Why do old Marshall heads have 4 inputs? / How to jump amp channels.

I know this might be the wrong forum but here goes. I feel like an idiot asking this question but a friend of mine has an old Marshall JTM-45 amp with 4 inputs on it. Why do these amps have 4 inputs? I thought they were single channel amps? He was talking about jumping the channels. What does this mean, why and how do people do this?

Mine's got 5.....

.....and goes all the way up to 11...

The main reason the Marshall has 4 inputs is because when they were copying Leo Fenders Bassman amp they wanted to do a good job. ;-)

Jumpering between inputs dates back to the old vox ac30s. It changes the sound in a nice way

And the original marshalls were indeed fender bassman clones

ORIGINAL: mike_mccue

Why did the Bassman have multiple inputs? Two chickens in every pot? Or was it two cars?

Because back then people would share an amp.

Thanks a million Ric. I'm still confused on the exact connections made. Could you explain it to me like I was 5 years old? Or just stupid.....

Could you explain it to me like I was 5 years old? Or just stupid.....

Imagine that the sound coming out of your guitar is like water in a pipe.

All of the water backs up when it gets to a little funnel just behind one of the jack inputs on the Marshall. You can put more water into its twin input jack, but you still have the same funnel behind it and no extra water gets through.

BUT, if you divert some of the water into the second channel it has another little funnel that lets more of the backed up water flow into the amplifier's sound reservoir. When you get more water flowing into the reservoir the water changes colour and makes even more wonderful sound shapes in the air around the amplifier's speakers.

ORIGINAL: Fog

ORIGINAL: Jonbouy

Mine's got 5.....

.....and goes all the way up to 11...

don't laugh but Liam outta Oasis thought it was a factual film..hehe

And I still think Liam is a spoof...

This is a great explanation, thank you! But I'm still seeking basic instructions on the physical connections when you jump channels. Go slow now, I'm kinda dumb.

Simple - plug yer axe into Ch 1, top input (we'll call this 1/1) - take a short patch cord and come out of the lower input (1/2) and plug it into either of the channel 2 inputs.

Try the various combinations and see whatcha like......

ORIGINAL: ricstudioc

This is a great explanation, thank you! But I'm still seeking basic instructions on the physical connections when you jump channels. Go slow now, I'm kinda dumb.

Simple - plug yer axe into Ch 1, top input (we'll call this 1/1) - take a short patch cord and come out of the lower input (1/2) and plug it into either of the channel 2 inputs.

Try the various combinations and see whatcha like......

PERFECT!!! Thanks Ric! You da MAN.

What sounds kind of strange is plugging an input into an input...you would think you would need an output source to have any signal? Obviously there is something coming out of that jack.

4 Input Long Explanation

This applies to all 4 input Marshalls and most 4 input tweed era & black/silver face Fenders.

Each channel has a 1 meg resistor to ground then two 68k resistors (1 from each jack) to the grids of V1.
Plugging into Jack #1 (upper jack on 4 input Marshall) puts the signal through the 68k resistors in paralell (net 34k
resistance) AFTER the 1meg resistor to ground.
Plugging into Jack #2 (the lower jack) shorts the 1meg resistor to ground placing one 68k to ground, AFTER the other 68k,
then to the grid of V1. In effect a 136k pot turned halfway down. A net loss of 1/2 the input signal.

Injecting a 1kz 101mv test signal into C1 J1, I measure 100mv at the V1b grid.
Injecting at C1 J2 yeilds 50mv on the V1b grid.

Chan 2 shows the same test results.

Chan 1 would be the "bright" or I think Marshall called it "High Treble" channel in the catalogs, although on the
faceplate it is just Volume I and Volume II. The inputs being labelled Inputs I and II.

When you Input to C1 J1, jump Chan #1 jack #2 to Chan #2 jack #1, you experience some signal loss.
V1b measures 94mv
V1a measures 86mv

Jumping C1 J2 to C2 J2, you experience a more significant loss.
V1b measures 75mv
V1a measures 25mv

The test suject is a 1985 4 Input JCM 800, labeled 1959 on the chassis, that evolved from the "classic" 1959
(this would be the 100 watt Super Lead Plexi) Marshall circuit, which is itself a modified so-called '59 Bassman.
Which is, I believe, a Western Electric (Bell labs) circuit. It was common practice for tube manufactures/distributors
to make working circuits available to amp manufacturers.

Hope I've made myself clear.
Having played Fenders since 1966 and scoring my first 100 stack in '71, I forget we don't always share the same experiences
and points of reference.
I guess that's why the manufactures get away with selling "classic" "historic" "vintage reissues" that are anything but reissues of
historic, vintage, classics.

I've been working on a video series of "Classic" Fenders and Marshalls for my web site. I'll post again when I have
that up and running.

Yours in the qwest...

I wish I had my old silvertone rig, A top with 2 channels/ 4 input jacks and a speaker cabinet with two 12s I think, and a space to store the head.
We plugged in a guitar a mike and my bass into it and placed the cabinet in front of the band , instant PA and backline .Ahh the good old days...
any one remember that combo? I think the head was a tube type ? and the tolex was silver ....

Bill,
I must admit the engineering side of brain was working when I read your post, 'cause I immediately thought "the signals are exactly the same". Shame on me. Since I have a 4 Input on the bench I did a quick test, and lo & behold; YOU ARE CORRECT SIR! There is a measurable difference in the signal each channel "sees". Now, can this be balanced out with the individual Volume controls? That would require subjective listening tests and could keep us busy all day long. I have a decent old Paul and a '71 50 watt, if you have a fine old Strat maybe we should get together sometime. If someone has some good beer, fine wine or excellent sippin' whiskey, certainly they would be more than welcome to attend as well!
And you are correct again about the LOUD part. This '85 is putting out 100 watts before the Volume pot hits 2. My 50 is the same, full power almost immediately. This all with a 100 millivolt input sighal. Which I find pretty well approximates "average" pickup levels.
I have some video up HERE if anyone's interested. Needs more footage and voiceover, but illustrates the difference in Bill's input ideas, and how quickly this comes to clipping.