Marshall JCM900 2x12 help with adding speakers!!!

lamf68

I have been using a JCM900 2x12 combo for the past few year, and whilst it's not to everyones taste, for the stuff my band does I love it.

my questions are probably simple to the amp minded out there, but as I kno nothing about ohms and voltage and stuff like that I have to ask!

anyway, here is a pic of the back panel.

questions are:

1. what is that switch that says Mode High/Low for ?

2. what do I do as far as adding an additional 2x12 cab or 4x12.

I appreciate this probably sounds retarded to everyone, but in the past

I've never added to a combo, as ive  always had heads and cabs!

any help greatly appreciated again.

ICBM

1. It runs the power valves in either Pentode (high power - full) or Triode (low power - about half) modes. Triode also sounds softer as well as being quieter.

2. Connect the cab to the other speaker socket and set the impedance selector to 4 ohms. It doesn't matter if the extra cab is 8 or 16 ohms. If it's a 4x12" with the 4/16 switching, use the 16-ohm input. Remember to switch back to 8 ohms if you go back to using just the combo!

By the way, that panel graphic which says 4(16) ohms on the impedance selector is one of the most misleading and dangerous on any amp - it is NOT a 16-ohm output setting, it's for if you rewire the amp inside to give the third - currently unused - tap. (The heads are the other way round, with 8 and 16 accessible and 4 unused.) I always deliberately clean off the bracketed part of the decal to avoid confusion... running an amp at 4 ohms into a 16-ohm cab or vice versa is a very bad idea, but that decal gives the idea they're the same thing to many people.

siraxeman

I have had 2 4502 50w Dual Reverb combos. First one I used for almost 10 yrs...liked it actually and don't agree with the bad rep these Dual Reverbs get (for having clipping diodes in the dirt channel) they still sound good to my ears and for me that is the bottom line. Now oddly the 2nd one I got a year n half after selling my first because I came across it in a smack converters for £175 and thought..?!! Do they know what they're selling here so cheap? So I got it...and I've no idea why unless it was the valves (all JJ's) or it had been some way modded...but it definately sounded noticably better than my first one. Both were mk I versions running 2x EL34s to. Anyway I sold it a year later when I got a DSL 50 head and 1960AV cab..Doubled my money!

JCM900's aint bad sounding amps IMO!!! Enjoy it!

mudslide73

^^ Thanks for that. I've been using a 2502 with a 1922 on the 4 ohm setting but you're right, I did think the (16) was a 16 ohm output.

I've got V30s in the combo and G12Hs in the cab. I had Shane Dolman fix the FX loop and some noise problems and it sounds pretty good for the stuff we do.

ICBM

siraxeman said:

I have had 2 4502 50w Dual Reverb combos. First one I used for almost 10 yrs...liked it actually and don't agree with the bad rep these Dual Reverbs get (for having clipping diodes in the dirt channel) they still sound good to my ears and for me that is the bottom line. Now oddly the 2nd one I got a year n half after selling my first because I came across it in a smack converters for £175 and thought..?!! Do they know what they're selling here so cheap?

Maybe they plugged into it and thought it was a Valvestate .

Actually I don't mind the sound too much - I prefer them to the JCM2000s, even the DSL.

The diode clipping is not a major issue to me, even though it's true - although in fact it's far more than that, the preamp is essentially two solid-state pedal circuits with a couple of valves for the tone stack and the reverb mix stage, and is a pretty good definition of a hybrid circuit. But it's not much different to running a SD-1 and a DS-1 in front of a valve Marshall anyway, and a lot of people don't have a hang-up about that...

ThePrettyDamned

Loved the jcm900 slx. Bloomin great sounding thing, I passed up the chance to own one for the price of the cab alone because 'these metal head amps are worthless'.

I'm such a nob at times.

lamf68

Huge thanks for the input lads, much appreciated.

seanfreeman

Could someone help
I have jcm 900 1x12 combo and want to add a 2x12 cab. I have had this cab for years and don't really want to go out and buy another one

Anyway the back panel is the same as pictured above and the cab only offers 4 ohms
If I change the switch to 4 ohms on the jcm and connect to the cab would I need to take out the combo speaker jack or do I keep it connected? Silly question I know but don't want to damage anything

Thanks

Sean

ICBM

If you're not cranking it right up it's safe to use both. If you are, slightly better to disconnect the internal speaker. Set the amp to 4 ohms either way.

You could also rewire the cab for 16 ohms if you would prefer to have the option of both more than one or the other. You can do that without replacing the speakers, just connect them in series instead of parallel.