It's been a while. Can you fill us in on your progress Paul when you get a chance?
Good timing; I was just going to spend some time today pulling together my scattered notes - I've been out of town a good deal lately, and grabbing a few free hours here and there to work on aspects.
This thread has a bit of a split personality, so in this post I'll only talk about a the new amplifier design being developed which will implement the fully shunt-regulated high voltage power supply concept. At this point, it is an entirely new design, not an upgrade of the original limited-production semi-kit of several years ago. Doc B. and I have conferred extensively on where to go with this, and concluded that the regulated approach should be matched with an equivalent level of performance, reliability, and utility.
One consequence is that this design is optimized for performance, rather than being an experimental platform. Not that you can't play with it of course, but we'll put better, more expensive parts in without worrying about making it easy to change them. Here are a couple examples:
1) All the power supply capacitors are going on a single large PC board - the "capacitor farm". They'll be premium long-life parts, and the layout places them away from the heat-generating parts for longer life. Probably on the same board there will be a set of sequenced power control relays and timers.
2) The cathode bias and the master current source bias resistors both get hot and dissipate a lot of power. So this time I am moving them to a second "heat-sink farm" board, well removed from the capacitors and with a large cooling vent immediately adjacent. At this time I expect to use Caddock TO-220 resistors on large heat sinks. It will be difficult to replace these with (for example) your favorite Mills NIWW, but on the other hand the Caddocks are damn good resistors to begin with.
3) On the other hand, I have allocated a large space around the tubes for the capacitors that carry signal current - interstage, parafeed, and cathode bypass. Those will be easy to swap out - this is still a Bottlehead amp, after all!
The performance improvements are relatively small. Based on the recent BeePre work, I'll try a second generation regulated filament power for the output tube, along with regulated heater power for the driver/shunt regulator tube. That should reduce hum and noise, and also overall operating point stability - which is IHMO a major part of why regulation sounds good. The operating points are optimized for performance, without compromise for available voltages or a variety of transformers and chokes. Again, not that you can't play around with iron, just that the design is built around specific parts. I intend to put in some better-quality signal capacitors from the start, as well.
For convenience, I want to have output impedance switches and input level trim to optimize system balance. And the power sequencing relays and timers will protect the output transformer from being magnetized by startup and shutdown transients, so the amp should sound good very quickly - no more "run it with music for 30 minutes before critical listening"!
For reliability I have already mentioned the cooling flow and parts location design. There are a lot of electrolytics in the low-voltage power supply, but with temperature control and careful parts choice (and some conservative, mil-spec design practices) you can get a theoretical lifetime of well over 100,000 hours. The sequenced power will extend the life of the tubes, as will regulating the filament and heater voltages.
Appearance is still being worked on. I'd love to have all the connections on the back panel, and a hefty thick front panel with only the power switch, perhaps with wooden side panels. It looks like a basic chassis plate 10 inches by 16 inches will accommodate the layout fairly well, and could be done optionally sideways for rack mounting if anyone actually wants that. But at this point, all that is speculative.
I should mention that there are some other features and directions being worked on; I can't go into them until I get some more research done, but the bag of tricks is not yet empty!
