Somewhere around here someone recommended a TO66 device
for this as a driver. I"ll need 2 x NpN and 2 x PnP.
Next question when replacing outputs, if there are stability problems,
then what do we adjust? the Emitter resistors Make them a bit larger?
What about some more feedback? collector resistors a bit smaller?
Emitter resistors:
These emitters resistors Mac selected work hand in hand with the VI limiting circuitry. So any tampering with their values and you will need to re-engineer the VI limiting so it behaves properly, And you will have to re-calculate the forward bias of the outputs to compensate for the different emitter values also.
Now this is not a impossible task but its an impediment to your getting the amp running like originally designed by Mac, which seems to be your first order of business from what I have been reading and following in this thread.
Feedback:
Feedback while solving some issues can cause others also. Unless your set to deal with all the possible negative issues along with the positive ones then I would hold off on this until absolutely needed...Again this will be impeding your original focus of just getting the amp to run as built by Mac first....
Stability issues:
These sorts of problems come in all colors and types, and they must be addressed as they appear.
I would be poorly guessing to suggest that you do about anything at this time because I do not know of what issues that may befall your new parts endeavor with this amp.
You may get by without any major issues at all. Then again you might just be creating a very troublesome set of issues, that will be very hard for any qualified person to de-construct and resolve on a open forum.
Usually instability issues require hands on, eyes on and equipment on to properly diagnose and then compensate for.
I like to think I am good at what I do, but even I feel very uncomfortable trying to take on such problems in a online situation. Hands on is the only way I would even try to be of professional help to you.
I have seen over on the DIY where several learned people try to debug a amps problems. It can get to be a bloody mess as engineers argue amongst one another debating whats the best avenue of resolution. While everyone's ideas have their own merits, trying to get two or more engineers to agree is like trying to get a jury of 12 to convict...lol...
TO-66 devices:
I have a very old source of parts, and they have some TO-66 devices on hand. The dust on the parts plus their original dates codes with verify their authenticity. < and the fact the business has been their for almost 40 years collecting these devices as surplus >
I will thumb back thru the thread and pick up the 2N numbers and make a call and let you know if I can get you the four devices you say you need. Count on getting a PM from my no later then tomorrow concerning this matter.
And not to add fire to any debate already looming in this thread. More modern TO-220 devices would properly be the best way to go, even if the old parts can still be had. The newer parts would most likely be more robust, and they could be made to fit the TO-66 pattern on the sinks with some ingenuity and resourcefulness on their install.
Nothing will be easy on any Mac gear this old IMHO, and any original parts will be stupidly priced due their short supply and of course the speculators that may be holding on to these devices for just such a sale situation.
I tried to find a 250K gain pot recently and sent the owner back to Mac to get one at the ridiculous price of ~$70.00 shipped for a $2.95 2 watt carbon gain pot. But if he wants it to fit the glass faceplate he has no other source possible that I could find in two hours of searching world wide. There were aftermarket replacements available at $129.00 a copy, and they were specifically listed as mac upgrade replacements. So you see Mac isn't the worst at parts, the aftermarket is simply stupidly over-priced, But they believe they got you by the balls, so their is no mercy...
Mac wants almost half of what it costs to re-cap the whole amp brand new from Mouser for one gain pot...Aftermarket wanted ~$8.00 less then the whole re-cap costs for one gain pot.
I would stock NOS parts forever too, if I made margins like these on just parts lol lol lol...My conscience won't let me do such things...and I don't sell parts, I just do service....Boy did I pick the wrong end of this business...lol...
Hope some of my efforts are helpful to you....