Speaker Load on a JCM900 4100 Hi-Gain

[cudamax2343]

I have a 4x12 16ohm Cab. My used but New JCM 4100 900 Has Only A 4 or 8 ohm load on the Back.

These can not be Run with a 16ohm load?
What Am I Not Seeing?
Can't believe their is no 16ohm Load Switch.

Are These Made only to be Run with the Matching
1936 2x12 8ohm Cab? or Any 2x12 8 or 4 ohm Load

Can Anyone Help me out and Set me Straight

 

[invertedjesus]

some moron blacked over the (16) next to the four. if you're going to use a 4 or 16 cab, put it to the left. if you're going to use an 8 ohm cab, put it to the right.

 

[cudamax2343]

Thanx's Dude. I Pulled out the Magnifying Glass and it Does Say 4/16 ohm.

So it's to The Left Position For my Cab.

Send me An Address so I Can Send you 20 Dollars

You Can Pickup a Hooker on Me.

 

[mlordin]

Send me An Address so I Can Send you 20 Dollars

You Can Pickup a Hooker on Me.

Hooker for 20 bucks - where?

 

[cudamax2343]

Out Here a 20 Will Getch-a Ride Date. They are Everywhere.

and They all Know Me.

When my Truck Goes Down a Certain Road

by a Certain Place of Town. They All Come a Running

For thecrazygreek--cudamax

 

[cudamax2343]

Reminds me of an old Saying

AHooker Does What She Does For Free

Cause She Likes Too and She Likes To Do it.

You Only Pay-em to Leave.

 

[Purgasound]

The 900 heads share the same rear panel print as the combos. The (4) only applies to the combo versions of the amp. The heads have 16ohms and 8ohms only. The combos have 4ohms and 8ohms only. It's a design flaw for sure, but you can confirm this information via the Marshall website handbooks.

 

[invertedjesus]

The 900 heads share the same rear panel print as the combos. The (4) only applies to the combo versions of the amp. The heads have 16ohms and 8ohms only. The combos have 4ohms and 8ohms only. It's a design flaw for sure, but you can confirm this information via the Marshall website handbooks.

wait wait wait, are you serious? so I've been playing my head at 16 ohms into my 4 ohm cab...?

 

[T-Bird]

Hi.

The 900 heads share the same rear panel print as the combos. The (4) only applies to the combo versions of the amp. The heads have 16ohms and 8ohms only. The combos have 4ohms and 8ohms only. It's a design flaw for sure, but you can confirm this information via the Marshall website handbooks.

Might want to get Your facts straight before making claims though.

Quite a lot of manufacturers, Marshall among them, uses regular 4/8/16 OT's with only two options on the switch. In the Marshalls case the 4 and 16 Ohms are ALTERNATIVES, hardwired inside the chassis. You can have either, but not both without further modifications.

Take a look at the page 4 on the schematics. On the top right hand corner You'll find the OT and the accompanying note 4.

http://www.schematicheaven.com/marshallamps/jcm900_dualrev_100w_4100.pdf

cudamax2343, looks like a tech has changed the tap to the 4Ohm instead of the 16 and MARKED it as well. Just like it should be.

If You are competent tech, you can do the mod yourself. If You are in doubt about your abilities, take the head to a tech.

Regards
Sam

 

[steelhorse]

I think AV has done his due diligence T-Bird, because simply because those configurations are alternatives does not mean that is predominantly the case.

 

[Purgasound]

Hi.





Might want to get Your facts straight before making claims though.

Quite a lot of manufacturers, Marshall among them, uses regular 4/8/16 OT's with only two options on the switch. In the Marshalls case the 4 and 16 Ohms are ALTERNATIVES, hardwired inside the chassis. You can have either, but not both without further modifications.

Take a look at the page 4 on the schematics. On the top right hand corner You'll find the OT and the accompanying note 4.

http://www.schematicheaven.com/marshallamps/jcm900_dualrev_100w_4100.pdf

cudamax2343, looks like a tech has changed the tap to the 4Ohm instead of the 16 and MARKED it as well. Just like it should be.

If You are competent tech, you can do the mod yourself. If You are in doubt about your abilities, take the head to a tech.

Regards
Sam

Sounds kind of like you're busting my balls.

I know what you're saying but I don't really agree with how you're saying it.

