ohms confusion

T

theD1CKENS

*insert clever title here
what's the deal with matching(or mismatching) ohms? it's never come up for me until recently.

I have an Epiphone ValveJunior amp(5 watts) and a Crate Blue Voodoo cab(200 watts, I believe). the amp has 3 outputs on the back at 16, 8, and 4 ohms, 16 being the one run into the amp itself by default(it has it's own 1/4" jack that can be disconnected, which I do when I run the cab). the cab has a selector on the back, which can switch between 16, 8, and 4 ohms.

when I use just the amp, I leave the 1X12 connected to the 16 ohm output. when I run the cab, I disconnect the 1X12, and run out from 4 ohms into the cab, with the switch set to 4 ohms, also. this seems to give me the most volume, and allows me to push the amp harder before it starts to REALLY break up...I think

basically, what's the difference? I want to get the most out of this set-up, volume-wise, tone-wise, and otherwise...without blowing anything up. I haven't really read the fine print on the back of the amp, but I'd guess it's irrelevant which output I use. wouldn't the factory speaker be able to handle any of its own outputs with no trouble? same deal when I run the cab, as long as amp and cab are matched, does it matter?

on an unrelated note, how the EFF does a tiny little 5 watt amp have enough juice to push a 200 watt cab?! it doesn't seem to have trouble pushing 2 cabs, either, full-stack style, tho I have no idea how many watts the other cab is. not mine, sounds like poo and don't need that much amplification, anyway.

apologies for the long post
 
well ...... how many speakers in that Vodoo cab? 4X12"? I'm not sure how they have it switchable between all 3 imedances but anyways .....

Amps will usually put out more wattage into a lower load. Think of electricity like water flowing thru a hose. A lower load is like a bigger hose ........ easier to flow thru so more water (electricity) can get thru thus ..... more wattage coming out of the amp. So since it is putting out more wattage thru the same speakers it probably does seem a little bit louder but only a little bit as you'll see in a minute.

And that has to do with why a 5 watt amp can sound loud thru a big cab.
To sound twice as loud takes 10 times the wattage. Might not believe me right now but look it up ..... 10 times the power to get twice as loud.
Looking at it a different way ..... that means a 5 watt amp is only half as loud as a 50 watt amp. "eek"
So that's why it only sounds a little bit louder if at all, because at most you're getting an extra couple watts out of it.

As for which to use ....... theoretically an amp should be fine at any load it's rated for so since they have a 4 ohm tap, shouldn't be a problem and, in fact, I have a Valve Jr head and I frequently run it into a 4 ohm load.
However, the conventional wisdom says that the 16 ohm tap uses all of the transformer and so you get all the tone the amp has to give by using the 16 ohm tap.

So if the 4 sounds best to you I wouldn't worry about it too much but the amp will work harder into 4 ohms and you might want to keep a fan on it.
 
Most cabinet wattage ratings are for the amount of program the speakers can handle over a set time period. 5 watts in a tube amp is much more than people realize. Its louder with the multi-speaker cabinet because you are pushing more speaker area.

Volume is always relative to the environment its in. My old studio had a vocal booth that you could put a 100 watt Marshall in and turn it up. You could still go in there with it though, as the room was as dead as a tomb. Too much so....great for spoken word though! And loud guitar amps
 
Thanks guys.

I`ve heard something weird about the wattage to volume ratio, tho I don`t really get it. Anyhoo, the BV is a 4X12, and its got this little red 3way switch for ohms, and also has 3 jacks, one of which may be an output to run the bottom cab(mines the angled top cab), but the other 2 can handle all the ohms options, depending on which input you plug into. Hard to explain without pics, but I assure you it can handle all 3.

Basically, the way I`m understanding it is lower ohms=more volume, and higher ohms=better tone, correct? What about the amp? It should be fine on any ohm setting, or should I look into that more. I LOVE that amp(it`s a combo amp, btw) and would hate to screw it up somehow.

I don`t necessarily think 4 ohms sounds the best, but it appears to get more volume out of it. That`s just the first way I hooked it up,
and haven`t fooled around with the other options

Thanks for the quick response guys. That`s why I keep coming back here:)
 
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if it is louder it's not gonna be by much.
Once again .... the rule is to get twice the volume you need ten times the power. So with a 5 watt amp there's simply no way you're getting more than another watt or so.
So I would run it whichever way sounds best to ya.
Yes, you should be able to run it into 4 ohms just fine since it's rated for that if that's the setting you prefer.

As for the cab ..... no ...... it's not hard to explain ..... I get it perfectly .... switch that changes the load ..... got it.
It's just that I know cab wiring and I'm not quite sure how they go about getting all 3 loads from 4 speakers. This isn't relevant to your question so don't worry about it but with 4 speakers .... if they're all 16 ohm speakers then series/parallel gives you 16 ohms and parallel gives you 4. To get 8 you'd have to do something weird wiring-wise .... maybe even cut out a speaker. I wonder if they have a mix of impedances in those speakers ..... I'll have to look that up.

