Hi Mark,
The only tube that isn't used on the balanced inputs is the first one on the far left of the chassis (which is a 12AX7A).
PS don't pay too much for those Mullard 12AT7WA's - there are literally tens of thousands out there (UK military surplus from the 1980's) - one UK dealer alone has over 35,000 listed in their current inventory - current UK stock would be about $12 each (PM me if you want details of a trustworthy UK supplier as I have just bought a batch of these).
Also, be a bit suspicious of any that have 'Mullard' actually written on them, the actual tubes should only have their 'CV' numbers which identify the military type (in this case CV4024) and other lettering which identify the supplier etc.
Any that have additional Mullard logos screened on the opposite side of the tube seem to have this added afterwards to justify a premium price over the normal items - I bought some of these and they are definitely genuine Mullard items from the tube codes and physical appearance, but someone has added the Mullard name and logo afterwards (this is on the 'wrong' side of the tube and will face the back of the MC275 as well as being the 'old' style shield logo which would not have been current in the 1980's. It is also printed with a paint which will not scratch off with a fingernail, genuine markings are chalky and easily rubbed off).
At the moment they are a good bargain to stock up on at a price equivalent or cheaper to current new Russian or Chinese tubes, but don't be tempted to pay a premium price for them as they are not at all scarce....
