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08-17-2019, 08:13 PM
Hey
Maybe it's a coincidence that your Rc-b =330k and the screen voltage sampling resistor in the bis reg is 330k? The two are actually unrelated as in fact is the case for the Rs around the BJT bias switches. You have to think of the output of the bias reg as a voltage source and the BJTs simply as switches, tying that voltage source to the bias-set network, or isolating the voltage source from the bias-set network.
The Rs around Q9,10 are selected to allow sufficient base current that the BJTs can pull the bias pots as close to the bias reg output as possible.
There are lots of ways to have mixed bias. TUT4 shows swept fixed to cathode bias using RmX methods. Another approach is to have a variable Rk and a tracking bias reg with adjustable tracking ratio. You could also make things completely manual but safe by using much higher Rk values. Older STUDIO amps had such a feature controlled via the Fiixed-Cathode switch and a Heat switch, later combined on one 3-way switch as Tweed-Mix-Modern.
Maybe it's a coincidence that your Rc-b =330k and the screen voltage sampling resistor in the bis reg is 330k? The two are actually unrelated as in fact is the case for the Rs around the BJT bias switches. You have to think of the output of the bias reg as a voltage source and the BJTs simply as switches, tying that voltage source to the bias-set network, or isolating the voltage source from the bias-set network.
The Rs around Q9,10 are selected to allow sufficient base current that the BJTs can pull the bias pots as close to the bias reg output as possible.
There are lots of ways to have mixed bias. TUT4 shows swept fixed to cathode bias using RmX methods. Another approach is to have a variable Rk and a tracking bias reg with adjustable tracking ratio. You could also make things completely manual but safe by using much higher Rk values. Older STUDIO amps had such a feature controlled via the Fiixed-Cathode switch and a Heat switch, later combined on one 3-way switch as Tweed-Mix-Modern.
