setting ohms correctly???????

Franz Wa · 08-18-2009, 02:16 PM

Hey everyone and very VERY sorry to have to ask those basic questions but i can't find my answers anywhere (seems to obvious!!!).
Well i am totally new to the Marshall technology (used to play in a vox ac-30through tons of pedals before).
I just bought a jcm 800 2205 and 1960a cab (where original speakers were replaced by v30's).
Once again my questions are very basics but i know that if matched uncorrectly the cab and head, i can blow many things (Drake transfos...)
So i want to plug my head in my single cab.
1st... an you confirm that the 2 output on the head are identical? (one to plug in A cab the other to eventuallu plug in B cab) I can use either one or the other the signal is exactly the same?
2nd... i,ve read that when you use a single cab in mono(instead of full stack) you can set both the head and cab on 4 ohms and plug the speaker cable in 4 ohms input of the cab.
But most of the users of marshall head + cab seem to set amp and cab on 16 ohms ?? So is there something better ?
Thanks.. any help would be much appreciated since i have the amp but didn't even dare trying it !!!!!!!!!!!

MartyStrat54 · 08-18-2009, 02:37 PM

Quote:

Originally Posted by Franz Wa

Hey everyone and very VERY sorry to have to ask those basic questions but i can't find my answers anywhere (seems to obvious!!!).
Well i am totally new to the Marshall technology (used to play in a vox ac-30through tons of pedals before).
I just bought a jcm 800 2205 and 1960a cab (where original speakers were replaced by v30's).
Once again my questions are very basics but i know that if matched uncorrectly the cab and head, i can blow many things (Drake transfos...)
So i want to plug my head in my single cab.
1st... an you confirm that the 2 output on the head are identical? (one to plug in A cab the other to eventuallu plug in B cab) I can use either one or the other the signal is exactly the same?
2nd... i,ve read that when you use a single cab in mono(instead of full stack) you can set both the head and cab on 4 ohms and plug the speaker cable in 4 ohms input of the cab.
But most of the users of marshall head + cab seem to set amp and cab on 16 ohms ?? So is there something better ?
Thanks.. any help would be much appreciated since i have the amp but didn't even dare trying it !!!!!!!!!!!

First you need to verify that the cab you have has been wired to 16 ohms. The speakers have been replaced, so there are no guarantees that it is 16 ohms. If they used 8 ohm V30's, it could be an 8 ohm cab. Take a multimeter to check this.

Generally, when using two, 16 ohm cabs, you run one cable from the first cabinet and hook it to the second cabinet. Then a cable is run from the second cabinet over to the 8 ohm output of the amp.

Quote: I've read that when you use a single cab in mono(instead of full stack) you can set both the head and cab on 4 ohms and plug the speaker cable in 4 ohms input of the cab. End Quote. I know that some of these cabs have a switchable load option. If the mono mode is a true 4 ohm load to all four speakers, then you can use the 4 ohm output. A half stack is almost always a 16 ohm load and I would recommend running it in 16 ohms.

As I said, the first thing you need to verify is that the cab has 16 ohm speakers wired in series-parallel for a 16 ohm load.

Franz Wa · 08-18-2009, 03:57 PM

Hey Marty !
Thank you so much for your answer.
Once again as basic as this task seems i'm not sure i know how to do it.......
I do have a multimeter but wherever i measure the impedance i do not have 16 ohms nowhere.
Where should i measure that impedance? at the 16 ohm input on the cab ?
How do i do to measure the impedance of one speaker? do i have to unplug all electic wire connected to the speaker before i do that ?
Once again thanks very much

Franz Wa · 08-18-2009, 04:54 PM

Byt he way... i just founf an email from the guy who did the mod and sold me the amp !!
Don't know if he can be trusted though.. but according to what he says... everything seems to be perfectlu wired...
If you don't mind helping me with my basic questions on how to measure the impedance i'd love to check that by myself just tomake sure everything is indeed wired perfectly!!

«... The speakers are 60watt 16 ohm wired perfectly with 16ohm mono or 8 ohm stereo, 240watts total...»

