Why Interstage coupling?

Simple, it is the best way for vacuum tube circuit to coupling signal from source to load or another stage with minimum loss and hold music integrity to the highest level.
Interstage coupling in vacuum tube amplifier has always been desirable due to several advantages as follow:

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.
" Inductor offers highest gain obtainable from that gain stage while the total gain of gain stage with resistor will depend on resistance. The higher the resistance, the higher the gain with higher power loss and higher output impedance.
" Inductor is a passive device that can store energy from power supply, much like a reservoir, that has ability to supply transient energy to load (in this case the vacuum tube itself). Resistor is also a passive device but it could not store any energy within them so the fluctuation of voltage to be feed to vacuum tube will vary more.
" We could eliminate a coupling device (usually capacitor) by introducing second winding in the same core of plate load inductor, it becomes Interstage transformer.

2. Elimination of coupling device
The use of Interstage coupling offers the reduction of one major part in gain stage; that's coupling capacitor. Coupling capacitor works against DC (usually high voltage) to reach the grid of next stage but allow AC signal (supposedly audio frequency) to pass. Coupling capacitor has always been the subject of discussion regarding its tonal characteristic impurity between each type and brand. Some audiophiles even notice the tonal different of capacitors of same brand same type same value but different voltage rating!
Secondary winding of Interstage transformer acts and behaves much like the primary winding; that's to extract magnetic force and convert into electrical signal. Turn ratio governs the proportion of voltage to be extracted by secondary winding.
Magnetic coupling in Interstage transformer is the best energy transfer mechanism for low frequency (audio frequency) that can be best viewed same as the force working in most dynamic loudspeaker drivers.

3. Better than Direct Coupling Method
Direct coupling allow DC + AC signal to pass to grid of next gain stage. This may look good but severe oscillation in subsonic region, hum and RF intermodulation usually cause instability of the circuit due to its extreme bandwidth. This will degrade the performance of the circuit considerably and cause harsh/bright sound.

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.