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I just noticed that ever since I bought my Peavey 6505+ I had it set to 8 ohms, but my JCM 900 was set to 4 ohms mono(8 ohms stereo). Could that be what was causing a wavy sound when my tubes were warming up? Should I set my head and cab to 4 or 16 ohms?

This is my first tube head, so I'm confused about the mono/stereo settings on my cab. I mean, if my amp is at 8 ohms, and my cab is set to 4 ohms mono/8 ohms stereo, then should I plug my amp to the 8 ohm mono/16 ohm stereo?

I know it's a big no no to put the Marshall in stereo. So what setting will work best so I don't blow anything or burn out my tubes faster?

I seriously need help in lamen's terms! I've read a lot on Google sites and all I got were hotshots arguing over their expertise. So all new tube newbies were still confused.

You have to know how much resistance your pulling.
If your running an 8 ohm cab set to 8.
If your running two 8 ohm cabs set to 4.
I know this sounds funny but 8+8=4 & 4+4 =2
I don;t know why they even have a 16 ohm setting unless everything is disconnected and your just running a hot spot or something like that.
Honestly, 16 ohm cab seem to be pretty rare.

If you have a standby switch on your amp you should be doing the warm up in the standby mode.
Puts less wear on tubes and speaker components from what I've read.

Ok, I think I got it. So in mono it says 8 ohms, and stereo it says 16 ohms. I don't use stereo from the one head, so it looks like I was plugging into 4 ohms before(8 stereo) with my amp set to 8 ohms. Confusing, but I got the backwards math. My Ampeg says 16 ohms, but is only a 200 watt cab, so I guess I'll be ok.

Thanks for the help Sans!

EDIT: My morning fatigue might have just sounded a little off. I'm still wondering why the ohms are cut in half on this amp. Wish I would have gotten a JCM 800 instead.

As long as you amp settings match your load your set.
My Carvin has the ohm switch as well. It's 1 x 12 combo so long as I'm running that one 8 ohm speaker I set for 8. If I stack a second 8 ohm speaker I have to switch to 4 ohm.

My PA has 4 ohm per channel minimum. On the two monitor channels I can run an 8ohm monitor and two hotspots per side L/R.

My three way mains pull 4 ohms so only one per side L/R.

Do you have mono and stereo switches on your cab too?

Forget I said that, you have a 1x12. I need a nap.

I get it though, more power from the amp than required and you're screwed. I see. You have to cut down the amp ohms when you're using more speakers.

No.
We run everything in mono.
I can set my PA up to run the mains in stereo but I really don't see any point in it.

I'm no pro but to have both mains running right and left channels just don't make sense to me.

That's what was getting me, the whole mono/stereo thing. I never used any amps that worked that way before.

Stereo?


Running a PA in Stereo mode is great for panning the band if you have a creative sound man. Same difference as mixing down your songs on a DAW. to stereo or mono. And stereo FX like reverb or delay run in stereo really sound big. I always run my boards in stereo. I usually keep vocals close to center but pan the instruments.

If you cab is 4 ohms then set the head to 4 ohms, in my cab when I set it to stereo, if you plug in one input your only using 2 speakers, I have used 2 heads into one cab using the stereo setting.

Ok, well here's another rookie question: Do ohms have anything to do with sound, and do ohms effect volume levels at all?

And, is it better to go with 4 or 8 ohms mono?

Thanks in advance!

AH balance is the goal IMO.
Ohms is the measure of resistance to electrical flow.
As long as the application is balanced the circuits will work appropriately.
If not ... keep a fire extinguisher handy.

From what I have researched as long as you have a quality power source and quality speakers you can get quality sound.
I have seen and heard recommendations that for an 800 watt cab you need 1600 watts of amplification so you do not have to push your amp to distortion levels to get volume.

But to be honest I'm only running 300 watts @ 4 ohms per channel on my mains. And as long as I'm not in a huge open space that seems to be plenty.

Here's the deal.

