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I am installing a fixed bias in a 6L6GC 100w amp. The bias tap from the PT is 60VAC which I measured. I am using what seems like a very popular design. I got it from the SLO schematic and I see it on alot of 6L6GC 100w amps with fixed bias. The voltage divider is 15k first and then 56k with a 25k trimmer. I am also using two 10uf caps to get rid of the ripple. I am getting -83v after the diode and a max neg voltage of -70v. With this set up I cant even get close to what I need. I eventually replaced the 56k with a 15k and now have two 15k's as the voltage divider but I am still a bit out of range. Is this normal? Is there a different way I should be lowering the voltage like a resistor before the diode? I was looking at the Shinrock transformer website because they have a schematic of a SLO amp they sell and their bias tap is 60vac and they were using the 15k, 56k+25kVR voltage divider and they got -72 after the diode and -59 bias. Help me understand and get this right.
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c'up,
Recheck all your solder joint and measure the resistance of the parts you're using cuz if you've got two 15K resistors in series across the last cap and are taking the pot voltage from that source (or from across the diode before the caps, doesn't matter) then you should have a 2:1 voltage divider and exactly 1/2 the voltage you start with. Should work as is but I bet you've got a bad solder joint.
Rob -
Rob,
Finally, thanks for a reply. I actually put a 50k pot in replacing the 25k and then a 10k instead of the 56k. It got me in the area but it is at the low end of the potentiometer and the current really jumps at the slightest twist of the trimmer. I am definitely going to check the solder joints. Is it possible to verify a cold joint with a DMM? I usually just resolder everything in question.Comment
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