Well first of all, I have to say that I've been studying electrics for a while now, and I am thoroughly impressed with how helpful everyone is. I've read tons of message board replies that say something to the effect of, "You shouldn't do it that way because of this, but if you want to, here's how you would do it." It's very fun to learn with so many willing teachers out there.

@Roly:

I realize that doing my own design is a huge mouthful, but I'm really enjoying the challenge, and right now I'm more interested in learning the theory of how all the different pieces come together. I still might actually build a proven project for my first one, but right now I have a newborn and lack a shop, so it's easier to get the learnin' in.

Anyway, I've done some more homework, and here's what I've come up with. First, I'm not sure where I got 68uF from. I think I did the calculations before I figured out everything that was going to be in my circuit, and missed some decimal places maybe? Anyway, I t = C V, so C

_{reservoir} = I t / V.

(.4A) x (1/120) / 1.75V [5% of 35] ~1900uF, so I'd round to 2000uF. I don't know if I should have used peak or RMS voltage, but this one comes out higher, so I'd go with that.

For the bypass capacitors, I found an old magazine article (

http://www.rfcafe.com/references/popular-electronics/bypass-capacitor-jan-1962-popular-electronics.htm) that said Xc should be about 1/10th of the cathode resistor, so in my case Xc = 56. Since the lowest guitar frequency is 80Hz (or maybe 70Hz if I play in drop D, which I do sometimes), I'll pass everything over f=60Hz.

C

_{bypass} = 1 / (2 x pi x 56 x 60) ~ 47uF.

I still might be confused about the mains fuse value, but I might have it: My transformer will drop the voltage by a factor of .3 (36V / 120V = .3, so V

_{secondary} = V

_{primary} x .3), which means it will raise the current by the same ratio (I

_{primary} / .3 = I

_{secondary}. So to figure the primary current draw, I

_{primary} = I

_{secondary} x .3 (I did some equation rearranging on paper that I didn't type out).

So if the secondary current is 400mA, multiplying by .3 will give me a primary current draw of 120mA, and I should choose a fuse based on that number.

I didn't realize the 12au7 was not in production anymore. I found an article comparing different tubes at low voltages (>12 V) where he did a bunch of measurements and determined that it had the best performance. I'm not married to it though, so I will change it to the 12ax7.

I put the 80ohm resistor in there because I figured that would limit the current to the required 150mA. But on reflecting on other schematics I've seen in which they connect directly to the 12.6V (or 6.3V), I suppose that the filament itself might limit the current, so the resistor wouldn't be needed.

I did not calculate the Johnson noise of the circuit. I read about it in one of the many articles/books I have read, but I haven't studied it very deeply at all; I mostly just knew it existed and that I might have to take it into account. But I do appreciate your point on the impedance mismatches. I had not considered that (obviously), but now it jumps out at me while I'm perusing various schematics, so thanks for putting it in my head.

The second grid-bias resistor can be removed because the grid is already referenced to ground via the gain pot.

Speaking of the gain pot, I'm not quite clear about how it loads the anode. I get how the two resistors are in parallel and how that makes 20k. I don't know what that means as far as how the anode is being loaded. Does this "count against" the output impedance of the first triode, or the input impedance of the second?

@phatt

I haven't had a chance to look at your schematics yet (my computer has been weird about not loading webpages lately), but I'm really interested in your 555 design. That's the chip that kinda made everything click for me when I was first learning about transistors. And I appreciate your encouragement about working with high voltages. I actually feel a lot better about it since I started with this project, but I'm really into it now, so I figure I might as well finish it. I'll see if I can get it recorded when I'm done and maybe it'll surprise you and sound sorta good--and by that I mean maybe it will make a sound.