Soliciting advice on mods for Soundcraftsmen FET PR1800

[cyclecamper]

I have 5 of these old professional-model amps now. They have a relatively conventional FET output section, and a pulse-width-modulation PWM switch-mode fully-regulated power supply. About 900 watts a channel into a low-impedance load. I sold my more powerful class "D" amps because I like the sound of these better.

I hosted the schematics on my internet-access-provider webspace...seems I got some hosting space with my internet account:
http://mysite.verizon.net/gregorypinkowski/PR1800-schematic.pdf
Please let me know if it works.

I have built a few amps, but my formal electronics background is high-school level though I have a few years in semi-pro audio (working for bands). Any comments on this amp are appreciated. I don't intend to even address the power supply...an all-discrete switch-mode supply is a rare thing. Maybe some bypass caps across the big power supply output caps?

Ignore the last page, that's the input for the home model without balanced XLR inputs or the phase inverter for bridged mono operation.

First please look at page 3 (of 8), right at the input, capacitor C1 labelled NP 25/25 (note comment in title block to assume microfarads). That looks like a good prospect to upgrade. But for a reasonable f3 -3DB point and keeping the phase shift out of the audible range I'd think a 4.7 microfarad polypyoylene cap would do into 16K. Did they make the non-polar elecrolytic so big just because they're cheap and they didn't need to limit bandsidth here? Is there any point in my using something bigger than a 4.7, like a 10 microfarad? I'm looking for a cheap good-sound cap, since I'll need 10.

Any other caps I should replace? Perhaps in the feedback loop? Power supply filter caps are still working OK for now.

(still on page 3) I see R26 is a 180 ohm, and the note says "BIAS 400V" so I guess that means the bias is only .002 amp? That's barely even on. After reading Nelson Pass lately (a lot of it over my head) I can't help but wonder whether I could increase that bias...maybe a LOT (assuming the fans work and the power supply doesn't freak out). Would that be crazy? What does R20 do, adjust bias? Or is it DC offset? Any comments on the bias for the 12 output FETs would be appreciated.

Page 4, the output section...what about the C1 at the output? I don't really know what kind of cap it is...should I investigate?

I really like the sound of these things. Much better than the similar related PCR800 family of shoebox amps (which use 2SK135/2SJ50 instead of the 2SK176/2SJ56 of the PR1800). But I always seem to like the sound of big amps idling...I often wonder whether that's jus the headroom or perhaps I like that big amps act class-A for the first few watts? Would I like them even more if I biased them up to really run class A for some appreciable portion of the signal?

And my last question: I can wire up my speaker loads for almost any impedance, and I understand the basic issues...damping factor etc. So, assuming a reasonable match, is there any advantage to running these amps bridged mono (besides getting away with running fewer heavy-guage speaker wires)? Some people have tried to convince me there's reasons some forms of distortion cancel when bridged, as if a symettrical circuit, but I don't see that as true or I don't understand it. Any insight appreciated.

[cyclecamper]

OK now I bought another well-worn one and put a deposit down on 3 more, so I will have 9 of them. That's 216 output FETs and 9 fully-regulated power supplies. Assuming I beef up the cooling appropriately (fans) how risky is it to attempt to increase the bias for class-A operation?