Marshall JCM900 model 4100 head 100 watts any good?

[Slobrain]

Hey Folks,

I have a chance to get a Marshall head model 4100 jcm900. I never played thru one and its on craigslist for around $350.00 but its kinda ugly due to someone painting the front panel black. Kinda beat too.
Are these any good? a Marshall at $350.00 is pretty low and cheap compared to all other Marshalls. This one uses the 5881 power tubes.

Maybe a good candidate for a strip and rebuild?

Anyone like these , hate these???

any good???

Thanks

Slo

[Chuck H]

I used one for several years when I was gigging. I liked it a lot. It's the standard Marshall sound with a little diode clipping to add spice. A popular mod is to just remove the diode clippers. Since I was playing a lot of hard rock I found the dual master setup more effective than channel switching. I don't think you can go wrong at $350.

EDIT: You will never get good clean tones out of it stock. It's strictly a rocker.

EDIT 2: I had the 2100 not the 4100. Looks a little different. The 2100 doesn't have channel switching or reverb.

[soundguruman]

It's actually pretty good if you modify some of the circuit.
"as is" it does have some problems...like...no bass response.

The clean channel is not clean, until you take out the LED clipping diodes.

There is some whistling oscillation because the reverb tank send return wires are too close to the input jack...
but moving that away, it seems to cure the oscillations.
I kind of want to put a metal shield in there, that seems to work better than just moving the wires.

and the effects loop is the wrong level for USA effects pedals.
The effects loop power supply needs some beefing up.
Otherwise the effects loop tends to create very high frequency oscillation, which kills the tone of the amp.

And for the effects loop, you want an isolated power supply for the effects pedals.
Otherwise there is a ground loop between the front panel effects and the rear panel effects loop.
(when the effects run off the same power supply)

But you don't need to strip it and start over.
A few tweaks makes it a very useable amp.

In fact, the ones I tweak are fairly good sounding.
The 40 watt triode output makes it tolerable at a lower volume level.

[Slobrain]

Hey SGM,

I was looking at the schematic and its a SS front end, kind of odd seeing that for a Marshall. But I think I know what they were trying to achieve. This amp is pretty ugly, someone painted the front panel black, If I can get it from him for $300.00 I might buy it. Tweaking can be cool, or just gut it and make another amp altogether too.

An old Ampager named Joe L. I think took one of these and built a SLO 100 into it. I seem to remember him doing that. I actually wouldn't mind building something into it as well but since the layout is 3 preamp and 4 power tube sockets in the middle of the chassis sort of making it something else a bit limited.
I probably wouldn't mind building a hot rod Plexi circuit in it but then again I like channel switching amps too.

I have had a bunch of Marshall over many years, just never got into the 900 series, then again I could maybe build a Lerxst amp into it too. lots of ideas but not sure which one would be real fun...

so did you set up the clean channel cleaner and the hi gain better than the stock sound? seems one complaint is no bass, the other complaint is it sounds like a dual recto which I don't like those at all...

If someone ever built some circuit boards that fit the chassis and made these amps sound awesome they could do well in that business...

Cheers

[Alan0354]

I have the 4101 Dual reverb 1X12 combo, not good. I had it fixed and original, sounds pretty bad. I try to use it as a reference, but it's just bad......as in bad. It is so cheap for good reason.

[Joe L]

An old Ampager named Joe L. I think took one of these and built a SLO 100 into it. I seem to remember him doing that. I actually wouldn't mind building something into it as well but since the layout is 3 preamp and 4 power tube sockets in the middle of the chassis sort of making it something else a bit limited.
I probably wouldn't mind building a hot rod Plexi circuit in it but then again I like channel switching amps too.

SLOBrain! Good to see you.

Actually, it was a JCM 800 channel switcher combo with reverb that followed the 2205 schematic. I recently ripped out the old SLO p2p parts and built a great sounding 2550 Silver Jubilee circuit using just the Lead channel controls. That left two tube sockets and the clean channel controls free so I'm experimenting with a EF86 clean channel. The great thing about this mod is that transformers are the same between the 2205 and the 2555.

As far as the 900 goes, I met up with Bad Co's tech at one of their gigs in New Orleans and Mick Ralphs was using two heads and four bottom cabs. The sound was just ok with the 900s lacking bass and having a definite SS buzz to the sound. I fired up my rig at home when I got back that night and it sounded better. But of course, Micks playing was fantastic so all the sayings about it being in the fingers apply. I just know he would have sounded better on my rig.

So... If you come upon a donor 2205/2210/4212 JCM 800, jump on it! As far as the 900, change the circuit, replace the puny OT and you might have something.

[Alan0354]

If you can rip up the circuit board, build a new amp into it, I guess it's worth while. It'll be better if you can talk the price down more. I am planning to rip the guts out in my JCM900, to me, it's beyond hope. But the PT and OT are good AND the chassis is of high quality thick sheet metal. That itself worth the money in my book. I am planning to do a single channel Plexi with master volume in the distance future.

