73 Marshall Major help

[Music Saves lives]

I have recently bought a 73 Major. The labels on the back and front panel are mostly worn off. I have some questions about the jacks and switches. On the front there are 3 switches on the left hand side like stand by switches. Besides the power on what do these three switches do? I know one should be power and one should be standby. Is the other an impeedance selector?

On the back panel there are 4 speaker jacks. Are two of these for phase flipping or are these different impeedance taps. Please forgive the rookie questions. I have everything hooked up to two 4 X 12 cabs and all sounds good. Just don't want to fry a transformer. Any help would be greatly appreciated. q

 

[Gene Ballzz]

Pics will help us help you! Front and rear panels, zero in a bit on the switches and voltage/impedance selectors on the rear! What imedance are your cabinets?
Just Askin'
Gene

 

[Matthews Guitars]

Front switches: Left to right, polarity, power, standby. Same as a model 1959 Superlead or 1987 50W lead amp.

Back panel: All speaker jacks are identical in function and phase. Speaker impedance selector is used to pick the impedance,
which applies to all jacks.

Beware the impedance selector. If it's the plug style, be sure it connects very firmly in its socket. If it's easy to pull it out, don't run the amp unless you get a tech to install a different speaker impedance selector because the original style plug is responsible for a lot of fried output transformers. It's basically a little piece of junk that loses all its contact tension over time, making the connection between output transformer and speakers unstable.

 

[VERVEHAMMER]

Congratulations on the 73' Major!

Just for that, I'm gonna jet over to youtube and watch

Deep Purple's BURN

 

[vomheimlin]

Polarity, Power, Standby

 

[VERVEHAMMER]

Always Inspect and
Insure Correctly Installed,
Fuse Values
Fuse Types

Protect yourself
Protect your Amplifier

Service with Safety!

 

[Fernieite]

And congrats!

Nuff said...

 

[shooto]

Pics!!!

 

[Music Saves lives]

Plugging into 16 Ohm imputs on A and B cabs

 

[Music Saves lives]

Should polarity switch be up or down? The plug has a ground prong, in other words it is a 3-prong plug that goes into wall outlet that was installed by an amp tech before I gaught.

 

[Music Saves lives]

I am blind and the 1st person who helped me could not see labels. The hardest thing for me to figure out is the layout of back panel. Any descriptions would be appreciated. aThis is a prety rookie question, but I am wondering where the impeedance selecter is in paticuliar and if the phase can be flipped so that I can pair with a Silver Jubilee and have in phase w/o flipping phase in signial chain.

 

[Dave_11]

Bumping this up because you said you can't see and need some help to know what is going on with the back panel.

I'm a novice as far as Marshall amps go, but I can tell you what I see. Hopefully someone more knowledgeable will answer some of your other questions.

Back panel, going from left to right.
First there is a knob that looks like a modification from a former owner. It is placed right in the middle of the Marshall Major label.
Next there are two jacks that looks like you have your speakers plugged into. There is no label for these, but I looked at some other Majors online and they were the same, with no label. The serial number is below these, which reads A5788E.
Next is what looks like the impedance selector. It is labeled "OUTPUT" above it, and "IMPEDANCE" below it. I cannot tell what it is set to. The angle of the photo obscures it. Also below the impedance selector there is a white paper or tape label that reads "LEAD". This looks like it was put on at the factory.
Next are the two speaker jacks, and they are labeled "SPEAKERS" below them. There is nothing plugged into these.
Next is the fuse. It is labeled "H.T." above and "FUSE AMP" below. There is also a piece of plastic label tape that someone put on below that reads "1.5 AMP".
Next is the power cord. That is all in the picture.
Hope that helps.

 

[Gene Ballzz]

@Music Saves lives
First, I would confirm that the OUTPUT IMPEDANCE selector shows "8" in it's window (given two 16 ohm cabinets) and more importantly that it plugs in quite firmly, not loosely! These amps put out massive amounts of voltage and available current and an intermittent connection can be apoalyptically catastrophic! These IMPEDANCE selectors are notorious for getting loose, requiring the tightening of their "grippers" or replacement with a more modern style, positive action switch.

Next, I would be using the jacks labeled SPEAKERS, instead the jacks near an obvious modification, making them questionable as to their actual function, without opening the amp up!

And Then, I never, EVER trust those horrible, switchable Marshall jack plates. They are also notorious for causing/creating/allowing intermittent connections that can BLOW UP amplifers and/or their output transformers! These MAJOR amps contain extremaly high voltages and should never be opened up and/or internally messed with by casual observers or novices. Some of the voltages are higher than most meters can safely measure. These voltages can KILL you instantly and safety is NO ACCIDENT!

This Major is truly a "Big Boy" amp and must be treated with the respect it deserves, or it can/will turn itself (and maybe you) into a smoldering, messy pile of components!

Should sound amazing when it is right though! I'd get it to a professional tech, before using it much, just to confirm it's health and to determine what mods or other madness have been done to it!

Simply Crankin'
Gene

 

[jmp45]

Should polarity switch be up or down? The plug has a ground prong, in other words it is a 3-prong plug that goes into wall outlet that was installed by an amp tech before I gaught.

Doesn't matter, plug grounds. I have a 72 major, polarity, standby and power same thing. I ignore the polarity.

Like Gene says, 8 ohm selector for 2 16 ohm cabs. Doesn't matter which of the four jacks.

 

[Matthews Guitars]

Looks like you MIGHT have an added effects loop and a level control for it. In this case, you might have your speaker leads plugged into the effects loop, rather than into the speaker jacks. If you turn the amp on like that, you could seriously damage the amp. (Blow up the output transformer.) You don't want to do that. That'd be about a 500 dollar repair bill.

So plug your speaker leads into the two jacks on the RIGHT side of the back panel.

The two jacks on the left side MAY or MAY NOT be speaker jacks. They may be for an effects loop. And you want to be sure before you run the amp.

You do want to be sure about what's what. Remove the chassis from the cabinet, make some photos of the insides of the chassis around all the back panel jacks, and post them so we can check it out.

 

[jmp45]

So plug your speaker leads into the two jacks on the RIGHT side of the back panel.

I agree just to be sure. My 72 any of the 4 outs can be used.

 

[Dogs of Doom]

I'm thinking that, that knob on the back is a MV?

 

[Matthews Guitars]

I'd wonder if it was a level control for an effects loop. That's why I say that it'd be best to take the chassis out, photograph what's inside, and post the pics here. Better be safe than smoke a VERY expensive output transformer!

I can see from here that the left two jacks are spaced much closer together than the right two jacks, and there's NO WAY that's factory!

 

[Dogs of Doom]

the reason I say that, is because he said that he's getting good sound out of it as-is, so I'm assuming that he's plugged in to those jacks & getting plenty of sound. If they were an effects loop, he wouldn't be getting sound like that...

On the back panel there are 4 speaker jacks. Are two of these for phase flipping or are these different impeedance taps. Please forgive the rookie questions. I have everything hooked up to two 4 X 12 cabs and all sounds good. Just don't want to fry a transformer. Any help would be greatly appreciated. q

 

[Matthews Guitars]

True, I hadn't caught that part. It's still a really good idea to figure out what's in an obviously modded amp.