Amp-cab Impedances Mismatch (or Not?)

[ilarranaga]

Hi guys,

I bought a JCM900 (4100) amp and a JCM900-1960A cab both together and both used (secondhand) 2 years ago. The man who sold me the amp and the cab told me to use "from this jack in the amp to this jack in the cab with the switches like this". He was doing it in that way for around 10 years. I did not think more about it and just followed his instructions.
I never understood why I should plug the cable in this way so I decided to learn about it. To do that I read the thread "Marshall Amp Forum > The Amps > Marshall Amps > Speaker Load on a JCM900 4100 Hi-Gain".
Well, here starts the question.

I realized that I was using the 4 ohms input in the cab with the switch in mono. I realized that I had the switch in the amp in (4) 16 ohms and that this actually should be 16 ohms since my amp is a head. This is what I understood in the thread that I had mentioned.
So... mismatch!! Very bad!! I am using the 16 ohms output in my amp to a 4 ohms input in my cab. But they are working in this way around 12 years and the amp is still there, it works.

This fact made me to think that perharps the amps was modified to offer a 4 ohms output changing the cables on the OT as is showed in the thread I mentioned. But NO!! It seems to be configured as a 16 ohms output.
Maybe the cab was re-cabled and the 4ohms input was working as 16ohms... NEITHER!! I measured the impedance with a multimeter and is around 4 ohms (around 5,5 ohms to be more accurate, but the multimeter is not the best...).

I am really concerned about this problem...
Am I really using a wrong 16ohms-4ohms configuration?
If that is the case, why is the amp still alive after 12 years?
Can I measure with a polimeter or is it any way to know for sure if my amp is offering a 16 ohms or a 4 ohms output?

Thanks a lot!

 

[V-man]

1. to cover/rehash the obvious: there are 2 settings on a JCM 900 head: (4)16, and 8. Sadly Marshall's negligent printing is misleading and those settings are 16 & 8 ONLY.

2. JCM 900s are remarkable amps. My 4100 forgave me of nearly EVERY rookie mistake in the book, and when my ignorant newb ass pushed it too far, I was out the cost of a couple fuses and the Hi/Lo power switch in back.

3. Just because it went well so far is no guarantee it will continue to do so. You may be lucky due coddling the amp, where at louder volumes, bad things would happen, or your number may not be up yet. Even if your number never comes up, what excuse is there to operate the amp incorrectly on purpose? You learned it's wrong, that it's putting undue stress/wear on the components, and it might become an expensive repair. If nothing else the cavalier attitude may one day inadvertantly carry over into a less forgiving, more expensive amp and you will wear two pairs of sneakers out kicking your own ass for lax standards of care.

4. Give the amp a warm pat on the headshell in affirmation of its forgiving nature (so far) take care of it from today foward, and remain consistent with your tube amp operation... consistently alert and cautious.

 

[ilarranaga]

Thank you for your great response, V-man.

I agree you, I should change the way I use my amp and connect all properly, but I would like to ensure that the amp's OT is configured to 16 ohms (not 4 ohms). If I had bought the amp new from a shop, I would change the connection and problem solved! But it is not the case and I do not know what modifications the amp has had.
Do you know any way to check / measure this?

Thanks a lot!

 

[Purgasound]

Take some pictures and post them on here. Especially detailed ones of the OT if you want us to tell you if it's wired correctly.

Peace out.

 

[Voodoo Amps]

Hi guys,

I bought a JCM900 (4100) amp and a JCM900-1960A cab both together and both used (secondhand) 2 years ago. The man who sold me the amp and the cab told me to use "from this jack in the amp to this jack in the cab with the switches like this". He was doing it in that way for around 10 years. I did not think more about it and just followed his instructions.
I never understood why I should plug the cable in this way so I decided to learn about it. To do that I read the thread "Marshall Amp Forum > The Amps > Marshall Amps > Speaker Load on a JCM900 4100 Hi-Gain".
Well, here starts the question.

