HELP! Connecting JCM800 4104 to Marshall MX212R Cabinet

[Héctor Tamales]

I could get into specifics, but long story short...what is the correct and proper way to connect my JCM800 4104 combo into a Marshall MX212R cabinet?

My 800 combo has 2, 15ohm speakers and the extension cab has 2, 12" 8ohm speakers (150 watts). What I did was dial my combo down to 4ohms and an hour later into playing, I smelled burning plastic and shut it off and disconnected it immediately.

Since then, it has been playing fine by itself, but I would like to use this extension cabinet. So there's the dumb a$$ story of mine for the year (thank goodness it's almost over).

(Please see attached photos for specifics)

Thanks for the help folks!

Hector

 

[99_Revolutions]

Are you wanting to use the speakers in the combo along side the mx cabinet?

 

[Chris-in-LA]

Your cab says it’s an 8ohm load. So, two 8ohm loads, the 4ohm setting should have been right. Not sure what was wrong with that.

 

[Héctor Tamales]

Are you wanting to use the speakers in the combo along side the mx cabinet?

Yes. So that would be 4 speakers total (2 in combo, and the 2 in MX cab).

 

[Geeze]

If you have an DMM check the speaker cab at the jack to confirm it's load. If it is 4 ohm it will probably read 2.9 ohm or so. Also check for the dumb things like the extension cord not touching or being near a tube or tranny.

Russ

 

[Héctor Tamales]

Your cab says it’s an 8ohm load. So, two 8ohm loads, the 4ohm setting should have been right. Not sure what was wrong with that.

Well, I had it set to 4ohm setting when I plugged in the cab. Should it be on the 8ohm setting when addingthe MX cab? I placed it back on 8ohm setting for the 2 speakers in the combo.

 

[Héctor Tamales]

If you have an DMM check the speaker cab at the jack to confirm it's load. If it is 4 ohm it will probably read 2.9 ohm or so. Also check for the dumb things like the extension cord not touching or being near a tube or tranny.

Thanks! I don't have a DMM. I used a new speaker cable. What I should have stated originally was, I dialed the combo to 4ohms to plug in the MX cab. Maybe it should have been on 8ohms instead? When I do that, I now have 4 speakers ( 2 in combo and 2 in MX cab) to properly disperse power to.

Russ

 

[Chris-in-LA]

Well, I had it set to 4ohm setting when I plugged in the cab. Should it be on the 8ohm setting when addingthe MX cab? I placed it back on 8ohm setting for the 2 speakers in the combo.

4 ohms if you are using both, the cabinet AND the internal speakers. Put a meter on the cabinet and verify that it’s a 7 to 8 ohm load.

 

[Jethro Rocker]

Well, I had it set to 4ohm setting when I plugged in the cab. Should it be on the 8ohm setting when addingthe MX cab? I placed it back on 8ohm setting for the 2 speakers in the combo.

That was correct, if indeed the extension cab has a total of 8 ohms as per above posts.
Burnung plastic could have been anything else.

 

[Leonard Neemoil]

This probably sounds dumb, but are you sure that you're using a real speaker cable and not an instrument cable?

Not sure if an instrument cable used as a speaker cable would cause your problem but it's definitely not something anybody should do.

 

[Chris-in-LA]

An instrument cable can cause an OT to blow.

 

[scozz]

This probably sounds dumb, but are you sure that you're using a real speaker cable and not an instrument cable?

Not sure if an instrument cable used as a speaker cable would cause your problem but it's definitely not something anybody should do.

Doesn’t sound dumb at all Leonard, I’ve seen guys make that mistake.

 

[Leonard Neemoil]

Doesn’t sound dumb at all Leonard, I’ve seen guys make that mistake.

Thanks, I hear ya.

The only reason I said it might be dumb is because the op references "speaker" cable. That being said, to many folks a cable is a cable is a cable, so to some, instrument cables could be considered speaker cables.

One of my old bandmates insisted on using instrument cables to run his valveking stack because they're lighter, "more flexible", and "are the same damn thing". They must use some awesome OTs in those because the few years we were in a band the thing never fried. And we(I) play(ed) all out balls to the wall old-school metal at max volume.


Haven't talked to him in years, wonder if that poor thing is still putting up the good fight.

 

[Héctor Tamales]

This probably sounds dumb, but are you sure that you're using a real speaker cable and not an instrument cable?

