Now we are peeling the onion. Mike, I like your explanation and your train of thought. It is useful and provides some insight. Especially for the bypassed cathode case. But what if B+ had no caps to provide an AC path to ground? In practice, circuit theory is full of approximations that are used to simplify analyses. Good approximations greatly simplify the math, allow for easier understanding, and also provide very good accuracy. It's a win-win in many cases.romberg wrote:I *think* the theory behind this is that the B+ is connected to ground by BIG filter caps at each power supply node. So... an AC signal is gonna be sent to ground by these caps just like any other noise/ripple. To a AC signal (with a freqency that is not super low) the B+ is a short to ground through the power supply filter caps. Or... I've got it wrong .JoeCon wrote: Yeah seeing the B+ as a ground for AC is a new one for me. But now that you said that it makes more sense.
Mike
An idealized ground is considered or defined to be perfectly at 0 Volts. This means that it has no alternating component or noise component. An idealized DC voltage supply, B+ for instance, is defined as a constant voltage. It also has no alternating or noise component. Ideal sources don't exist in real circuits. There can be plenty of noise and alternating components on both. We are all familiar with the ripple and sag of the B+ source in guitar amps. For the purpose of analysis, it can be useful to approximate ground and B+ as ideal, constant voltages. So, why is B+ considered AC ground? Because we define it that way in the approximation. It is fixed and cannot alternate. It's not fully accurate, but it's a good approximation in many cases. The caps we are discussing are a good case for this approximation. On the other hand, if we want to calculate how much of the B+ ripple is going to be present in the amplifier output, then the ideal source approximation becomes problematic.
By the way, this thread shows that there are usually quite a few different ways to think about how a circuit works. These are normally worth exploring.