I realized that I was using the 4 ohms input in the cab with the switch in mono. I realized that I had the switch in the amp in (4) 16 ohms and that this actually should be 16 ohms since my amp is a head. This is what I understood in the thread that I had mentioned.
So... mismatch!! Very bad!! I am using the 16 ohms output in my amp to a 4 ohms input in my cab. But they are working in this way around 12 years and the amp is still there, it works.

This fact made me to think that perharps the amps was modified to offer a 4 ohms output changing the cables on the OT as is showed in the thread I mentioned. But NO!! It seems to be configured as a 16 ohms output.
Maybe the cab was re-cabled and the 4ohms input was working as 16ohms... NEITHER!! I measured the impedance with a multimeter and is around 4 ohms (around 5,5 ohms to be more accurate, but the multimeter is not the best...).

I am really concerned about this problem...
Am I really using a wrong 16ohms-4ohms configuration?
If that is the case, why is the amp still alive after 12 years?
Can I measure with a polimeter or is it any way to know for sure if my amp is offering a 16 ohms or a 4 ohms output?

Thanks a lot!

In the hopes that it helps; You can safely go out of the head and connect to an equal or higher rated impedance. IE: you can go out of the amp head at 4ohms and plug into a 4, 8 or 16ohm cabinet. It is not recommended to reverse this as it runs the power tubes a lot harder. The higher the volume is set the sooner/more likely the odds are you will cause a failure. I should also note that if you connect things improperly but do not play on it then no damage will occur. The higher the volume the more likely damage is to occur.


Have a great day!
Trace

 

[ilarranaga]

Ok American Viking,

I am going to be able to upload the photos on saturday, plese hold on ;-)
I compared yesterday this photo in the other thread and my OT wiring, and it is exactly the same, it seems to be 16 ohms. I believe that this wiring is the default wiring of this head (I saw it on some schematics of the 4100) , but I will upload the photos of my amp on saturday anyway.

Just in case, does the fact of having the same wiring mean something?

regards.

 

[ilarranaga]

Voodoo Amps,

that is the reason I am concerned why. If all in the amp and the cab is no-modified, I have the amp 16ohms and the cab 4ohms. Really bad.
I use the amp with the master volume of both A and B channels on the mid-range normally. Perhaps this has not been enough to damage the amp?

thanks

 

[JayCM800]

I doubt someone modded or rewired the amp to run on 4ohm (i don't see for what purpose or advantage). Plug it all on 16ohms, cross your fingers, hope it doesn't blow up!

 

[Purgasound]

In the hopes that it helps; You can safely go out of the head and connect to an equal or higher rated impedance. IE: you can go out of the amp head at 4ohms and plug into a 4, 8 or 16ohm cabinet. It is not recommended to reverse this as it runs the power tubes a lot harder. The higher the volume is set the sooner/more likely the odds are you will cause a failure. I should also note that if you connect things improperly but do not play on it then no damage will occur. The higher the volume the more likely damage is to occur.


Have a great day!
Trace

If you're going to go out of an amp head at 4 ohms and connect to a higher impedance/resistance than that means the amplifier is delivering more current than the speaker cabinet will accept. This current has no where to go but back to the transformer so how is this safer????

 

[Voodoo Amps]

If you're going to go out of an amp head at 4 ohms and connect to a higher impedance/resistance than that means the amplifier is delivering more current than the speaker cabinet will accept. This current has no where to go but back to the transformer so how is this safer????

Let me preface this by saying; I do not wish to get into any kind of heated tech debate and my only intent with posting was to be of help.

Respectfully; perhaps referencing or acquiring some books on transformer design that outlines or focuses on how output transformers react with power tubes at various primary and secondary impedances. In the interests of being clear, I am not saying this as a dig and nor am I saying this to engage in any kind of a flame war (sincerely).


Hope it helps;
Trace



Hope it helps;
Trace

 

[ilarranaga]

Take some pictures and post them on here. Especially detailed ones of the OT if you want us to tell you if it's wired correctly.

