Bogen Update

Spent the past couple days mostly working on the Bogen. I decided to skip the modified Fender tone stack and instead I'm going to put discrete independent tone controls throughout the amp.

All that's currently built so far.

I've done the usual tweaks - add something then take it out, take something out and then put it back. My thought for a treble control was adding a Vox "Cut" control - a cross-line low pass filter tied across the grids of the output tubes. You can find it on the Vox AC30 schematic. Pretty clever! If you'd like to try it out, this post explains the math simply. I get the feeling that Thévenin equivalent circuits would be handy for stuff like this, but I'm trying to fight it for now. 

Anyway, the cut control didn't work too well for me - the treble roll off was nice, but I think it was loading down the output too much to be useful. I might be back to this eventually, but there are a few other places to hang a treble pot, so I'll try those first.

 I went through the (admittedly simplified) math to design a bridged-t notch filter for a mid control. Not entirely my idea; check out the Gibson GA-30RVT schematic. Man, old Gibson amps were so creative for guitar amps, at least in hindsight. Fender amps were straight from the RCA playbook, apart from that tone stack. Anyway. 

So I went through the math and came up with a great bridged-t, and then I checked Merlin's preamp book and he pointed out that the taper is kind of awkward on the control as Gibson had it, and presented an example of a slightly different bridged-t with a great gradual cut which didn't shift the notch frequency. So I took that. It's fantastic, really changes the character of the amp.

The heater voltage was over 7V, so the antiparallel diodes on one leg brought that down to a more reasonable 6.6V. Still haven't figured out a good place to put the heater elevation.

The overdrive sounds a little nicer without the cathode bypass resistor on the output tubes, but I prefer the transient response and higher output of having it in. Definitely something to remember if I ever build a high-gain cathode-biased amp at some point.

The bootstrapped cathode follower sounds much better with a hefty grid stopper. That 100k is dragging the high frequency response down to ~7 kHz, thanks to the massive gain, so I expected I'd need a bright cap with the gain pot set low, but it seems fine without it. Probably doesn't help that I play Telecaster and  Jazzmaster clones. 

Current plans

So, that's what I'm planning on doing next. Might tweak the cathode follower's load; the compression is nice, but now that everything's built I've found that maintaining headroom is quite a balancing act. The cathodyne phase inverter's main downside (w.r.t. guitar amps) is that it doesn't generate any gain, so all your voltage swing has to come from the preamp. It's a good match for the 7591s here (they're biased at around 18V - almost as easy to drive as EL84s with twice the output power and a decidedly 6V6/6L6 tone) but I wish that 6C4 were its dual triode equivalent (12AU7) instead. Actually, given that the heater voltage is running so high I might have been able to drill the chassis before I started and swapped in a noval socket. Ah well. Or I could undo the paralleled input stage, but it sounds so good I really don't want to. 

The issue right now is that the lion's share of the voltage gain is coming from that one bootstrapped stage before the cathode follower. Surprisingly the overdriven tone is really good too, and I think the cathode follower smoothing it out is a big part of that, but it also helps that the power tubes are breaking up so soon. Higher values of load resistor on the cathode follower should help the headroom a little bit.

I could probably talk about this amp all day, but depending on how it sounds after I get the bass and treble controls wired in I think I'm going to knock the input stage's gain back down to where I wanted it originally. It's hard to explain why, in words; there's just something about how the headroom feels. 

And yet I'm already looking forward to the next build. I'm thinkin a badass Antek toroidal power transformer, maybe wire the Crate amp up for a pair of EL34s instead of the quad of 6V6s it has now. It wants so badly to be a Marshall, might as well let it...