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I'm new to tube amplifiers, but I'm trying to learn more by comparing schematics of various amplifiers. From what I can immediately gather, and I'm probably wrong, is that with tube amp designs you don't see too much of the typical HP filtering going on. What I mean is using a coupling capacitor followed by a resistor to ground. This is used commonly in stompbox designs that I've seen, but what I see primarily in tube amp designs is a fairly large resistor in series with the signal and then a jumper capacitor strapped across it which passes the high end.

I also see this arrangement on the Cathode bias, and from what I've been able to research, the larger the capacitor, the more low end you get? What would happen if there was no Cathode bypass capacitor at all? Much less gain? Are there any tube amplifier circuits that utilize this?

Yeah, what you're describing is a pretty common tweak. Put the bypass cap to V1 on a switch and you've got a fat switch. Changing the coupling caps from .1uf to .047 or even .022 can attenuate unwanted lows. A lot of 5E3 Deluxe players will do this mod. The bright switch cap or treble cap is another fun one to play with. A higher value will pass more upper mids, giving you a more British sound. Raising or lowering the amount of negative feedback can also make a big difference. It actually sounds like you've got a pretty good handle on this stuff.

Thanks. Heres going a bit deeper. I understand the concept of negative feedback, but I am having trouble understanding why there would be negative feedback, and why strapping a cap across the bias resistor would affect that. I could see if the cap was acting as a short to the output, but that doesn't seem to be how it is set up.

I think you're getting different concepts crossed. Negative feedback, while it may occur locally, is usually a term to describe global negative feedback in the power amp (presence and depth/resonance controls). This is independent of the cathode bypass capacitor. Also, HF shelving is used, but my background is high gain cascaded tube amp designs where HF can become a large problem. As you look at more complicated amps, you will see more techniques be used to control the tone shaping aspect of the amp. Some are almost excessive, but they all move the curves.

I'd recommend starting with Randall Aiken's site, and the Valve Wizard site for background reading. The Music Electronics Forum also has a large volume of tube amp discussion from theory to troubleshooting. Most of what I have learned has been from those three sites in combination with classic texts (RHD4, RCA tube manual) and practical hands-on building.

Thank you for the clarification. My confusion came from a website about tube amps that said the cathode bypass cap is there to strengthen the audio signal (or a part of it) and increase gain due to the higher fidelity. It quoted the difference to be due to less negative feedback on the gain stage. I guess what I read may not be the best source of information. I will look up the site you mentioned, thanks.