Circuit terminology

[vibratoking]

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.

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 .

Mike

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.

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.

[romberg]

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?

Ah yea. I had a feeling I was a bit off target. Thank you so much for your explainations in this thread. It has been a real learning experience.

I think B+ is called B+ because in the early days this referred to the B battery. And of course a battery powered circuit does not have any caps like you point out. But it does supply a steady voltage.

It makes sense that the important bit is the voltage across the cap and not the specific (relative to 0V) voltage on either side. I guess the next layer of the onion involves me cracking open my old Electrisity and Magnetism text and revisiting my calculus. But I think my curiosity can be stated by the approximations you mentioned.

Thanks again,

Mike