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In Reply to: RE: Fixed bias problem, and no, it is not the tubes. posted by DAK on January 11, 2017 at 18:00:49
Bad resistors in the bias supply series (dropping voltage) circuit. Have a schematic or picture?
Follow Ups:
The bias circuit consists of the 6.3vac filament trans in the center, then to the choke about 800 ohms followed by 50uf then to 5K w/ 50uf to 10k w/100uf 50k pot.
My bias supplies use a three section filter and a filter capacitor on the output voltage adjustment pot. It produces less than .001 volt AC ripple. I use 3K dropping resistors. At 140-160 volts initial bias voltage I would use 15K dropping resistors if it has a three section filter. 22K if a two section filter. Your transformer will supply 50ma current.
That is a estimated guess on resistor values, but supplies a stabile and stiff bias supply. I really need to see the amplifier schematic.
Right, i measured the residual AC on my bias supply and it is around there .002volts. The only thing that is out of the ordinary is the filament transformer which got hot after 45 minutes. It is a used transformer so maybe a question mark as to its reliabillity? This trans is connected to a .8A 6.3v tap. It is not unbelievable that previously it may have be over worked. If it was actually abused could that cause it to not maintain it voltage regulation?
Edits: 01/12/17
But only in circuit. They don't appear abused but i will check them again to be sure. And both sides have the exact same problem. I can bias them correctly at the start and then they both drift equally on both sides. thanx
Edits: 01/11/17
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