Not sure if an instrument cable used as a speaker cable would cause your problem but it's definitely not something anybody should do.

Hey, just seeing this. Yes, all I use are speaker cables for these type of connections. I understand you had to ask. So aet to 4ohms you think?

 

[Leonard Neemoil]

Hey, just seeing this. Yes, all I use are speaker cables for these type of connections. I understand you had to ask. So aet to 4ohms you think?

If the extension cab is 8 ohms then it must have either two 4 ohm speakers or two 16 ohm speakers. Not two 8 ohm speakers as you stated in the op. That part mixed me up until I looked at the picture of the input jack.

So you're trying to use a 16 (15) ohm load for one output jack and an 8 ohm load on the other output jack, right?

I didn't think something like that was suggested for Marshalls. I always thought that the impedance had to be matched on both output Jack's.

Anyway, so if I remember how this works, you are combining a 16 and an 8 ohm load = 24 ohms total. If the amps output jacks are parallel, then the amp would see a 12 ohm load. That's IF I remember all the series/parallel stuff correctly. It's been a long time.

If I am correct, as it's been so long, I can't remember if the impedance switch should be set to 8 or 16. I forget if it should be slightly higher or slightly lower than the load the amp sees. I've never done this but have heard of others that do it. One way is supposedly safe and the other is an amp killer.

I always match the impedance for each side when using any tube amp. Solid state doesn't matter as far as I've been told but I don't know personally.

So that being said, you could disconnect one of the extension cab speakers, if they're 16 ohm speakers, and run just one 16 ohm speaker along with the stock speakers. As long as the amps output jacks are wired in parallel, your amp will see a load of roughly 8 ohms, so set the impedance switch to 8 ohms. Again though, that's assuming I'm remembering everything correctly. You shouldn't do anything I said until someone else chimes in and confirms my statements.

Sorry I can't remember exactly and I can't really answer your question.

 

[fitz288]

So aet to 4ohms you think?

Combo pic from OP shows 2 x G12M70 15ohm.
Only (correct) way to wire that is parallel 8 ohm.

MX212 (if stock) would be 2 x Seventy-80 16 ohm parallel 8 ohm

So the answer is: yes, set the amp to 4 ohm with the internal & external 8 ohm loads both plugged into the combo.
* Unless there's something different in the MX212!

 

[Leonard Neemoil]

Combo pic from OP shows 2 x G12M70 15ohm.
Only (correct) way to wire that is parallel 8 ohm.
View attachment 98999
MX212 (if stock) would be 2 x Seventy-80 16 ohm parallel 8 ohm
View attachment 99001
So the answer is: yes, set the amp to 4 ohm with the internal & external 8 ohm loads both plugged into the combo.
* Unless there's something different in the MX212!

Yes, duh! I got mixed up and was thinking a single 15 ohm speaker in the combo. Doh!

4 ohms is correct.

So that doesn't explain the burning plastic smell though.

OP should definitely get a meter. It's an essential item anyway imo.

Myself personally, I'd check the impedance on all 4 speakers and at the plugs they're connected to.

 

[fitz288]

Yes, duh! I got mixed up and was thinking a single 15 ohm speaker in the combo. Doh!

4 ohms is correct.

So that doesn't explain the burning plastic smell though.

OP should definitely get a meter. It's an essential item anyway imo.

Myself personally, I'd check the impedance on all 4 speakers and at the plugs they're connected to.

Sound advice to always check speakers or cabs for impedance.
Keep in mind that resistance on a meter is not impedance.
On the Seventy-80's in the MX cab, for example:

While rated at 16 ohms each, their resistance is going to meter at about 10.4 ohms resistance each.
Don't be shocked if the cab reads about 5.2 and not 8.

 

[fitz288]

So that doesn't explain the burning plastic smell though.

Also true.
4101 manual says jacks are parallel, so 4 ohm selector for using both internal/external is correct.
Smell when only using extension cab is odd.
I've got no explanation for that.

 

[Héctor Tamales]

Combo pic from OP shows 2 x G12M70 15ohm.
Only (correct) way to wire that is parallel 8 ohm.
View attachment 98999
MX212 (if stock) would be 2 x Seventy-80 16 ohm parallel 8 ohm
View attachment 99001
So the answer is: yes, set the amp to 4 ohm with the internal & external 8 ohm loads both plugged into the combo.
* Unless there's something different in the MX212!

Thank You!