Bodie with zener-assisted cathode bias

I finally got around to trying the zener trick I mentioned a few months ago. Bodie's idle bias voltage was around 21V, and that surged up to 31V under heavy overdrive. A pair of 12V 5W zener diodes in series now clamp the cathode voltage at 24V. Maximum clean output power has risen from 19W to 23W. On the 'scope, crossover distortion is now only barely present during overdrive and the overdriven tone has increased rather dramatically. I think next time I'm going to have to try just straight fixed bias; 7591s lose so much in cathode bias. Not that I really need another 10-15W of output, but the tone is much improved. I can probably lower the plate voltages to something more sane than, say, 480V to keep the output level reasonable.

I'm going to be tweaking this amp for years, I just feel it.

For now, I still need to come up with a decent bass control. While I really like the "independent tone controls scattered throughout the preamp" approach, there aren't many very good one-knob bass controls. Sure, you can take the baxandall stack apart and just use the bass control from that, but it didn't work too well. It may be time to try a flat tilt control in place of just a dedicated bass control. That would give the user the option of cutting treble early and/or late, which would open up the preamp distortion characteristics a bit.

I'm tempted to start looking at FFTs of the frequency response to see if the NFB loop is causing any high-frequency strangeness. Given the (deliberately) limited bandwidth of the amp, square wave analysis is somewhat problematic, but it seems to indicate there's some unwanted phase shifting at high frequencies resulting in excess treble.

Then again, I might not notice that if I weren't using these vintage EV SRO speakers. It might even be good with a particularly dark speaker, but I feel a well-designed amp should work well with any speaker, though that may be something of an impossible goal.

Anyway.

It seems I lucked out picking a voltage for the zeners to latch the cathodes to; the plates are happy at the dissipation they're subjected to under heavy overdrive. I wonder if I could go a little colder though, get a little more squish out of the output section.

And I'm still thinking about the power supply. It's working fine, but could it be better...?

Yep, years of tweaking ahead.

New Crate Plans

Alright, so, I've decided to take the Crate rebuild in a different direction - Vox land. I was wondering if anyone would be interested in a single channel AC30 with no reverb or tremolo and it turns out yes, they call it a Trainwreck Rocket!

Now, of course, I'm going to have to deal with heat. I suppose I would have anyway (seeing how I don't work for Crate and thus am not comfortable hanging tubes below a completely sealed chassis) but cathode bias ups the heat load I'm going to have to dissipate. Also I'm going to put in (switchable) sag resistors to emulate tube rectification. Needs moar recs!

Here's what I've come up with so far. Still a lot of tweaking to be done. It's based on what people assume is in a Trainwreck Rocket, as well as the original AC30 top boost channel. I've added a handful of tweaks throughout; we'll see how they work.

• Parallel first triode. Why the hell leave it unused. Same gain, lower noise. 
• Arc protection on the cathode follower to help the tube survive switch-on.
• Grid stopper on the cathode follower. Just something I feel like trying. Figured I'd split the difference between 10k and 1M for starters. 
• Reduced grid leaks on the LTP to reduce noise. Reduced cap across the second input to help recovery from blocking distortion.
• Lar/Mar PPIMV
• Sizeable grid stoppers on the power tubes for better distortion.
• Power tubes biased in pairs, partially to spread the heat around, partially so one pair can be pulled. I might do individuals, we'll see.
• Zener-assisted bias for the power tubes. Cathode-biased 6V6s seem to distort nicely anyway, but this might be worth looking into.
• Switchable sag resistor for a "tube rectified" feel. Might make these bigger.
• Heaters referenced to power tube cathodes for free elevation. Humdinger to get the last of the noise out.
• 1k screen grid stoppers. 
• Grid stoppers everywhere. 10k for starters. 

Layout is going to be a big pain. All the tubes are in the center of the chassis, so I'm going to have to get clever. I'm tempted to start drawing it up in DIY Layout Creator or sort of CAD program. I may just use paper. It's going to have to be all point-to-point, but since this isn't going to be as much of a ground-up as Bodie was I'll be able to plan better.

Welp, I've been spending even less time in front of the computer lately, but I'll post again when I've got more to go on.