No indication on the speakers themselves if ther're 8 or 166 ohms...
Thanks

MartyStrat54 · 08-18-2009, 04:59 PM

Quote:

Originally Posted by Franz Wa

Hey Marty !
Thank you so much for your answer.
Once again as basic as this task seems i'm not sure i know how to do it.......
I do have a multimeter but wherever i measure the impedance i do not have 16 ohms nowhere.
Where should i measure that impedance? at the 16 ohm input on the cab ?
How do i do to measure the impedance of one speaker? do i have to unplug all electic wire connected to the speaker before i do that ?
Once again thanks very much

Okay. First make sure that the multimeter is working correctly. If the battery in the multimeter is low, it will give false readings. Just take a foot-long piece of wire and put the test leads on each end. The meter should be set on the lowest range. You should get a "0" reading meaning a short. The meter is then working properly. You should be able to stick a cable into the cab on the other end, put the red test lead on the tip and the black test lead on the shaft. You should get a reading at this point. It could be 8 ohms, or it could be 16 ohms (+ or - 2 to 3 ohms). If you are not getting any such reading there is a problem. To read an individual speaker, you only have to take one wire off (either red or black) and put your leads on each speaker terminal. Again, you should get an 8 or 16 ohm reading (+ or - 2 to 3 ohms).

More than likely the cab is wired series-parallel. If you can post a clear picture of the wiring, I will be able to tell how it is wired. Otherwise, you will have to do an online search for a wiring diagram. There are many on this forum. You could do a search here first.

I looked real quick and here is a good one. Go down about five or six posts on the first page. You will see a post by King Kong. He included a picture. Is your cab wired like this?

SPEAKER WIRING

Franz Wa · 08-19-2009, 12:50 AM

Sorry for the late response ! Busy evening !
Ok !! hum.... I ' m so ashamed i could'nt figure out by myself how to measure the impedance... So obvious when you say it!!!!
Anyway.... when i plug a cable in the 16 ohm input with the cab set to mono the multimeter reads 14.6 ohms.. when i plug the cable in the 4 ohms input it reads 6.2 ohms..... and when i switch the amp to stereo and measure the impedance in the 2 inputs.. it reads 8 ohms...
So i guess everything is perfectly wired right?
The head is the JCM 800 2205... There are 2 outputs on the rear panel.... i do not understand the reason of that since that head is not stereo. Can i use ANY of these 2 outputs if the ohm selector is switch to 16ohm ??

try to insert pics pf wiring

and outputs

Sir Don · 08-19-2009, 01:57 AM

It appears that the box is wired in series/parallel, so no problem there. Ohm readings tend to be on the lower side so the 14.6 seems right for 16 Ohms, 6.2 seems a bit high for 4 Ohms but I would think all is ok. The head has two loudspeaker outputs so you can connect two speaker cabinets to it. Two 16 Ohm cabinets - set your amp to 8 Ohm output, one 16 Ohm cabinet - set your amp to 16 Ohms. If you use one cabinet you can use either output from the amp head. Hope this helps. Cheers.

MartyStrat54 · 08-19-2009, 09:59 PM

Quote:

Originally Posted by Sir Don

It appears that the box is wired in series/parallel, so no problem there. Ohm readings tend to be on the lower side so the 14.6 seems right for 16 Ohms, 6.2 seems a bit high for 4 Ohms but I would think all is ok. The head has two loudspeaker outputs so you can connect two speaker cabinets to it. Two 16 Ohm cabinets - set your amp to 8 Ohm output, one 16 Ohm cabinet - set your amp to 16 Ohms. If you use one cabinet you can use either output from the amp head. Hope this helps. Cheers.

I agree. The 14.6 ohm reading is perfect DC reading for 16 ohms. Set the cab to 16 ohms and I see in the picture that your selector is already set to 16 ohms and you are good to go. Just remember, if you want to run another 16 ohm cab, you will need to drop the impedance to 8 ohms on the amp.

