#1
hey guys. I do not know much about all the "OHMS" thing. I have a 100w Marshall JCM 800 hooked up to a 4x12 Marshall 1960 AV. I really do not need it that loud, so i was planning on getting a Orange 2x12 cab. The Orange 2x12 is 120w at 16ohms (no idea what that means), can i hook up the Marshall JCM800 (100w) to the Orange 2x12 cab? I just want to make sure it works because the last thing i want is to blow out the cab. If you need more information to determine it just ask. Thanks alot guys!
#2
Based on electronic resistance formulas related to power transfer, if your speaker cabinet is rated at the same Ohmage as your amp's speaker out, power transfer is maximum. If your speaker Ohms are twice that of the amp's output Ohms, the power transfer is effectively halved.

So if your amp's output is 8 Ohms, use a 16 Ohm speaker cab for half power. If your amp's out is 4 Ohms, use an 8 Ohm cab and so on...

Does your JCM 800 have a choice of output Ohms? Ohms is marked by the Omega symbol, if you weren't sure.
Last edited by carpepax at May 24, 2008,
#3
it'll be A-OK. as long as the cab is really 120 watts at 16 ohms. you can always use a head with a lower ohm rating than the cab you're running it through, it's just when the cab is rated lower than the head that you run into problems with things going bang bang.
Gear:
ESP Eclipse 300
Jackson RR3
Bugera 6262 head
Trace Elliot straight cab
Boss NS-2
Boss DD-3
#5
yeah my jcm800 has 3 choices for 4ohms, 8 ohms, and 16 ohms. Right now I have the head set to 16 ohms, and I have my 4x12 Marshall (280w handling) set to 16 ohms (Marshall cab has a 4 ohm option too). The 2x12 orange is rated at 120w and only has the choice of being at 16 ohm. So it should work if I just leave my marshall 100w Tube head at 16 ohm and plug it to the 120w (16 ohm) Orange 2x12?
#6
Quote by carpepax
Based on electronic resistance formulas related to power transfer, if your speaker cabinet is rated at the same Ohmage as your amp's speaker out, power transfer is maximum. If your speaker Ohms are twice that of the amp's output Ohms, the power transfer is effectively halved.

So if your amp's output is 8 Ohms, use a 16 Ohm speaker cab for half power. If your amp's out is 4 Ohms, use an 8 Ohm cab and so on...

Does your JCM 800 have a choice of output Ohms? Ohms is marked by the Omega symbol, if you weren't sure.

that doesn't work the same when dealing with an impedance matching output transformer in a tube amp, like the JCM. If you mismatch in either direction, it lowers the power rating, but not in half.

example using 100W amp rated at 8ohms:

PLoad = current^2 x RLoad
100W = I^2 x 8ohm
I = 3.5 amps. This is what the output transformer is designed for in this example matching impedance correctly.

Pload = (Vsource^2 x Rload) / (Rsource + Rload)^2 = (Vs^2 x 8) / (8 + 8)^2 = 100W
Vsource = 56.5 V

I = Vsource/(Rsource + Rload) = 56.5 V/(8ohm + 8ohm) = 3.5 amps using a matching impedance cab of 8ohm

PL = load power = 100W rated at 8 ohms
Vs = voltage source (fixed in the circuit) = 56.5V
Rs = impedance source (fixed in the circuit - impedance matching output transformer) = 8 ohm

_____________________

mismatching using 4 ohm cab*

I = 56.5V/(8ohm + 4ohm*)= 4.7 amps which is more current than the output transformer was designed for.

The power formula is:
Pload = (Vsource^2 x Rload) / (Rsource + Rload)^2 = (56.5^2 x 4ohm) / (8 + 4)^2 = 88W
______________________________________________
mismatching with 16ohm cab*

I = 56.5V/(8ohm + 16ohm*)= 2.33 amps

Pload = (Vsource^2 x Rload) / (Rsource + Rload)^2 = (56.5^2 x 16ohm) / (8 + 16)^2 = 88W
"The fool doth think he is wise, but the wiseman knows himself to be a fool." - W.S.
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#7
^Wow. Thanks for that clarification. I've only just begun electronics study, having taken a "Fundamentals of DC/AC circuit analysis" class, working with resistors, capacitors and inductors mostly. Both the DC and AC halves of the course were crammed into one 16 week semester, but I wasn't as confused by it as by what you wrote.

The point was that higher speaker Ohms than amp output Ohms means less than full power. It seems we can agree on that, ya?
Last edited by carpepax at May 24, 2008,
#8
Quote by jrm42099
So it should work if I just leave my marshall 100w Tube head at 16 ohm and plug it to the 120w (16 ohm) Orange 2x12?


All the electronic explanations aside... Yeah, that's all you have to do.

However...

Quote by jrm42099
I have a 100w Marshall JCM 800 hooked up to a 4x12 Marshall 1960 AV. I really do not need it that loud, so i was planning on getting a Orange 2x12 cab.


There are many good reasons to switch from a 1960AV to an Orange 2x12, but a reduction in volume isn't one of them. If you really want some Marshall crunch at a reduced volume, you need to look into an attenuator or a smaller amp.
You Don't Need a halfstack.

You Don't Need 100W.

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#9
Quote by slatsmania
All the electronic explanations aside... Yeah, that's all you have to do.

However...


There are many good reasons to switch from a 1960AV to an Orange 2x12, but a reduction in volume isn't one of them. If you really want some Marshall crunch at a reduced volume, you need to look into an attenuator or a smaller amp.


This is the logical thing
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#10
Quote by carpepax
^Wow. Thanks for that clarification. I've only just begun electronics study, having taken a "Fundamentals of DC/AC circuit analysis" class, working with resistors, capacitors and inductors mostly. Both the DC and AC halves of the course were crammed into one 16 week semester, but I wasn't as confused by it as by what you wrote.

The point was that higher speaker Ohms than amp output Ohms means less than full power. It seems we can agree on that, ya?

haha, sorry about that, it's just easier to show the math sometimes to someone that knows the formulas. And yeah, agreed.

Quote by slatsmania
All the electronic explanations aside... Yeah, that's all you have to do.

However...


There are many good reasons to switch from a 1960AV to an Orange 2x12, but a reduction in volume isn't one of them. If you really want some Marshall crunch at a reduced volume, you need to look into an attenuator or a smaller amp.

+1
"The fool doth think he is wise, but the wiseman knows himself to be a fool." - W.S.
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