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05-08-2023, 07:56 PM
Hi Guys
Below is the statement that is incorrect, particularly the section beginning with "so". The part prior to that is also inaccurate inasmuch as the 0C3 is not tracking plate current; rather, it is following plate supply fluctuations that result from signal-dependent variations of that supply.
""all it does" is regulate screen grid voltage by tracking plate current and maintaining a constant voltage difference between plates and screen grids so the grids are never positive relative to plates"
Otherwise, what you did is as I described but using a tube instead of solid-state. That is a nice variation of tech but gives few options for setting loudness since Vs has been reduced by a fixed amount. You can get lucky with things (fixed loudness reduction) and maybe you have for your needs?
A simple way to make this full variable is to have a voltage divider across Va that has a variable portion (a pot). The pot wiper ties to a mosfet gate that has D to filtered Va and S to ground via a high-value resistor, and also ties to the screen-stops. The screen voltage will be any amount lower than Va that you need for loudness adjustment and it tracks the sag of Va. Again, this is just a PS-TT variation that works with cathode-bias. Add a tracking bias reg for fixed bias.
Below is the statement that is incorrect, particularly the section beginning with "so". The part prior to that is also inaccurate inasmuch as the 0C3 is not tracking plate current; rather, it is following plate supply fluctuations that result from signal-dependent variations of that supply.
""all it does" is regulate screen grid voltage by tracking plate current and maintaining a constant voltage difference between plates and screen grids so the grids are never positive relative to plates"
Otherwise, what you did is as I described but using a tube instead of solid-state. That is a nice variation of tech but gives few options for setting loudness since Vs has been reduced by a fixed amount. You can get lucky with things (fixed loudness reduction) and maybe you have for your needs?
A simple way to make this full variable is to have a voltage divider across Va that has a variable portion (a pot). The pot wiper ties to a mosfet gate that has D to filtered Va and S to ground via a high-value resistor, and also ties to the screen-stops. The screen voltage will be any amount lower than Va that you need for loudness adjustment and it tracks the sag of Va. Again, this is just a PS-TT variation that works with cathode-bias. Add a tracking bias reg for fixed bias.
