PJ, thanks for the explanation. I don't understand how a constant current source works, but thought Ohm's Law had to apply in some way.
Yeah, a current source replaces a resistor (fixed, resistive impedance at the frequencies we care about), with an impedance that is much higher.
Since you're cool with Ohm's law, we'll look at the voltage amp stage in the defunct Foreplay III as an example:
The stock circuit drops about 3.3mA across a 22.1K resistor from a 150V power supply, so 75V appears on the plate of the first stage.
When we install a C4S in this circuit, the 22.1K resistor is removed and replaced with a current source set to about 3.3mA. Consequently, the same plate voltage appears, but the load impedance that that gain stage sees is now several million Ohms (instead of 22,100 Ohms).
It isn't necessary to understand the workings of the current source, but rather that it allows us to circumvent the restrictions of Ohm's Law a bit. If we wanted that same gain stage with a 1M plate load resistor, 75V on the plate, and 3.3mA of current, we would need a 3,200V power supply and a 1M 30W resistor. When you go through the analysis, you need hundreds of dollars worth of parts to build a 3.2kV power supply, then you need those resistors too (they are about $100 each), then the pin spacing on the 9 pin socket would cause a lot of arc over before the tube drew current, etc.
