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    Stereo 70 bias problem

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    dondi52

    Posts : 3
    Join date : 2011-06-09

    Stereo 70 bias problem

    Post by dondi52 on Thu Jun 09, 2011 11:26 pm

    Hello. I am new to forum. My Stereo 70 began acting up a week ago and I am hoping for guidance. When I turn it on first time in the day, the bias shoots up past 1.5 to 1.75 or so, stays there for a minute, and then in the course of 10 minutes settles down to 1.5 or even drifts a little lower. After this it holds pretty steady. I have replaced the selenium rectifier with the appropriate diode, and the bias supply caps too. Not sure what to try next. Thanks in advance. Dan

    j beede

    Posts : 317
    Join date : 2011-02-07
    Location : California

    Re: Stereo 70 bias problem

    Post by j beede on Thu Jun 09, 2011 11:52 pm

    Identical behavior in the bias for both channels?

    dondi52

    Posts : 3
    Join date : 2011-06-09

    stereo 70 bias problem

    Post by dondi52 on Fri Jun 10, 2011 9:36 am

    yes, the problem is the samee in both channels.

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Stereo 70 bias problem

    Post by Bob Latino on Fri Jun 10, 2011 12:08 pm

    Hi,

    There could be an issue with your quad cap (DC power storage). The cap could be old and it may take a minute or so to "reform" each time the amp is turned on. The amount of bias needed is related directly to the B+ high voltage. It is possible that the voltage "wanders" or won't stabilize for a minute or two because the cap has an issue. If you have the original quad cap, you should replace it.

    Bob

    dondi52

    Posts : 3
    Join date : 2011-06-09

    Re: Stereo 70 bias problem

    Post by dondi52 on Fri Jun 10, 2011 9:27 pm

    Hello, and thank you so much. The quad cap is indeed the original, and I will see about getting it replaced

    Dan

    Gary H

    Posts : 2
    Join date : 2011-07-12

    sound valve bias resistor value

    Post by Gary H on Tue Jul 12, 2011 4:43 pm

    Hi everyone I have a Sound Value ST 70 which has had a burn out. The bias resistors are unidentifiable does anyone know the value of these

    Gary

    GP49

    Posts : 718
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Stereo 70 bias problem

    Post by GP49 on Wed Jul 13, 2011 11:52 am

    Gary H wrote:Hi everyone I have a Sound Value ST 70 which has had a burn out. The bias resistors are unidentifiable does anyone know the value of these

    Do you mean the resistors in the bias voltage string, with the bias adjust pot in the middle?

    I don't remember for sure but wasn't the Sound Values (Sound Valves, later on) an exact clone of the stock Stereo 70? Even if there was a slight difference, using the stock Stereo 70 values for those resistors...except for upping their wattage rating...would get you close enough. The bias adjust pot will likely take up any slight variance..

    tubes4hifi
    Admin

    Posts : 1261
    Join date : 2008-11-30

    Re: Stereo 70 bias problem

    Post by tubes4hifi on Wed Jul 13, 2011 12:36 pm

    two 10K resistors, one on either side of the pot
    here's the original Dynaco ST70 manual if you don't have one . . . .
    http://www.tubes4hifi.com/DynaST70.pdf

    Gary H

    Posts : 2
    Join date : 2011-07-12

    thank you

    Post by Gary H on Wed Jul 13, 2011 5:14 pm

    Thank you very much for your help I'll let you know how it turns out
    Gary

    dynacoman

    Posts : 13
    Join date : 2011-09-03

    Re: Stereo 70 bias problem

    Post by dynacoman on Sat Feb 18, 2012 10:04 pm

    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.

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