Good luck .. have fun!
 
Hey Lt. Bob,
Been a long time since I did this stuff but it seems that:
16 ohm speakers x 4 in series = 16 ohms
16 ohm speakers x 4 in parallel = 4 ohms
16 ohm speakers, 2 sets in parallel then each set in series = 8 ohms
 
And if I seem to remember from the day, if your amp had a low ohm output and you put that into a high ohm speaker array, you could damage the amp because of the resistance. Basically, the power backs up at the amp like a bottleneck because the speaker array had more resistance. Just to be safe, I always matched output and speaker to the same impedance.
 
For safety, it is usually ok to go up and down one step. So if the Amp is 8 ohms, it is safe to run at 4 ohms (down one), and 16 ohms (up one). 2, 4, 8, and 16 are the usual steps. I match them whenever possible (i.e. use the 8 ohm out from the amp for an 8 ohm cab). But, doubling or halving the resistance won't hurt the amp any.
 
Washburn100 said:
Hey Lt. Bob,
Been a long time since I did this stuff but it seems that:
16 ohm speakers x 4 in series = 16 ohms
16 ohm speakers x 4 in parallel = 4 ohms
16 ohm speakers, 2 sets in parallel then each set in series = 8 ohms
Click to expand...

16 ohms X 4 in series =64 ohms. The rest is completely correct.

They are using a series of resisters to match the impedances.
 
you see! everything was going great til you guys got all technical on me:p
I'll just play around with what sounds good, and keep em matched. I'm guessing the volume difference I was 'hearing' was all in my head...it wasn't much, and it's all about the tone anyway
 
Don't run a 4 ohm cab with your amp set at 8 or 16 ohms.It can damage your amp because your not providing enough power to the cab.If you have your amp set on 4 ohms though you can easily power a 4,8,or 16 ohm cab.Just match the amp with the cab.

If you run both cabs you need to make sure that you adjust accordingly.
 
mcmetal said:
Don't run a 4 ohm cab with your amp set at 8 or 16 ohms.It can damage your amp because your not providing enough power to the cab.
Click to expand...
that's not correct. There is no required amount of power that has to go to the amp and it's pretty common in hi-fi circles to just choose whatever impedance setting on the amp that sounds best to the ears.

The reason setting the amp too high(say 16 ohms) into a lower load (say 4 ohms) is that the amp is 'looking' for a certain amount of resistance and can conceivably put out too much power into a lower load. That's the same reason you don't want to run an amp into a load it's not rated for.
For instance Orange amps are only rated down to 8 ohms and that Epi is not rated to 2 ohms. If you try to do that, the amp tries to put out more power than it's capable of and that can damge certain components in the amp.
Personally I do prefer to match the output setting to the cab and it certainly CAN damage an amp to do otherwise but not because it's not putting enough power to the cab. It'll be because the amp's trying to put out too much power.
 
Lt. Bob said:
.....The reason setting the amp too high (say 16 ohms) into a lower load (say 4 ohms) is that the amp is 'looking' for a certain amount of resistance and can conceivably put out too much power into a lower load. That's the same reason you don't want to run an amp into a load it's not rated for.
Click to expand...

That's not quite right. It doesn't matter, as long as the fear of God has you on your toes and not mismatching too terribly. It's reflected impedance and the potential problems associated with flyback voltage from the secondary that gets multiplied in the primary of the output transformer. Flyback and the resulting arcing can ruin an output transformer. An open load with a tube OPT is bad. A shorted load in a transistor output stage (with no transformer) is bad.
 
Now you guys are talking way over my head...

DON'T run 16(or what have you) into 4(or what have you)...but what about running the amp at 8(arbitrary) into the cab at 16? That would be like "big hose, little stream" and should be fine, right?

It's driving me nuts now! My gf had surgery and is laid up. Guess who gets to take care of her and NOT experiment with all this new info
 
Just because it amuses me to muddy the waters even more:

Tube amps generally handle shorts better than opens while transistor amps generally handle opens better than shorts. That's why attenuators have switches to choose tube or transistor.
 
bouldersoundguy said:
Just because it amuses me to muddy the waters even more:

Tube amps generally handle shorts better than opens while transistor amps generally handle opens better than shorts. That's why attenuators have switches to choose tube or transistor.
Click to expand...

Ummm....what?:confused:
 
I remember a time, not so long ago, when rock n roll was simple. Just plug and play. What's this about 'shorts, and 'opens?
 
Open circuit = a wall outlet with nothing plugged in

Short circuit = the same outlet after you stick your tongue into it :eek:

Rock and roll was never that simple because tube amps have always had impedance settings. Transistor amps don't need to worry about impedance (well within reason, if you plug in a 4x16 ohm in series you aren't going to get much volume out of your 64 ohm cab, and most aren't happy with much less than 4 ohm), and any reasonable quality transistor amp should be internally protected against a short circuit on the output.
 
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