Bogen Update

I sat down to wire up the phase inverter and man, I was just on a tear - got the phase inverter, the cathode follower and the bootstrapped gain stage preceding it, and the gain, master volume, and presence controls all wired up and working beautifully. Oh, and I tweaked the power tubes. All that's left is:

• the first (paralleled) gain stage,
• heater elevation,
• tone stack,
• tweak the power supply,
• tweak the NFB,
• tweak everything else.

Yeah, that's all!

Dear god, never again.

Yeah, that's a real mess. It's working and it's stable but man is that ugly. I really wish they'd mounted all the tubes in line so I had room for a turret board or something. This chassis has been a huge pain every step of the way, and "local star" grounding isn't helping appearances any. With the gain and master volume all the way up there's a faiiiint hint of hiss with your head right next to the speaker, but I still have to wire up the first gain stage and the tone stacks, which will likely both add noise, but I'm also going to elevate the heaters, which should reduce (some) noise. Well, it'll reduce hum, but there isn't any hum (yet.) I'm using 1W metal film resistors throughout and that's probably keeping the hiss at bay. Carbon comps would be a nightmare.

The noise performance should still come out better than average, but there are a couple grounds that I got lazy on (presence pot right to main star ground, OT grounded to main star instead of local ground where NFB returns) so I could wire those differently if need be.

Here's the latest schematic:

Not a whole lot has changed:

• I had to flip the OT primary leads because (surprise!) the negative feedback turned out to be positive feedback with the leads the way they were. I swear to god, output transformers are the USB connectors of the tube amp world. No matter how sure you are you're putting it in the right way, you've always got to flip it (often twice.)
• I did change the 7591s' grid stoppers to 220K. The datasheet value for the max grid circuit resistance for cathode biased 7591s is an optimistic 1M. Ordinarily people make the grid leak resistors larger, but thanks to the low output impedance of the cathodyne there's no real reason to do that. 220K seems large for a grid stopper, and historically it is, because a grid stopper forms a low-pass filter with the input capacitance of the tube it's connected to. In a triode stage you might start losing some highs, but the beautiful thing about beam tetrodes (and pentodes) is the alarmingly low input capacitance thanks to the screen grid. The ordinary values of 1-5K roll off everything above ~300 kHz. With 220K I'm rolling off everything above ~72 kHz, which is nearly two octaves above the limit of human hearing. Considering the average guitar speaker rolls off everything above ~6kHz, we're not in any danger of losing any "airiness" or "sparkle." Hell, I might take the grid stoppers up to 470K, giving me a roll off above 30kHz and reducing the blocking distortion even more. The blocking isn't bad right now, but I'm curious. Plus, NFB stability, right? Yeah, like I need more excuses to tinker...
•  The 7591 cathode bypass cap is 1000 uF because it's the closest value I had handy. 
• The NFB resistor is too large.
• The presence cap is too large.
• The cathode follower's cathode is sitting at 211V; definitely have to elevate those heaters.

Like the overeager child I am, I wired up an input jack across the gain control and took it for a test drive. Understandably, a guitar straight in isn't getting me anything close to full output, but using the line out on a practice amp as a crude preamp worked.

How it sounds right now:

• So much high treble and low bass. I was testing this with a jazzmaster clone, so that wasn't helping, but I won't be adding a tone stack lift any time soon. Pretty sure that bright cap will never get engaged so I might not even bother wiring it up. But what else would I use that switch for? Hmmmm....
• Not a whole lot of sag - the power supply seems pretty stiff, even though it's a voltage doubler with a modest reservoir cap, and the power tubes are cathode biased. Then again, I did add that whopping cathode bypass cap, and there's still another pair of triodes to install. Not that a couple milliamps of constant class A current draw will make a huge difference, but we'll see.
• The clean tone is full, lush, sparkly, you name it. I do miss built-in reverb, but I've got pedals and I probably overuse reverb anyway. A great clean sound was the primary goal here, and that's absolutely a home run.
• The overdriven tone just barks, with a slight bit of beating from the blocking distortion. The transient response is great. Right now I'm pretty much only hearing power amp overdrive and it's fantastic, but I've got to get the preamp finished before I know the whole picture. The blocking distortion adds a splash of ugly and I think it might be better with a little less of that. Maybe not, and maybe I'm just stuck with it anyway. 

Been a good day in the basement!