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Unread 01-27-2012, 04:29 PM   #1
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4 ohms question

Is 4 ohms from the amp, safe for a 4, 8, and 16 ohms cab? I have to use a cab and I don't know the cab's ohms. I want to play it safe, and I think that by using 4 I should be safe. Am I correct in thinking so?

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Unread 01-27-2012, 04:39 PM   #2
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Lower the ohms higher the output current. Safer is to set to 16 ohms, IMHO. (If it works same as general electronics)
Do not take my words for sure.
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Unread 01-27-2012, 04:52 PM   #3
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You have to match the ohms of the cab to the ohms of the amp. Most amps have a switch in the back that you can set for 4, 8, or 16. If you have an 8 ohm input on your cab, set the switch on your amp to 8.

There's probably something you could do to check the resistance (ohms) of your cab with a multimeter. Look into that, but don't expect to see a reading of 4, 8, or 16 ohms dead even on the multimeter's display - the number will fluctuate so it'll be close to 4, 8, or 16.
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Unread 01-27-2012, 05:03 PM   #4
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I usually do match with my own cabs but I have to use someone else's cab and they don't know what the ohms are and I don't think I can check at the show.

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Unread 01-27-2012, 05:28 PM   #5
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Do you know what model the cabinet is?
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Unread 01-27-2012, 05:29 PM   #6
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Unread 01-27-2012, 05:40 PM   #7
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I don't know anything about those. Running 4 ohms into a mystery cab probably won't hurt it, but to be safe you could always use the other band's head, too.
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Unread 01-27-2012, 05:48 PM   #8
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do *NOT* set your amp to a higher impedance (higher ohm setting) than the cab. that will stress the amp (which is a lot more expensive to replace than a cab) because the cab will be trying to draw more power than the amp is supplying and will heat up more than usual.

if impedance matching isn't an option, *always* set the amp to a lower impedance than the cab. it will result in lower power output from the cab than having it matched but at least you won't blow your amp.


that said, there is an easy way to find out what the impedance on your cab is:
1/ find a friend who has a multimeter
2/ plug a speaker cable into the cab (but don't plug the other end into anything)
3/ set the multimeter to the 20 ohm range
4/ touch one probe to the tip of the speaker cable's free plug (the short roundish part past the plastic ring), touch the other probe to the plug's sleeve (the long metal part beneath the plastic ring)

the probe will give you a number that is neither 4, 8, or 16. that's because it will be giving you the cab's RMS value based on the multimeter's DC output for measuring impedance but speakers are designed for an AC input.
if you multiply the number it gives you by about 1.2-1.4, it should give you a value close to 4, 8, or 16. that is the impedance of your cab.
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Unread 01-27-2012, 05:50 PM   #9
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Mismatching a bigger load onto a smaller output (16 ohm cab to 4 ohm head say) is safe.

It generally isnt recommended that you mismatch by more than a factor of 2, but tbh the chances of you doing any damage are pretty small. Just don't do it the other way round as it puts the output section of the head under strain.
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Unread 01-28-2012, 01:22 AM   #10
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Thanks for the help guys!

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Unread 01-28-2012, 03:32 AM   #11
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Theres alot of forum myth regurgitation going on in this thread, i hope to clear this up a bit for everyone (and cut through some of the bull)

Lower impedance will generally stress the power tubes gradually as volume increases.

Higher impedance will generally stress the power tubes less as volume increases until the powertubes are forced into cut off at high volume levels. Up until the point where the impedance mis-match creates voltage spikes onto the plates of the power tubes called flyback.

This flyback occurs due to the energy stored as a 3ft diameter magnetic field around the OT - this field can collapse and create voltage spikes that are MORE than your B+ - WAY more than what the amplifier can create.

Power tubes have a set impedance they expect to see on the primary of the OT. the secondary is a voltage step down and a current step up - at full tilt you will see voltages around 70V but currents in the 60A range.

There are alot of factors involved like tube quality, B+ voltage, the volume you are playing, the OT core resistance, OT inductance, winding isolation material, and many more. Its too hard to say which is better or which is worse - in reality neither are good - impedance is nothing but opposition to current flow and a false resistance in the imaginary plane - what you want is a matched impedance so the impedance differences cancel and you get max power transfer. Impedance is NOT one setting flowing current better than the other.

If you have to mis-match, set the amplifier equal to or lower than the load that is expected so that the load is higher than the amplifier sees. lower impedances will most likely stress your power tubes and shorten their life. Higher impedances are OK to a point, but eventually it becomes Russian Roullete - you could cook your OT on a flyback spike and fry fuses, sockets, tubes, and wiring galore.

Hope this helps to clear a few things up.
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