Are you going to build it out on that PCB you designed? That would be a treat.
Looking at your first stage tube circuit, I've tried using a diode (and an LED too) for the cathode bias and measured more THD than with a plain resistor. Does the CCS load on the plate help alleviate some of the distortion?
I'm still looking at the positioning of the PT, OPTs and Choke for the rebuild. The PCB requires planning, or at least thinking about the whole layout before you start. That's one of the reasons I'm attracted to it, because of the discipline that it enforces, but it's more inflexible than point-to-point. I might go for a combination approach, split the PCB into a number of subcircuit assemblies. I liked the layout I had in the first build, it was just too cramped. I'd like to follow the general scheme for the PCB, but there are problems. It might be necessary to have holes in the PCB to allow wires from the top of the chassis to pass through to the component side of the PCB. I would like to stick with a single-sided PCB because I can make it myself (no through hole plating), a doublesided PCB that size would be expensive to have manufactured. I can't get PCB mount dual-gang 4-way switches, so I'd have some PCB mount parts passing through the front panel, but the 4-way switches would have to go to headers. I can't use a 10 * 6 PCB which I would like to because although the chassis I'm looking at is 10 * 6, it has a lip on the bottom, so actually inserting the PCB is problematic.
While the toroidal transformer suits me, it was cheap and simple, in some ways I'd prefer to revert to the back-to-back arrangement I drew in the other thread. I'd like to have 12V and run the heaters in series because the DC would be a lot less fussy with a peak voltage ~17V, there'd be more overhead for the regulator, and you'd only need one, because the current would be halved. You'd be able to use a 110-0, 110-0 primary on the first transformer which would make the build directly accessible to builders in the US, but sourcing transformers which are available in both Europe and the US is a pain. I'd really prefer to have both power transformers PCB mounting which adds to the difficulty if anything.
It's all a bit of a puzzle. The way I usually deal with these conundrums is to let the problem stew around in my head until my subconscious solves it. Sometimes a new piece of information comes to light which pushes the decision in a particular direction. Maybe in this case it's not possible to get an absolutely optimal solution. I'm just not going to go at it like a bull at a gate like I did the first time round.
The theory says that the CCS plate load results in a horizontal load line. This should result is a reduction in distortion, the spacing between where lines on the characteristic intersect the load line becomes more constant. Whether this pans out in practice, I don't know.
I'll look at replacing the diodes with resistors in the sim. and see what effect it has on the THD. I'm just following the recommendations from Morgan Jones' book and the suggestions I've had here for the moment. Not that I feel that the sim. is to be taken as gospel anyway...
Transistors for the CCSs arrived.