Peace out.

Hi,

here are the photos of my amp. I hope they will be enough to know the output configuration of my amp. Do not hesitate to ask for more photos in case you consider it.

Thanks a lot!!

 

[Ken]

In the hopes that it helps; You can safely go out of the head and connect to an equal or higher rated impedance. IE: you can go out of the amp head at 4ohms and plug into a 4, 8 or 16ohm cabinet.

This is only true for amps with a SS power section as they don't have an output transformer. The subject is a 4100 DR which has a tube power amp. With a tube amp, this is not only wrong, but dangerous!!! The WORST case for a tube amp is an open speaker out, which means the higher the mismatch to the high side on the cabinet, the worse it is. A 16 ohm load on a 8 ohm out is absolutely a bad idea in the long run, even if nothing blows up right away.

Even mismatching a tube amp to the low side is not a good idea. (On a SS amp, this will fry the power transistors in short order). While a 2 ohm load on a 4 ohm output on a tube amp is a bad idea, it's not as bad as the reverse.

Bottom line: match impedences correctly. Your OT can last a lifetime when you do.

Ken

 

[Buggs.Crosby]

i would test the cab with a meter and see what it says.....for all you know it is wired internally at 16ohms using the 4ohm jack................i would disregard everything that has been posted so far until you know if the cab is correct

that aside the amp definitely has something going on with it......R6 seems to have been altered and is that resistor coming off the purple on the tranny original?

 

[diesect20022000]

mine never had a resistor off the trnsformer like that. That's odd.

 

[Buggs.Crosby]

the 1st pic he posted has a pair of them

 

[Purgasound]

mine never had a resistor off the trnsformer like that. That's odd.

It is typical on the 900 series. It is a NFB wire connected to the 4 ohm tap of the transformer. The resistors are only there for a tonal change. They can be removed and they are only found on the 5881/6L6 models. Those resistors are not present on the EL34 amps.

 

[Purgasound]

From the pictures of the OT the wiring is stock so yes, your 16(14) side of the switch is dedicated 16ohms. If you were to move the green wire to where the purple wire is it would be a 4 ohm.

Also the piggybacked resistor/capacitor combo on R6 is normal for 5881 amps as well.

 

[ilarranaga]

From the pictures of the OT the wiring is stock so yes, your 16(14) side of the switch is dedicated 16ohms. If you were to move the green wire to where the purple wire is it would be a 4 ohm.

Also the piggybacked resistor/capacitor combo on R6 is normal for 5881 amps as well.

Hi,

thanks for your response. So it seems to be a 16 ohms output connected to a 4 ohms load. I will plug the jack in the 16ohms input of the cab from now on.
Just to know it, do you know any way to measure (with a multimeter for example) the output configuration of the amp like I measured the input configuration of the cab?

regards

 

[Purgasound]

Well since the amplifier doesn't really have a load to measure you can't really...

The easiest way I can explain it is the output transformer is basically a winding that is tapped in at three different points. These are the 4 ohm, the 8 ohm, and the 16 ohm taps. Though the transformer itself isn't going to be 4 ohms or whatever, the amount of the winding used is what adds a bit of resistance which will negate output current. This current needs to have a destiny which is why the impedance needs to be matched so the amplifier doesn't send more current than the load can absorb. When this happens the current extra current that the speakers won't accept is reflected back into the transformer and it will fight to find a way out either by overheating the transformer or shorting out and/or arcing and punching holes in the insulation. In other words, match impedance or else! These things won't fail immediately but they will not tolerate certain mismatches for long. Maybe this picture will kind of illustrate what I'm talking about. (this pic doesn't pertain to your amplifier it's just one I found with a quick search)

I don't think I explained this very well or used the most proper terms but you should get the general idea. I am at work at just trying to type fast and move on so it looks like I'm doing something. Peace out.

 

[paunch23]

why does marshall even have that (4) next to the 16 below the impedance switch????
so if i understand this correctly a jcm900 head is actually either 8or16Ω, right??