Tweet
Hello folks,
I am confused about something here regarding class A biasing vs Class AB. I understand that the tube needs to be biased higher in Class A, as it needs to conduct for the full drive waveform + & - probably in the middle of the tubes load line for symetry.
In the RCA receiving tube manual (or any for that matter) they list the max plate dissipation for idle conditions in Class A. IE:EL84
Max values for Class A: Plate dissipation = 12Watts @ 300v plate voltage.
That is the equiv of 40ma cath current.
I find many of these little amps absolutely pushing the max of 12watts at idle with the scenario listed above. I've also seen where the idle wattage is quite a bit higher than that.
Would you try to reduce the idle wattage to lengthen the tube life, or just leave it as is. Some of these are set up this way with the stock tubes right from the factory and if you darken the room, sometimes you can see the tube just beginning to red-plate.
In the case they combine self-bias with fixed bias (or just self bias) & you can reduce the idle dissipation easily. In the self-bias case you have to increase the value of the cathode resistor...BUT, you have to be careful it does not affect the gain of the stage. Kinda tricky sometimes.
How do all feel about this situation. I'm hoping there are others with similar questions that this dissertation might resolve.
Thanx, glen
I am confused about something here regarding class A biasing vs Class AB. I understand that the tube needs to be biased higher in Class A, as it needs to conduct for the full drive waveform + & - probably in the middle of the tubes load line for symetry.
In the RCA receiving tube manual (or any for that matter) they list the max plate dissipation for idle conditions in Class A. IE:EL84
Max values for Class A: Plate dissipation = 12Watts @ 300v plate voltage.
That is the equiv of 40ma cath current.
I find many of these little amps absolutely pushing the max of 12watts at idle with the scenario listed above. I've also seen where the idle wattage is quite a bit higher than that.
Would you try to reduce the idle wattage to lengthen the tube life, or just leave it as is. Some of these are set up this way with the stock tubes right from the factory and if you darken the room, sometimes you can see the tube just beginning to red-plate.
In the case they combine self-bias with fixed bias (or just self bias) & you can reduce the idle dissipation easily. In the self-bias case you have to increase the value of the cathode resistor...BUT, you have to be careful it does not affect the gain of the stage. Kinda tricky sometimes.
How do all feel about this situation. I'm hoping there are others with similar questions that this dissertation might resolve.
Thanx, glen
Comment