[Slobrain]

Hey Fellas,

Looks like the guy that wanted to sell the JCM900 backed out, probably a good thing since so many say these are not good amps. I did get lucky and find a 1984 Peavey Renown 2x12 in a pawn real cheap. Although its not a tube amp its still a good sounding SS amp. I sort of have a soft spot for those old Peavey SS amps from the early 80s as you can tell... I did club time with those in the early 80s metal dayz...

As far as a cheap tube amp to mod, I'll have to keep looking...

Cheers

[Slobrain]

Hey Joe,

Good to hear from you. I wasn't sure you were still doing mods. Its been along time since the Ampage days hasn't it... Do you still hear from Jeff S? Its funny that I remembered that Marshall amp you posted years ago and made to the SLO. It seemed to come back to memory when this guy wanted to sell his JCM900.

Glad to hear you did the Silver Jubilee circuit, those Jubilees were good sounding amps. Did you add the diode clipping circuit in it like it shows in the schematic? I think the new Lyrxst amps are based on the old silver Jubilee amps. If I could have gotten that JCM900 I was thinking about doing something different maybe like dual channel but I wanted something more hi gain but in the same sound of a JCM800 with a tube screamer in front type gain. You know, smooth gain.

I once talked to Lee Jackson in the late 90s and he told me that the Ampeg 501 was his take on a hot-rodded Marshall tone, I built that circuit into a bassman head back in maybe 1998 and it sounded good. If I remember correctly I added a diode bounding circuit to that amp past the tone circuit for more distortion and that thing seemed to have a heck of a lot of gain... but it was smooth sounding. The only reason I didn't keep it was the quest for the ultimate tone... and a clean channel too.

Slo

[soundguruman]

Hey Fellas,

Looks like the guy that wanted to sell the JCM900 backed out, probably a good thing since so many say these are not good amps. I did get lucky and find a 1984 Peavey Renown 2x12 in a pawn real cheap. Although its not a tube amp its still a good sounding SS amp. I sort of have a soft spot for those old Peavey SS amps from the early 80s as you can tell... I did club time with those in the early 80s metal dayz...

As far as a cheap tube amp to mod, I'll have to keep looking...

Cheers

A. The secret is these are actually really good amps, if you make a few simple changes. These amps can sound killer.

B. If you rip out the board and start over, you are doing it the hard way. It's completely unnecessary. You are wasting a lot of time and money for nothing.

C. Once the changes are made, it would please just about any professional guitar player. No complaints at all.

D. Don't trash great potential. It's a diamond in the rough.

E. There is nothing wrong with the transformers either, 100% useable.

F. Just because it's "popular" to rip the board out, does not mean it's the smartest idea. Knowledge is the key. Ignorance is the downfall.

[Slobrain]

A. The secret is these are actually really good amps, if you make a few simple changes. These amps can sound killer.

B. If you rip out the board and start over, you are doing it the hard way. It's completely unnecessary. You are wasting a lot of time and money for nothing.

C. Once the changes are made, it would please just about any professional guitar player. No complaints at all.

D. Don't trash great potential. It's a diamond in the rough.

E. There is nothing wrong with the transformers either, 100% useable.

F. Just because it's "popular" to rip the board out, does not mean it's the smartest idea. Knowledge is the key. Ignorance is the downfall.

Hey SGM,
I'll agree with you. Sometimes diamonds stay rough until the right person posts good mods, then the dam breaks loose... Just look at how the SLO 100 amp became so copied after the schematic hit the net in the late 90s... Now some guys are doing different spinoffs of other amps just from that one schematic that got loose years ago.

Maybe someone can setup a site called the JCM900 mod page. then these amps will become popular instead of so unpopular. Definitely good potential but someone needs to start the ball rolling to help these amps reach full potential.

Cheers

[soundguruman]

You don't need separate site. You can do it right here, in MEF.
The mods I propose have been developed by an experienced, schooled electric guitar player, and experienced Marshall amp technician. (me)

The trouble with "mods" done by technicians
Is that most of them have no clue how to play electric guitar.
They have no idea what a guitar amp is "supposed" to do, in the hands of a real guitar player.
AND how could they know?

So, this is where guitar player / technicians will really have a chance to accomplish something great.

*mods to the 4100 cost less than 1/10th of ripping the board out and starting over again.
* taking less than 1/10th the time...
and the amp will sound every bit as good, or better.

The main disadvantage to Music Electronics Forum is that it's 99% electronics
and the "music" part is overlooked.

However to a real musician, the "music" part is way, way, more important.
I mean, WHO are you building guitar amps for, anyway?
Time to wake up and smell the eighth notes.

The clean booming lows
and the crunchy crispy highs
are what a Marshall is all about.
But you can't detect that with a sine wave and an oscilloscope.

[allstarrr]

Hey Soundguruman, it's been many years now, but after talking so much about the right mods to do easily to the JCM900, how about actually posting what those are? I'd love to see and do those mods to the JCM900 I have. Thanks...

[Enzo]

Hi allstarrr, welcome.

Mr Guruman made himself unwelcome here some years ago and is not around. He was noted for posting broad opinions without any support or basis.