The tube is labeled V2. The resistor is labeled R1. The speaker is labeled SPK1. So now let's follow along with our water analogy. The faucet (our tube) is an active pressurized device. When we want to fill the tank, the faucet turns on and pushes water into the tank; the pressure to fill the tank can go up or down depending on where we turn the faucet handle: the higher we turn it up, the greater the pressure and the quicker we fill the tank. When we want to reverse the process and drain the tank in an amount and speed equal to that of the faucet filling it we have two problems: the drain hose (resistor) is a fixed size and the pressure to drain is dependent on the water level inside the tank which varies. Bottom line, we cannot fill and drain the tank at the same pressure and speed. This is because the fill mechanism is active (the tube) and the drain mechanism is passive (the resistor). This is what causes the asymmetry in a SET amplifier. The net result is distortion, where the bottom half of the waveform is not the same as the top half. What's pleasing about this type of distortion is it is primarily 2nd order and the ear finds it to be a warm and rich coloration.
There are ways to minimize this problem including making the drain resistor really small and adding feedback to compensate. Unfortunately, neither of these Band Aids really solves the problem. No, SET or single device amplifiers are not the best choice for power amplifiers when symmetrical low distortion music is the goal.
What's the right way to fix the problem? Replace the resistor with another tube. That configuration is called push pull and gets us closer. It looks like this.
In this method one tube pushes the current while the other pulls it. In our water analogy we still have a faucet to fill our tank, but instead of a simple fixed hose to drain the water, we now add a suction pump. The faucet pushes the water into the tank (loudspeaker) while the suction pump pulls it back out at the same speed and pressure as it went in. This lowers the distortion without resorting to a Band Aid.
Yet there is an even better means of doing this called complementary symmetry, but that's somewhat restricted to solid state devices which can pump or suck depending on which type of device you choose.
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