8950`s and M-2057`s are basically the same tube, the M-2057 was just a bit more robust.
Both tubes above require 12/13 volts on the heaters, the 6LB6`s use 6 volts on the heaters.
I have never owned one of these units, but if you look at the picture
at CBTricks
http://cbtricks.com/Amp/maco/maco_750b/index.htm
I only see 8 holes ( sockets ) in there unless the predriver is UNDER the deck.
If you look at the schematic
http://cbtricks.com/Amp/maco/maco_750b/graphics/maco_750b_redraw_sch.pdf
Shows a single 8950 pre-driver, 2 6lb6`s used as driver tubes, and 6, 6LB6`s used as finals
It looks to me like Maco did away with the Pre-driver, and just did not note it on the Schematic.
The M 2057 are so rare that I doubt you will find any for sale, and if you do, they will want gold for them, I have seen them sell for $100 each NOS
Even 8950`s are going for big money...good NOS ( new old stock ) sweep tubes are getting harder and harder to find.
6LB6`s are a little more common.
Search the web for prices, and there is Flea-bay, it is a crap shoot with tubes from there, butI have scored some good tubes there in the past.
( Read the Warning Below about working on your amp before anything )
Look here:
http://www.nj7p.org/Tube4.php?tube=6LB6
For the 6LB6
And here:
http://www.pmillett.com/tubedata/8950.pdf
For the 8950
This is the pinouts for a 6LB6/8950, pin # 1 and # 12 are the heater
pins, check the voltage at the socket`s with your meter, if you have 6 volts on the heaters for the finals and drivers, it is wired for 6LB6`s, if there is 12 volts there, someone mod`ed it for 8950`s
WARNING!
! BE DAMMNED CAREFUL WORKING IN THAT AMP WITH IT PLUGGED IN AND POWERED UP !!
If you are not experienced working with high voltage in tube amps, get someone to do it for you, that amp can kill you.
Really.
Even when powered off,and unplugged, the high voltage caps can hold a lethal charge.
73
Jeff