Ohms... just a simple yes or no.

[JCMRobbo]

Hello fellow Marshall enthusiasts I’m having a mare here and I hope that someone can just explain this as simply as possible so I understand this.
I have tried to understand ohms and Impedance and I just don’t get it and this lack of understanding comes as an expensive service bill....

All I want to know is the best set up for my JCM 900 100 watt head so that I don’t blow it up or damage anything Any further..

DO I....

Set my JCM 900 to (4)8 or 16ohms??

Into a 1960a 4x12? Set to what??

Or into a 2x12 With two 75watt 16ohms cones??

what will blow the head up and what won’t. I understand it’s about resistance and I’d appreciate the enlightenment so I don’t destroy my Marshall head and cab.

Just as simply put as you like, help a geezer out here hahahah

cheers!

 

[Jethro Rocker]

Well depemds on cab firstly.
Is it a stereo/mono 1960A??
If so, when set to mono it has a 16 ohm or 4 ohm jack. So uwe the 16 ohm jack and set amp to 16. You really just match amp amd cab.
If the 2x12 cab is wired in parallel, the bthe 2x16 ohm cones become an 8 ohm load total.
So.. guess what? Set amp to 8 ohms!!
That amp has a (4) zsetting which has to be switched inside. If not required leave it alone and it is an 8 ohm tap.

Do you need to know the whys and hows or just what to do?

 

[mickeydg5]

Sorry, there is no simple yes or no.
The impedance of both need to be match from amplifier to cabinet(s).
If the 1960 is set for or built for 16 ohms then hook up from that to amplifier 16 ohm setup.
If your two 16 ohm speaker cabinet is wired parallel then that would be 8 ohms total then hook up from that to amplifier 8 ohm setup.

The JCM900 100W heads usually have only 16 and 8 ohm output impedance switch.

 

[paul-e-mann]

Hello fellow Marshall enthusiasts I’m having a mare here and I hope that someone can just explain this as simply as possible so I understand this.
I have tried to understand ohms and Impedance and I just don’t get it and this lack of understanding comes as an expensive service bill....

All I want to know is the best set up for my JCM 900 100 watt head so that I don’t blow it up or damage anything Any further..

DO I....

Set my JCM 900 to (4)8 or 16ohms??

Into a 1960a 4x12? Set to what??

Or into a 2x12 With two 75watt 16ohms cones??

what will blow the head up and what won’t. I understand it’s about resistance and I’d appreciate the enlightenment so I don’t destroy my Marshall head and cab.

Just as simply put as you like, help a geezer out here hahahah

cheers!

The answer is..........it depends......

Best bet is put a meter on the cab to see what it is then plug into the amp to match. Its either gonna be 8 or 16 ohms, dont mix the two.

 

[What?]

Yes
No
Take your pick.
Neither will help you.

Simple answer: Match the impedance setting of the amp to the cab. If you don't know what the impedance of the cab is, use an ohm meter to find out.

 

[RLW59]

I'm gathering your head cost you money due to an impedance problem? You may not have been doing anything incorrectly.

The switching jacks on 1960's are notorious for failing -- the cab still works, but the impedance isn't what the switch and labels say. (Most of them never have problems, but enough of them fail that it's a well-known thing -- kind of like broken headstocks on Gibsons.)

If you've been connecting your cabs as per the first 2 replies, be very very suspicious of your 1960.
----------------------
I'm guessing from your post that you don't have a voltmeter, and think it'll be expensive and complicated to use. Checking cabs is easy, and basic meters are good enough for cabs -- like $10 at Walmart or Harbor Freight.

 

[JCMRobbo]

Thank you everyone this is really very helpful and I appreciate it! Once my 900 is back from the shop... grrrr i will MATCH the head and cab!

I will get a volt metre good shout.

again thanks all! I owe you all a virtual beer!

 

[Jethro Rocker]

Thank you everyone this is really very helpful and I appreciate it! Once my 900 is back from the shop... grrrr i will MATCH the head and cab!

I will get a volt metre good shout.

again thanks all! I owe you all a virtual beer!

What happened? Why is it in the shop?

 

[Chris-in-LA]

Thank you everyone this is really very helpful and I appreciate it! Once my 900 is back from the shop... grrrr i will MATCH the head and cab!

I will get a volt metre good shout.

again thanks all! I owe you all a virtual beer!

Make sure you get a multi-meter, one the reads volts and ohms.

 

[kebek]

Simple... post a picture of the ohms switch of your amp and another picture of the connectors in back of you cabs... with these pictures we can drive you safely to the optimal cconnection (this is valid if the amp and cabs were not altered to have different setings that the ones shown on those 2 pictures

 

[Jethro Rocker]

Simple... post a picture of the ohms switch of your amp and another picture of the connectors in back of you cabs... with these pictures we can drive you safely to the optimal cconnection (this is valid if the amp and cabs were not altered to have different setings that the ones shown on those 2 pictures

Simpler still. 16 ohm cab, 16 ohm switch. It isn't really all that complicated.

 

[Matthews Guitars]

Having a meter can confuse you because the DC resistance reading can be very different from the AC impedance of the cabinet.

But it'll be close. A "16 ohm" speaker will read a DC resistance of between 12 and 20 ohms DC.

An "8 ohm" speaker will read a resistance of between 6 and 10 ohms.

A "4 ohm" speaker will read not more than six ohms and probably not less than 3 on your meter.

But the right answer can't be as simple as you wish it was. The simplest possible answer is to match the speaker tap to the cabinet impedance as closely as is possible.

 

[EADGBE]

Set the amp to 16 ohms. And the 4x12 cab to 16 ohms. And do it while the amp is unplugged. And they say it's also a good idea to plug the speaker cable first into the speaker cab and then into the amp.

 

[Jethro Rocker]

Set the amp to 16 ohms. And the 4x12 cab to 16 ohms. And do it while the amp is unplugged. And they say it's also a good idea to plug the speaker cable first into the speaker cab and then into the amp.

If the amp is off which it should be that is completely irrelevant.

 

[EADGBE]

If the amp is off which it should be that is completely irrelevant.

No it isn't. Some people try to switch cabs while the amp is on.

 

[RLW59]

Volt meter, ohm meter, VOM (volt ohm meter), DMM (digital multi meter) are pretty much synonymous nowadays. All the cheap meters do AC, DC, volts, ohms, and continuity. Cheap to moderate usually don't do capacitance.

One member got confused and irritated when told to use a DMM. Thought it was esoteric jargon, an acronym normal people shouldn't be expected to know.
--------------------
VOM/DMM/whatevers will have a knob or switch that let's you select DC volts, AC volts, DC resistance. Select DC resistance (probably labeled "DCR").

Some have range knobs on them. So you have to know approximately what you're going to measure (tens's of ohms, hundreds of ohms, thousands of ohms). For cabs, select the lowest range.

Some are autoranging, so they automatically adjust. Handy if you expect to measure lots of other things, but not really necessary for measuring cabs.

 

[SmokeyDopey]

I think it is good practice to check if the cab is connected before plugging anything in.
It may be overly cautious, but sometimes one can be distracted.

Like putting on a seatbelt. If you do it enough times you just automatically do it. You sit in the car, and your hand automatically reaches for the seatbelt.

 

[Jethro Rocker]

No it isn't. Some people try to switch cabs while the amp is on.

I meant to plug jn cab first then amp with the amp unplugged.

 

[Dogs of Doom]

Simpler still. 16 ohm cab, 16 ohm switch. It isn't really all that complicated.

& "mono" on the cab...

 

[Jethro Rocker]

& "mono" on the cab...

Yup as mentioned right off., if the cab has stereo.