Would that conclution on PP stages transfer to fixed bias as well? Both beeing constant negative voltage applied to grid.
Battery bias is not adjustable in increments of less than 1.5V (unless you use a circuit with a potentiometer, and then you are going to be replacing the batteries every week or day).
A 1.5V change on a 2A3 or 300B grid (with transconductance of 5,500 uMhos), will change the plate current by 5.5mA.
In push pull, how do you match the 2 plate currents, when 1.5V bias increments cause 5.5mA changes in plate current?
IF the tubes are Very Well Matched At Your Chosen plate voltage and plate current, then
it is not a problem, just use the same battery voltage for each tube. If the tubes were not matched at plate voltages and plate currents you chose, then "your mileage may vary".
Push pull current imbalance is a bad thing.
The push pull output transformer is going to be near saturation with 5.5mA of plate current imbalance (i.e. 58mA in one plate, and 63.5mA in the other plate).
The transformer is going to distort bass notes, and that will also inter modulate with the higher frequencies too.
I use self bias on push pull amps. Tubes that are anywhere in the same ballpark will have very similar plate currents. But I do not rely on that, I purchase matched tubes.
The results speak for themselves:
I have had up to 60mA plate current, and the match between tubes was always within
500uA (i.e. 60mA and 60.5 mA). Usually they are within 300 to 400uA. This is one advantage of using self bias and matched tubes in push pull.
But don't get me wrong, Fixed (adjustable) bias can do this just as well, since it is adjustable.
My push pull amps with self bias are used in class A, and with larger signals in class AB1 (no grid current). Yes, the self bias can shift slightly with large sustained signals.
But in a practical sense, that depends on the music, 'how loud', and how much capacitance is used in the bypass caps.
For push pull amps using coupling caps and fixed bias, they also have bias shifts if the amp goes into AB2 (draw grid current).
The difference between whether the amp is in operated in class A, AB1, and AB2 has to do with how much signal is coming from the driver.
But some of us choose not to listen with the volume turned too high.
Just remember: "ALL generalizations have exceptions".
A 1.5V change on a 2A3 or 300B grid (with transconductance of 5,500 uMhos), will change the plate current by 5.5mA.
In push pull, how do you match the 2 plate currents, when 1.5V bias increments cause 5.5mA changes in plate current?
IF the tubes are Very Well Matched At Your Chosen plate voltage and plate current, then
it is not a problem, just use the same battery voltage for each tube. If the tubes were not matched at plate voltages and plate currents you chose, then "your mileage may vary".
Push pull current imbalance is a bad thing.
The push pull output transformer is going to be near saturation with 5.5mA of plate current imbalance (i.e. 58mA in one plate, and 63.5mA in the other plate).
The transformer is going to distort bass notes, and that will also inter modulate with the higher frequencies too.
I use self bias on push pull amps. Tubes that are anywhere in the same ballpark will have very similar plate currents. But I do not rely on that, I purchase matched tubes.
The results speak for themselves:
I have had up to 60mA plate current, and the match between tubes was always within
500uA (i.e. 60mA and 60.5 mA). Usually they are within 300 to 400uA. This is one advantage of using self bias and matched tubes in push pull.
But don't get me wrong, Fixed (adjustable) bias can do this just as well, since it is adjustable.
My push pull amps with self bias are used in class A, and with larger signals in class AB1 (no grid current). Yes, the self bias can shift slightly with large sustained signals.
But in a practical sense, that depends on the music, 'how loud', and how much capacitance is used in the bypass caps.
For push pull amps using coupling caps and fixed bias, they also have bias shifts if the amp goes into AB2 (draw grid current).
The difference between whether the amp is in operated in class A, AB1, and AB2 has to do with how much signal is coming from the driver.
But some of us choose not to listen with the volume turned too high.
Just remember: "ALL generalizations have exceptions".
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