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07-19-2023, 05:51 PM
Hi Guys
The 10uF bias filter cap values work okay in the resistively-derived plate-winding-sourced bias supplies where the 100k to 220k series R helps create a long time constant for the first RC section, and typically a 15k works against the second 10uF. In this classic design the bias-set network parallels the second cap. These light values allow the bias supply to vary as the screen and plate voltages vary, which helps maintain the q-point for the output stage over the signal cycle and output loading.
To me, those values are archaic and the high-impedance bias supply is irksome considering the importance of this supply. My preference is always to use full bridge rectification and have proper filtering. In the case of a Power Scaled amplifier, a well-filtered and stiff supply is preferred, as Power Scaling maintains the output stage transfer function and will do so better under these conditions.
The first RBXs issued used the archaic values but for a while now it is shipped with better filter values and lower-loss RC sections. Most hobbyists use it along with a Power Scale kit, but it can also be used simply as a bias supply for passively tracked circuits.
Any stiffness added or dynamic quality that you feel is lost by using more ideal circuit values that reduce noise, can be accommodated by other means. Best to design out the noise and then tailor the circuit to the tone you want.
Be truthful to tone NOT to circuits.
The 10uF bias filter cap values work okay in the resistively-derived plate-winding-sourced bias supplies where the 100k to 220k series R helps create a long time constant for the first RC section, and typically a 15k works against the second 10uF. In this classic design the bias-set network parallels the second cap. These light values allow the bias supply to vary as the screen and plate voltages vary, which helps maintain the q-point for the output stage over the signal cycle and output loading.
To me, those values are archaic and the high-impedance bias supply is irksome considering the importance of this supply. My preference is always to use full bridge rectification and have proper filtering. In the case of a Power Scaled amplifier, a well-filtered and stiff supply is preferred, as Power Scaling maintains the output stage transfer function and will do so better under these conditions.
The first RBXs issued used the archaic values but for a while now it is shipped with better filter values and lower-loss RC sections. Most hobbyists use it along with a Power Scale kit, but it can also be used simply as a bias supply for passively tracked circuits.
Any stiffness added or dynamic quality that you feel is lost by using more ideal circuit values that reduce noise, can be accommodated by other means. Best to design out the noise and then tailor the circuit to the tone you want.
Be truthful to tone NOT to circuits.
