These amps are from the days when things were innocent to the point of being kinda dumb. Rock stars didn't wear ear plugs in these days.
I was once coaching a group of kids for an after school program where they had a rock band. They played well. The final performance was to be for their parents at the end of the term. They were figuring out the order of the set and decided to put "Hotel California" 3rd out of 4th. I approved and said "yeah that's perfect timing for people to put their cigarette lighters in the air like this" They replied rather baffled: "people used to put their lighters in the air????"
I said "oh man, you kids will never have any idea how disgusting a rock n roll concert once was." One of the fathers looked at me with a nod and said "yup."
So these amps are some of the greatest inventions in modern history, or should I say, a great piece of evolution. Marshall merely copied Fender and his tweed Bassman amp and made it louder. Much, much louder. I've owned several of these, the first being a 1977 50 watter, the next being a 1974 100 watt Super Bass. Then I went through a series of Plexis. A 1966 Super Pa, '67 Super Lead, '67 JTM 50. These are hands down my favorite amps. The early ones simply look gorgeous. Heck, they made Jimi and Cream even more attractive to look at. That Gold face, black cabinet and silver grill along with Persian carpets screamed psychedelic.
This one is a later model from '75. All original except someone added an fan which it doesn't really need. It's also been converted to 6550 tubes. Big sound with those tubes. I do prefer EL-34 tubes in these but, this is one ballsy amp.
The blue cap is from this amp, find me the date code:
28th week of 1975. Now by this point in rock history, the stars were demanding amps to be brighter and have more distortion. This one isn't a Master Volume model, but it's dangerously bright. I don't like that. I prefer the sound of the earlier Plexi models. But, this is easy to alter. Just take out the damned bright cap across the volume knob on the bright channel. In this amp it's a .005. Try a 50pf or 100pf. Or leave it out so you can actually turn the treble knob up! I left the circuit alone cause this works for the owner.
Also try taking a patch cord and putting between the low input on the bright channel and the high on the normal channel. I used to do this. I'd turn up the normal channel then add the bright channel for just a bit of hair.
When this amp came to me it sounded terrible and had a nasty hum with plenty of crackly junk going on. I re-capped the power supply, changed a couple bad load resistors and cleaned it thoroughly. Also removed all the pots and cleaned the chassis where they are mounted to. This is routine. It gets gunky back there and causes problems cause this is where all of your signal path gets grounded to. Clean it. It will only take 5 minutes! Then make sure everything is nice and tight. Screws, nuts and bolts. Marshall amps, unlike Fender amps have mechanical grounds. In other words, the grounds aren't soldered to the chassis. They are soldered to lugs that are bolted to the chassis. For quietest operation, clean them all.
I replaced the output tubes with Sovtek 6550 tubes. The plate voltage in this amp is 525V! I wouldn't trust any modern EL-34 tubes with that kind of juice. I'm running these at a conservative 35ma each. The preamp tubes, 2 were RCA, one was a Telefunken, all 12AX7 of course. I left them alone except for cleaning the pins which were filthy. They sound great and no modern tube can match these. I don't know what the good people at Telefunken were doing but, their 12AX7 had a 100k+ hour life span. And this was a smooth plate version, highly sought after. As per usual, that tube had both sides pretty close to being matched. I've sold dozens of these and use them in my own amps. It's rare that I get a mis-matched tube, even at this age!
Also replaced the broken handle:
Some before shots of the old caps:
The filters are pretty easy to replace except for this one which feeds your preamp tubes. Fortunately by this point they put a longer ground lead on it so you can just pull it up from the chassis, unsolder it and solder in your new cap.
In this particular amp I took some liberties with the power supply. The main filters rather than use the stock 50x50 caps I chose to use 350uf caps built by JJ. In series this gives you a total of 175uf. The next stage I went with the stock 50x50 caps followed by a 32x32 feeding the phase inverter and 16x16 for the preamp tubes. My customer wants that thunderous low end. JJ builds drop in replacements that go up to 800uf! That's pretty extreme. I've built a couple Plexi clones that used this high of a filter and gotta say, nothing wrong with a tight bottom. I chose to use a much smaller 16x16 for the preamps just cause that's what the old ones use. It's a bit faster response supposedly.
So it was great to breathe life back into this fine amp. I miss the days when it was normal to rock one of these with a couple 4X12" cabinets. Voice of god I tell you! Nothing sounds this good to me. And the funny thing is for those who have a prejudice against these due to the years of bad music made by hair metal guys. These 4 input heads are actually a low gain amplifier. They can be FANTASTIC jazz amps. There is so much tone and they respond so well to your touch. I don't care for the later 900 and 2000 series at all. As my former boss Steve Melkisethian at Angela instruments said: "I know bad taste when I see it!" He refused to sell those 900 amps when they came out and he sold a lot of Marshall. I'm on the same page as he is.
See.... what else can I say about these? I worked on a Dumble once. Belonged to Robben Ford. I couldn't understand for the life of me why people pay $40k for one of those when these just sound so much better! I'd take one, sell it and buy a few Plexis myself, or pay for a nice vacation.........yeah, that's what I'd do.