Sounds like you're arguing with me about the way the amp comes from the factory.

You're assuming an amp is modded based off of the mark of a felt pen, which anyone on the face of this planet can mark all over an amp. It could very well be modded but we won't know without looking inside.

I know the impedance of the OT can be modded, but I'm not making blind claims, that's the way it comes from the factory. No offense to my buddy cudamax, but if someone could simply be modding output transformer impedances with no problem, they wouldn't be starting a thread like this or need our help.

I like you T-Bird, I just gotta to make sure we're all clear and on the same page.

...and cudamax, maybe you should open her on up and take a picture of the inside of the chassis so we can see if it has been modified or not to handle 4ohms instead of 16. I don't ever trust random marks on an amp so I wouldn't assume it has been modded without verifying it for yourself.

 

[invertedjesus]

So... I should stop using my 900 with my cab...? I'm actually pretty pissed about all of this. As much as I know about my gear, and thats never even crossed my mind.

 

[Purgasound]

So... I should stop using my 900 with my cab...? I'm actually pretty pissed about all of this. As much as I know about my gear, and thats never even crossed my mind.

Well what kind of cab do you have? If it's 8 or 16ohms you're good to go. If it's a 4ohm 4x12 you can easily rewire it to 16ohms in a few minutes.

 

[cudamax2343]

Yo Tbird/Sam, Real Good Point. Someone could Have Modified the OT to 4 or 8ohms only and if one did, I guess it would be Right to make a Mark to give one an Indication of Such.

Man Are you a Cop or a Detective. Thats a Good Point. Enough for myself to Want to Measure the output of the Tranny to See just Where its at.

Can one measure with a Meter to See? or I Guess I'll Have to Disassemble for Photos to Show to you Guys for a Final Judgment.

Man I love this Amp. It's the Hottest Thing I've Ever Had and Don't Want to be Running a 16ohm Load if the Trannys Switched For 4ohms Even thou I;d Love to Install a Mercury Magnetics Tranny. Just not Right Now.

Anyone willing to Instruct me on How to Measure at the Speaker Jack with a Meter to Find out Just Where its at?

Or are Photos a Easy and Safer Way?

Also it Should Sound Better if the Load is Mismatched now and Then Corrected, Right? and Thanx's For all the Input and The Great 2 Points that Have Been Offered and now I need to Verify for Sure of Course with Everone Else Help.

 

[Purgasound]

I think the photo's would be safer than trying to poke around in there with a meter, but either way you've got to open up the amp.

If corrected it should sound better, there may not be much of a noticeable change though. Matching impedance is related to the proper way of controlling electrical current to from the cab and amp in order for it to work properly without being damaged. This kind of relates to the question of some people trying to choose whether to be on a certain ohm setting to affect tone, which IMO there is no clear audible difference between the settings.

 

[cudamax2343]

O.K. It Will Take About an Hour For Photos.

It's not Faster with a Meter at the outside Speaker Jack?

Man First I Got to Have my Coffee and 4 Pain Pills

 

[invertedjesus]

Well what kind of cab do you have? If it's 8 or 16ohms you're good to go. If it's a 4ohm 4x12 you can easily rewire it to 16ohms in a few minutes.

its 4 ohms. should I pick up a thicker gauge of speaker wire? i got really bummed when I realized I have that thin dollar store looking crap in there.

and I've been running my head at the 4/16 since april... at high volumes... never had a problem, smelled anything, noticed any weird volume drops... maybe mine is outputting at 4...? Should I take off my back plate and see what kind of OT is in there?

 

[cudamax2343]

O.K. Here are Some Photos.

And I Found Something that Looks Like it Overheated

Any Idea's? or Do You Guys need More Photo's of Something Else?

 

[Purgasound]

It does look like it has had that conversion from the 16ohm tap to the 4ohm tap. I don't want to say yes or no for sure until someone else chimes in. There's other guys on here that know a lot more about wiring than I do.

As far as that burned up part, looks like a resistor after the bias resistor. It's hard for me to read the number on the PCB because it's a little blurry. Definitely looks as though it should be replaced and looked over by a tech to make sure none of the paths on the underside of the board are damaged.

 

[cudamax2343]

So Those Resistors on one of the Legs of the Output Tranny might be For a 4ohm output?