What's the deal with the power section in my Adcom amp?

[NavinRJohnson]

It's been over 20 years since I received an electronics degree I never utilized. Hopefully you guys can help jog my memory a bit as I really want to start getting back up to speed.
I've now taken on a more aggressive project, other than soldering pre-made boards, to help lubricate my old wheels. It's an older Adcom amp that had bad caps on the PSU that were repaired and still has some slight hiss issues that I'm working through. I'm sure I still have a few caps to replace and I need to clean up the rat nest wiring.

In the meantime I had a few questions about the power section and a couple transistors
1: The transformer has 4 output taps and is running the input (preamp?) section at 65Vdc and the power section mosfets at 55Vdc. This seems common among some of the "higher end" amplifiers. What's the advantage of running the front end at a higher voltage? Just lower gain so noise is less of an issue?

2:The +55V and +65V rails are connected just before R905 in the PSU section. A friend of mine suggested suggested this was to keep power to the mosfets on the input section during power down because there is more capacitance on the power section PSU board. That sounded like it fit the bill but, wanted to make sure I wasn't missing something else. Sound correct to you folks?

3:Q633 and Q635 right before the power section followed by the zener diodes. I honestly can't make heads or tails of what's going on there. Any ideas on the purpose of these transistors?

The full res schematic is at the end of the attached PDF if you can make out the image below.

 

[Jony130]

Q1 - To reduce the output stage saturation voltage. Thanks to this the front end is able to fully drive the output stage.
Don't forget that the output stage work as a common drain amplifier. And this means Vsaturation > Vgs(th).
Also notice that front end use cascode amplifier witch also have high Vsat.
http://www.apexanalog.com/wp-content/uploads/2012/10/AN48U_4.pdf

Q2 - I would say simple RC filter.

Q3 - Zener diodes are overvoltage protection diodes. Because MOSFET's has low Vgs_max 20V or so.
Q633 and Q635 are simple over current protection circuit.
http://users.ece.gatech.edu/mleach/lowtim/prot.html
http://www.thatraymond.com/downloads/solidstate_guitar_amplifiers_teemu_kyttala_v1.0.pdf

 

[NavinRJohnson]

Thanks Jony! I really appreciate the links and input.

Q1: I vaguely remember saturation voltage and I'll need to read up to understand this better. Thanks for pointing me in the right direction.

Q2: Not sure I understand how connecting the positive rails would help filtering in this circuit. Can you expound on this?

Q3: Something is wrong, I understand this with your links!

 

[Jony130]

Q2: Not sure I understand how connecting the positive rails would help filtering in this circuit. Can you expound on this?

I don't understand what you are trying to say?
And maybe R905 is also a inrush current limiting resistor when power up.

Q3: Something is wrong, I understand this with your links!

What is wrong ? It seems that they made a error on the diagram. And you need to swap the emitter with collector for Q635.