When you run an amplifier at 8 ohms, the amplifier is working easier than it would have to at 4 ohms. When you half the resistance of a load the current increases. If your amplifier can deliverer 28 volts into an 8 ohm load , it takes about twice the power to deliver that 28 volts into a 4 ohm load. Solid state amplifiers can easily handle low impedance loads. Tube amplifiers can't, just because of the nature of the beast. Therefore the tube amp speaker transformer has multiple taps on the secondary windings to match the speaker loads. Solid state amplifiers are relatively lenient when it comes to a mis match. Tube amplifiers are not as forgiving. Most tube amplifiers run at higher impedances than solid state amplifiers. If you run a tube amp without a load, you can arc across the primary of the transformer and burn a carbon path from the plate to the grids of cathode. Then you have to replace the socket, or scrape the conducting carbon path from the socket.
No such problem with solid state amps. While the tube amp has to change 450 Volts with a few hundred milliamp plate voltage into 28 volts at several amps. A solid state amplifier operates in the that current range naturally. That is why speaker impedances have dropped for a lot of applications, such as car stereo power amplification units. Most of those are designed to run at 2 or even 1 ohm.

The sound pressure. If an amp is designed to power an 8 ohm load speaker to full excursion, in a given set of parameters it will be as loud as an amplifier designed to push a 4 ohm load speaker to full excursion. Or if your amp is designed to push 28 volts into 8 ohms, it will be as loud as an amp that puts about 1/2 of that into 4 ohms.

In other words the speaker excursion will be the same, or it will push the same amount of air. After all, a speaker is basically an electric motor. Fuel pushing a piston.

When you parallel speaker cabinets or speakers, you must double the current to push the same amount of voltage into a given load. So what do you get? Twice the surface area pushing the air. So although it doesn't sound twice as loud due to the response of the ear it will be like running two amplifiers at the same volume.

I am greatly simplifying the voltage current relationships simply for the sake of explanation. As poor as I am at explaining stuff, I hope this helps.

Nice analogy Slacker G.

4 ohms is less resistance than 8 ohms.
At any given voltage - more current will flow through 4 ohms than 8 ohms.
If your amp is looking for 8 ohms resistance, and you only give it 4 ohms resistance, your amp is giving more power than it wants to, and will get hot.
Just for an analogy - a piece of copper wire is 0 ohms resistance.
Or, in other words - a short circuit.
.
Do that to your amp - approach a short circuit, and your amp will get warm, like a toaster.
.
An amp needs something to stop it from being shorted out. The resistance of the speakers does that. You amplifier wants the proper resistance.
> Too little resistance - your amp gets hot, and maybe eventually burns.
> Too much resistance - and it won't be as loud.
> Proper resistance - good volume, and cooler & safer.
.
If you connect two 8 ohm speakers in parallel ( + to +, and - to - ), you will have 4 ohms resistance.

If you connect two 8 ohm speakers in series ( into the positive of one speaker and out of the negative of that speaker - into the positive of the 2nd speaker - out of the negative of the 2nd speaker and back to the amp . . .
- then you will have 16 ohms resistance.
.
8 ohms + 8ohms in parallel = 4 ohms
8 ohms + 8ohms in series = 16 ohms.

Thanks for the amp 101 guys!

I've got the reverse math now, it's just the mono vs. stereo discernment that's pissing me off.

The thing that was confusing me is that on the back my speaker cab inputs for 4 and 16 ohms say 8 ohms left and right (obviously 8 in stereo only) below the inputs. But, since I'm working with one head I have to use mono for all four speakers to work. I just don't understand why Marshall made such a big jump in ohm choices in mono(4 or 16) no choice for 8 in mono, only stereo.

If I had two heads then I would have to push 8 ohms in stereo because that's my only choice on the cab(which is a no brainer) but I don't have two tube heads. So why does mono go from the smallest to the highest numbers?

My head has a 4, 8, and 16 ohm switch. And I want to be sure if 16 ohm on my amp is safe for 16 ohm mono.

So my final question is...am I better off with 4 ohms rather than jumping to the big 16 ohm? Am I better off setting my head to 4 along with my cab, because 16 seems a little crazy.

P.S. Sorry for all the questions, but I've never used a tube head after all these years, nor, had a cabinet with mono and stereo options. I just don't want to fry a tube or jack up a transitor.

P.S.S. I'll be sending you guys your consultation checks next week. Seriously, you guys have helped me a lot so far.