Happy Rockin'

08-18-2009, 02:16 PM	#1 (permalink)
Franz Wa Junior Member Join Date: Jul 2009 Posts: 8	setting ohms correctly??????? Hey everyone and very VERY sorry to have to ask those basic questions but i can't find my answers anywhere (seems to obvious!!!). Well i am totally new to the Marshall technology (used to play in a vox ac-30through tons of pedals before). I just bought a jcm 800 2205 and 1960a cab (where original speakers were replaced by v30's). Once again my questions are very basics but i know that if matched uncorrectly the cab and head, i can blow many things (Drake transfos...) So i want to plug my head in my single cab. 1st... an you confirm that the 2 output on the head are identical? (one to plug in A cab the other to eventuallu plug in B cab) I can use either one or the other the signal is exactly the same? 2nd... i,ve read that when you use a single cab in mono(instead of full stack) you can set both the head and cab on 4 ohms and plug the speaker cable in 4 ohms input of the cab. But most of the users of marshall head + cab seem to set amp and cab on 16 ohms ?? So is there something better ? Thanks.. any help would be much appreciated since i have the amp but didn't even dare trying it !!!!!!!!!!!

08-18-2009, 03:57 PM	#3 (permalink)
Franz Wa Junior Member Join Date: Jul 2009 Posts: 8	Re: setting ohms correctly??????? Hey Marty ! Thank you so much for your answer. Once again as basic as this task seems i'm not sure i know how to do it....... I do have a multimeter but wherever i measure the impedance i do not have 16 ohms nowhere. Where should i measure that impedance? at the 16 ohm input on the cab ? How do i do to measure the impedance of one speaker? do i have to unplug all electic wire connected to the speaker before i do that ? Once again thanks very much

08-18-2009, 04:54 PM	#4 (permalink)
Franz Wa Junior Member Join Date: Jul 2009 Posts: 8	Re: setting ohms correctly??????? Byt he way... i just founf an email from the guy who did the mod and sold me the amp !! Don't know if he can be trusted though.. but according to what he says... everything seems to be perfectlu wired... If you don't mind helping me with my basic questions on how to measure the impedance i'd love to check that by myself just tomake sure everything is indeed wired perfectly!! «... The speakers are 60watt 16 ohm wired perfectly with 16ohm mono or 8 ohm stereo, 240watts total...» No indication on the speakers themselves if ther're 8 or 166 ohms... Thanks

08-19-2009, 12:50 AM	#6 (permalink)
Franz Wa Junior Member Join Date: Jul 2009 Posts: 8	Re: setting ohms correctly??????? Sorry for the late response ! Busy evening ! Ok !! hum.... I ' m so ashamed i could'nt figure out by myself how to measure the impedance... So obvious when you say it!!!! Anyway.... when i plug a cable in the 16 ohm input with the cab set to mono the multimeter reads 14.6 ohms.. when i plug the cable in the 4 ohms input it reads 6.2 ohms..... and when i switch the amp to stereo and measure the impedance in the 2 inputs.. it reads 8 ohms... So i guess everything is perfectly wired right? The head is the JCM 800 2205... There are 2 outputs on the rear panel.... i do not understand the reason of that since that head is not stereo. Can i use ANY of these 2 outputs if the ohm selector is switch to 16ohm ?? try to insert pics pf wiring and outputs

08-19-2009, 01:57 AM	#7 (permalink)
Sir Don Senior Member Join Date: Aug 2009 Location: Adelaide, South Australia Posts: 200	Re: setting ohms correctly??????? It appears that the box is wired in series/parallel, so no problem there. Ohm readings tend to be on the lower side so the 14.6 seems right for 16 Ohms, 6.2 seems a bit high for 4 Ohms but I would think all is ok. The head has two loudspeaker outputs so you can connect two speaker cabinets to it. Two 16 Ohm cabinets - set your amp to 8 Ohm output, one 16 Ohm cabinet - set your amp to 16 Ohms. If you use one cabinet you can use either output from the amp head. Hope this helps. Cheers. __________________ My gear: 2204, 2210, 2555, 9005, 1959, 5005, 4103, 2100, TSL100 www.myspace.com/rockthebosss

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