03-11-2016, 02:00 PM
DA, I have looked at the load line you have kindly posted for the 12AU7 (ECC82), the 25kΩ load line correctly runs from (0V, 10mA) to (250V, 0mA).
But at your operating point, (138V, 4.5mA) approx, your swing for 25V (total) out is 1V on EACH SIDE of the -5V grid voltage. So gain is 25V / 2V = 12.5, not 25! This ties up with the ECC82 having a µ of 17, which is the maximum gain that can be achieved with RC coupling.
Ref the discussion about more kΩ in the anode load bringing us down to the curved foot of the characteristics, well yes I can see what you mean, but what matters is the distance along the load line that the that the curves intersect with the load line. If these intersectioins are equidistant, then distortion is peanuts. Lawrence pwdrive has illustrated this - his AB is equal to BC pretty nearly.
The anode characteristic (Ia against Va) for constant Vg - which is what the your graphs show - is indeed curved towards the foot. But it turns out that the mutual characteristic (Ia against Vg) - for constant Va - is curved the other way such that the two cancel out. And that's why the points of intersection referred to tend to be equidistant.
The µ of a triode is a pretty reliable and constant characteristic. But mutiual conductance gm isn't. So if you use a really low value of anode load (like 1kΩ) the gain is going to be small and virtually dependent on gm, with the prospect of distortion as you run around the foot of any curve. Whereas if you use a very large value of anode load, like 100kΩ, or even better a constant-current load or a really high-value inductor, then the gain approaches µ which, being constant, implies low distortion.
But at your operating point, (138V, 4.5mA) approx, your swing for 25V (total) out is 1V on EACH SIDE of the -5V grid voltage. So gain is 25V / 2V = 12.5, not 25! This ties up with the ECC82 having a µ of 17, which is the maximum gain that can be achieved with RC coupling.
Ref the discussion about more kΩ in the anode load bringing us down to the curved foot of the characteristics, well yes I can see what you mean, but what matters is the distance along the load line that the that the curves intersect with the load line. If these intersectioins are equidistant, then distortion is peanuts. Lawrence pwdrive has illustrated this - his AB is equal to BC pretty nearly.
The anode characteristic (Ia against Va) for constant Vg - which is what the your graphs show - is indeed curved towards the foot. But it turns out that the mutual characteristic (Ia against Vg) - for constant Va - is curved the other way such that the two cancel out. And that's why the points of intersection referred to tend to be equidistant.
The µ of a triode is a pretty reliable and constant characteristic. But mutiual conductance gm isn't. So if you use a really low value of anode load (like 1kΩ) the gain is going to be small and virtually dependent on gm, with the prospect of distortion as you run around the foot of any curve. Whereas if you use a very large value of anode load, like 100kΩ, or even better a constant-current load or a really high-value inductor, then the gain approaches µ which, being constant, implies low distortion.
