#1
I was trying to look into more information on this online but could not find any so here i am, the marshall jcm 900 MKIII Hi-Gain 2500 has a footswitch with it but what does it do if it is a single channel head? And what exactly does the effects loop do? i though effects loops were something you would get from like a line-6 pod and loop stuff with or something like that.
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Marshall JCM900 Hi-gain MII 2500
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#2
Effects loops are a very common feature in amps, can't tell you if they came out before or after the effects pedal, but most likely. It's a pretty simple concept. It allows you to put rack gear units, or time-based effects after the pre-amp.

Also
http://www.erikhansen.net/marshallmkIII.php
Provides some good info. It has footswitchable master volumes.
...
#3
The effects loop allows you to put stuff between the preamp and the power amp. It can be effects, but something else, too - for instance, hooking a guitar preamp into the fx loop in a certain way will make it so that in practice it's directly connected to the power amp.

The difference in whether the effect is in front of the amp or in the loop is huge. Think about it; if you have, say, an EQ pedal in front of the amp, it'll just affect the guitar signal going into the amp. If you have it in the loop, it'll EQ the whole signal as it's going to the power amp.
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#4
They are a single channel amp with two switchable master volumes - like an SL/X. The 2500 is the poor man's SL/X. It doesn't have a gazillion op-amps in front of V1 like a 4500 but it does have clipping diodes. Remove the clipping diodes and you've basically got a 2204 with two master volumes and an FX loop.
http://www.drtube.com/schematics/marshall/cd0109-iss4.pdf
Remove BR1 and D1 and you have a fine amp.

The SL/X is better though, it's a 2500 with an extra preamp tube instead of the clipping diodes. In fact the SL/X is actually still called a 2500 but it's a Mark IV, not a Mark III
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#5
Also. These things have no reverb but they do have an FX loop. Get yourself a nice reverb unit and run it in the loop. And stick a nice flat boost in front of it. A modded tubescreamer (bass mod) with the drive down and level up, or better still a Timmy or an MXR 10 band EQ. Something that will give V1 a kick in the guts without colouring the signal in a way you don't want. The EQ is the best way to go, you can set the tone for your lead "channel" to perfection. Stick it next to your MV footswitch so you can hit them both at the same time.
You'll have a massively versatile rig if you do that.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#6
But is it better than his Hiwatt?
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#7
Yeah. He only has a Lead 50, not one of the expensive ones. I'd take a MkIII over it any day. If it was one of the old Hiwatts, nah. An SL/X is the amp he really should get. It craps over all of them for what he wants.
The MkIII with the diodes out is pretty much a JCM800 2204/2203. If you wanted pure 80's hair metal the MkIII is the one to have. But those diodes have to go. You don't clip the signal, you boost V1. Which is of course what they were trying to do with the 4100/4500 with all those opamps. But they had clipping diodes as well. Ripping those out of a 4100 fixes them too.
The real way to do it is to run a pedal or a tube or gtfo.
The SL/X was the right way, a 12AX7 boost. Shame you couldn't switch it out.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
Last edited by Cathbard at Jan 27, 2012,