Since this info has been elusive at best I want to leave it here so the internet remembers for further readers. (and maybe not that experienced at this)
So, I spoke with Mike Pickett (great as always) and the instructions are like this:
Transistors were SJ6521A/22A and they were a special commission for Bryston, since the standard ones did not meet what Bryston wanted at the time. Some people still like the sound of those so you can always ask them to send those. Nowadays, the replacement would be MJ21193G/94G, made by ON SEMI. Mike said these actually have better specs and that you'd be alright with them. You can not combine them, tho (on each side). Each side must have all of the trannies the same type. Better off if the whole amp is the same type.
Now for the biasing procedure, it is done with the 2k2 pot. First off, let the amp idle for a while with no signal. When it gets to a stable temp, adjust DC offset with the other trim pot (Blue bourns multiturn 10K) and let it stabilize with each turn. When you have the DC set, then set the bias pot so the test points TP1 and TP2 measure each 12mV average. Let the amp sit after each adjustment for a while. Maybe 10 min or even 20 if you have time, until you have that average 12mV after it is idling for a while.
That is it.
So, I spoke with Mike Pickett (great as always) and the instructions are like this:
Transistors were SJ6521A/22A and they were a special commission for Bryston, since the standard ones did not meet what Bryston wanted at the time. Some people still like the sound of those so you can always ask them to send those. Nowadays, the replacement would be MJ21193G/94G, made by ON SEMI. Mike said these actually have better specs and that you'd be alright with them. You can not combine them, tho (on each side). Each side must have all of the trannies the same type. Better off if the whole amp is the same type.
Now for the biasing procedure, it is done with the 2k2 pot. First off, let the amp idle for a while with no signal. When it gets to a stable temp, adjust DC offset with the other trim pot (Blue bourns multiturn 10K) and let it stabilize with each turn. When you have the DC set, then set the bias pot so the test points TP1 and TP2 measure each 12mV average. Let the amp sit after each adjustment for a while. Maybe 10 min or even 20 if you have time, until you have that average 12mV after it is idling for a while.
That is it.