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
+3
Bob Latino
j beede
dondi52
7 posters
Stereo 70 bias problem
dondi52- Posts : 3
Join date : 2011-06-09
j beede- Posts : 473
Join date : 2011-02-07
Location : California
Identical behavior in the bias for both channels?
dondi52- Posts : 3
Join date : 2011-06-09
yes, the problem is the samee in both channels.
Bob Latino- Admin
- Posts : 3277
Join date : 2008-11-26
Location : Massachusetts
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
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
Hello, and thank you so much. The quad cap is indeed the original, and I will see about getting it replaced
Dan
Dan
Gary H- Posts : 2
Join date : 2011-07-12
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
Gary
GP49- Posts : 792
Join date : 2009-04-30
Location : East of the sun and west of the moon
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..
Roy Mottram- Admin
- Posts : 1838
Join date : 2008-11-30
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
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 very much for your help I'll let you know how it turns out
Gary
Gary
dynacoman- Posts : 12
Join date : 2011-09-03
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.
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.