I have to say this is one of the top informative threads on this entire site for me at least. Can't say i have read them all however
You have done a fantastic job at documenting your work so that the masses can be educated. Everyone who is going to refurbish their old stereo should read this thread first. I'd say it is sticky worthy to say the least.
As for the Au-719 upgrade, I can suggest to change the o/p bjts. Long story that I say why. Might add some controversy, but it is a discussion subject none the less.
This amp, like most sansui's were rated for 8 ohm loads, not 4 or worse 2 ohm. So it limits what speakers you can use for one thing and how many you can use at once.
The short story, is to use newer o/p bjts with larger SOA but have all the same or similar characteristics as the originals, which are IIRC 100W (2sc2581/2sa1106) devices in a TO-3P package, with ft of 20-30MHz. Onsemi,Semelab,Sanken,Fairchild are all available as improved subs. semelab make a 300W device in a to-3p package!! Quite an improvement from the original.
this is a list of what Mouser offers, digikey is selling sanken these days
http://ca.mouser.com/Semiconductors...yzhg3Z1yyziiqSGTax1shZer4gZ1z0j3y4Z1z0j3xnSGT
In EE reliability terms, it is a weak link. Weak links usually are places for failures or lessen the MTBF.
90Wrms into 8 ohms is still very loud BTW, so if you do not crank it, it might be a moog point and why the o/p devices have lasted so long.
It would have been nice if sansui used a pair of o/p's but they did not, so increasing the SOA in a new device is a real easy solution.
I and others have played around with this sansui design in simulations. i can post it for you if interested. I did up the Au-x1 design in ltspice. it is the same design toplology as used in many of the high end designs of the era.
For the owner(s) of the au-719, they can think of the o/p bjt upgrade as extra preventative maintenance, when they decide to change the big ecaps. It is not imperative if the device is not driven heavily, like for headphone use.
Cheers Rick