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I've been looking for one of these for about 14 years now, and finally got one off eBay last week (hopefully I wasn't bidding against anyone here! 
) and have been playing it a bit the last few days; right now it's now on the bench undergoing 'factory upgrades'. I just thought I'd share a few impressions about this evidently pretty rare amp (it's on page 46 of the Doyle Marshall book), apparently the most powerful production all-tube guitar (not bass) head Marshall ever made.
It's a channel-switching design; channel A is a non-master (!) channel that (to my ears) just nails the Malcolm Young sound, and channel B is the master-volume channel that (again, IMO) sounds muddier and muddier the more the gain is turned up, with a heavily-enveloped neck-pickup 'cow-tone' at max gain.
Hopefully the 'Ruby Zener mod' will work it's magic here, as I have no intention of ripping this collectible a new one but wouldn't mind making a minor mod or component sub here and there if it makes a major difference overall.
There's three FX loops (Ch. A, Ch. B, both), preamp & XLR balanced line out (the latter from a separate OT secondary winding), phone & XLR speaker outs, six GEC KT77's, and a combination of tried & true and very-unusual-for-a-Marshall circuits. And it really doesn't seem all that heavy, although having two handles surely helps.
Ray
) and have been playing it a bit the last few days; right now it's now on the bench undergoing 'factory upgrades'. I just thought I'd share a few impressions about this evidently pretty rare amp (it's on page 46 of the Doyle Marshall book), apparently the most powerful production all-tube guitar (not bass) head Marshall ever made.It's a channel-switching design; channel A is a non-master (!) channel that (to my ears) just nails the Malcolm Young sound, and channel B is the master-volume channel that (again, IMO) sounds muddier and muddier the more the gain is turned up, with a heavily-enveloped neck-pickup 'cow-tone' at max gain.
Hopefully the 'Ruby Zener mod' will work it's magic here, as I have no intention of ripping this collectible a new one but wouldn't mind making a minor mod or component sub here and there if it makes a major difference overall.There's three FX loops (Ch. A, Ch. B, both), preamp & XLR balanced line out (the latter from a separate OT secondary winding), phone & XLR speaker outs, six GEC KT77's, and a combination of tried & true and very-unusual-for-a-Marshall circuits. And it really doesn't seem all that heavy, although having two handles surely helps.
Ray