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06-03-2023, 11:23 PM
Hi Guys
1, VOLTAGE is varied between grid and cathode which in turn changes CURRENT from cathode to plate. It is how a tube behaves.
The circuit is variable cathode-bias.
2,Everyone confuses the BIAS METHOD with the BIAS CONDITION and also the fact there are two uses for the term FIXED-BIAS
FIXED BIAS strictly speaking means that the operating point for the output stage is solidly set or "fixed" and does not change during a signal cycle. This is achieved in two common ways.
FIXED BIAS is also a bias method using a negative voltage applied to the control grid with the cathode typically grounded. This DC voltage is applied to grid via a grid-leak resistor and the signl is applied to the grid via a capacitor, which blocks the signal source from the negative voltage. The signal adds and subtracts from the DC to allow the grid to have varying strength in the control of cathode current, which rises and falls with the signal.
CATHODE BIAS is a bias method that allows the tube to determine its own idle point (idle current) working against a cathode resistor and with the grid effectively grounded. Again, a grid-leak resistor between the grid and the bias reference (ground) allows addition of a signal to the grid.
In all cathode biased circuits, there is no current unless the tube is in-circuit. You cannot test the bias adjustment (if present) without the tubes as you absolutely must do with an amplifier using negative grid voltage control.
CATHODE BIAS should NEVER be called "class-A". Although the tube runs hot, as is typically the case for class-A, the tube can go out of class-A during the signal cycle, which it does in almost all cathode biased amps using a bypass cap across Rk.
The BIAS CONDITION indicates how much of the 360-degree signal cycle the tube contributes to.
1, VOLTAGE is varied between grid and cathode which in turn changes CURRENT from cathode to plate. It is how a tube behaves.
The circuit is variable cathode-bias.
2,Everyone confuses the BIAS METHOD with the BIAS CONDITION and also the fact there are two uses for the term FIXED-BIAS
FIXED BIAS strictly speaking means that the operating point for the output stage is solidly set or "fixed" and does not change during a signal cycle. This is achieved in two common ways.
FIXED BIAS is also a bias method using a negative voltage applied to the control grid with the cathode typically grounded. This DC voltage is applied to grid via a grid-leak resistor and the signl is applied to the grid via a capacitor, which blocks the signal source from the negative voltage. The signal adds and subtracts from the DC to allow the grid to have varying strength in the control of cathode current, which rises and falls with the signal.
CATHODE BIAS is a bias method that allows the tube to determine its own idle point (idle current) working against a cathode resistor and with the grid effectively grounded. Again, a grid-leak resistor between the grid and the bias reference (ground) allows addition of a signal to the grid.
In all cathode biased circuits, there is no current unless the tube is in-circuit. You cannot test the bias adjustment (if present) without the tubes as you absolutely must do with an amplifier using negative grid voltage control.
CATHODE BIAS should NEVER be called "class-A". Although the tube runs hot, as is typically the case for class-A, the tube can go out of class-A during the signal cycle, which it does in almost all cathode biased amps using a bypass cap across Rk.
The BIAS CONDITION indicates how much of the 360-degree signal cycle the tube contributes to.
