'm currently using the Russian tubes 6p3s-e these were biased at 1.25v (12 months old). The bias current drifted up to 1.6v on the left channel and 1.34v on the right channel and I can't seem to re-adjust it back down. How should I proceed with testing? The quad cap it's not the original it's new a 40/20/20/20 525v. I've gone thru the bias set rail and everything is good it also has the curico bias expansion done to it. A diode (uf4007) in place of the selenium rectifier(new). I've taken voltage readings from the 1st potentiometer at point 1 (-38.91)and point 2 (-25.90) If anyone can give me a little more insight here as to what I should be looking for to help me figure out why this drifted I'd greatly appreciate the help.
    Well I also now have the readings from the quad cap
    Tubes out rectifier in
    half circle 484v
    square shape 330v
    triangle shape 418v
    blank 484v

    Tubes in
    Half circle 371v
    Square shape 250v
    Triangle shape 418v
    Blank 365

    I changed out the rectifier and had similar readings.

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: st70 bias problem

    Post by Bob Latino on Tue Feb 21, 2012 6:54 am

    Your voltage at test point 1 is low. It should be -45 VDC to maybe -48 VDC

    Try this ..

    1. Replace both caps on the 7 lug terminal strip - one or both could be leaking if they are the original caps. The "bias drifting" that you mention could be a clue that either or both of these caps are leaking.

    2. On the bias expansion mod try using a smaller value resistor to parallel the 10K resistor between lugs 3 and 4 on the 7 lug terminal strip. Instead of a 15K use a 10K or a 7.5 K resistor.

    Bob

    dynacoman

    Posts : 13
    Join date : 2011-09-03

    Re: st70 bias problem

    Post by dynacoman on Tue Feb 21, 2012 9:17 am

    Thanks bob, you just made me think about the 2 caps on the rail there new but I did raise the value to 100uf from 47uf do you think this could be the problem? Also I read the neg. voltage off the 7 lug rail and here's what I found tp1 -75.5 tp2 -37.5 obiously the reading I supplied earlier were direct from the potentiometers. Please let me know your thoughts.
    Thanks again for the help!


    Last edited by dynacoman on Tue Feb 21, 2012 9:55 am; edited 1 time in total

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: st70 bias problem

    Post by Bob Latino on Tue Feb 21, 2012 9:30 am

    Raising the capacitance value of those two caps won't really have any effect on the bias range. Try altering the value of that resisistor between lugs 3 and 4 until the effective value of the total resistance between lugs 3 and 4 is somewhat lower.

    Bob

    dynacoman

    Posts : 13
    Join date : 2011-09-03

    Re: st70 bias problem

    Post by dynacoman on Tue Feb 21, 2012 9:58 am

    Just updated my post

    dynacoman

    Posts : 13
    Join date : 2011-09-03

    Re: st70 bias problem

    Post by dynacoman on Tue Feb 21, 2012 10:30 am

    Well know I'm a little closer 10k resitor install and I'm showing tp1 -45.2 and tp2 -30.1 I've got to wait 1/2 for the electronics store to open as I don't have any small value resistors in 3w. The bias is down to apron 1.2 and 1.0.

    dynacoman

    Posts : 13
    Join date : 2011-09-03

    Re: st70 bias problem

    Post by dynacoman on Tue Feb 21, 2012 11:18 am

    Ok, so I paralleled a 10k+15k = 6.2k and it wouldn't go past 1v max when setting the bias. tp1 dropped to -35v. So, I thought to just go back and replace the single 15k resistor with a new one and this may sound strange but, I was able to set the bias again to 1.25v on both tubes. I don't know if the original 15k resistor was bad intermittent or what I did ohm it out several time and it showed 15.1k but everything is working again. My bias range is aprox. less than 1v and up to 1.4v. seems ok. Any other tests you want me to preform let me know. maybe when I installed the new caps I didn't have an extremely good solder connection or something.

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: st70 bias problem

    Post by Bob Latino on Tue Feb 21, 2012 8:35 pm

    Yes - You could have had a bad connection on that resistor OR the resistor is going bad or changing its resistance when it is hit by current. Ideally test point 1 should be about -48 VDC and test point 2 about half that, maybe -24 VDC. Sometimes you have to juggle those two resistor values until you get those negative voltage values on those two points. The reason that you have to alter those bias resistors when you switch from the selenium rectifier to a diode is that a diode has more "forward voltage" than the selenium rectifier. Those two original 10K resistors were set up to give the correct bias "range" with the selenium rectifier, not with a diode.

    Bob

    dynacoman

    Posts : 13
    Join date : 2011-09-03

    Re: st70 bias problem

    Post by dynacoman on Wed Feb 22, 2012 3:37 am

    Thanks again bob for your advise everything is sounding great in my house once again!!!

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