1. Low loss in plate load circuit.
Plate load is needed to absorb energy generated by vacuum tube. Either resistance or inductance can be the load of vacuum tube circuit since they both have reactance to AC signal. The different is that reactance (from resistor) has DC resistance up to 10 times or more of the same reactance from Inductor. That's huge amount of energy loss when use resistor as load.
The active load (active component) as used in SRPP and other circuits could also form another type of plate load to vacuum tube circuit. It forms constant high impedance load much like a cathode follower circuit on top of the gain stage. (some say that it is much like push-pull circuit) The benefit of high gain with wide bandwidth is now achieved with around ? amount of raw energy consume in active load.
Plate load not only served as energy absorber generated by vacuum tube, but also served as signal generator.
As far as hi-fi vacuum tube circuit is concerned plate load is either a resistor type or active load type. Could that mean that they perform superior to inductive load? The answer is definitely not. The cost and size of high quality inductor to be used as plate load is enormous, around +100 times of that resistor or active load. Transformer may also weight +1,000 times more than a resistor. So it is very simple reason why 99.9% of all tube designers adopt for resistor load.
Beside from cost and weight issues, a good plate load inductor or transformer is very difficult to wind even to big tube amplifier companies. If we could deal with all of the commercial aspects that isolate the use of plate load inductor, there are several benefits from using plate load inductor as follows:
Inductor offers lower DC resistance than resistor of same loading. Typical
50 Henry 30mA may have internal DC resistance of around 300 ohms compare
to 5.0K ohms resistor load. This means that if 30mA of current has to
be drawn through a 5.0K ohms resistor, it will generate 150 voltage drop
across that resistor and that is around 4.5 watts heat dissipation. While
Inductor will drop only 9 volt equivalent to only 0.27 watt heat dissipation.
This is how the efficiency is gained from the use of plate load Inductor.
Better than Direct Coupling Method
Successfully designing and implementing Direct coupling circuit are very difficult. If one claims that his Direct coupling circuit works good, the other one may find it working differently if he try to copy the circuits by using same parts and same schematic since bias adjustment and operating point of next gain stage will depend largely on tolerance or parts and each individual tube characteristic.
Direct coupling with plate load resistor is out of discussion since it passes the signal (left over energy from dissipating by load resistor) with appreciable amount of loss already when compare with the use of plate load inductor. Direct coupling with plate load inductor (Choke Load direct coupling) is better but still pass energy left over from voltage drop across the load, not the signal energy within the core to be transferred.