Impedance is to an AC signal (and a guitar signal or amp output is a form of AC) and resistance is to DC.

The speakers have an impedance rating.

With a tube amp, because it has an output transformer, you have to match the impedance that will work with the output transformer to the impedance of your speaker or cabinet. While you can sometimes mismatch without damage, it's always best to match with a tube amp.

Solid state amps lack the output transformer. Their power amps work in a linear fashion. Meaning the less the impedance (or AC resistance) of the speaker, the more power the output section of the solid state amp can generate. But there will be a point when the impedance is too low where the power transistors will try to work harder than they are capable of working, and they will burn themselves out.

That is why solid state amps can safely have higher impedances, because all it will do is make the amp work less hard. But in tube amps, the tubes will always work as hard as they can, and when they don't have the right impedance load hooked to the amp, they can be damaged, or damage the output transformer.

I hope this helped.

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ben adrian
Now Doing Amp Repair in San Francisco
ben@feedbackloop.net
the gear

OK. I'm sorry if this was said earlier. Its late


I have a marshall mg 100 watt head. It has a min 4 ohm impedance.

My cab is a behringer ultrastack at 400 watts (4 100 watt jenson speakers).

The cab has a switchable 8 ohms, or 16 ohms stereo. I was wondering how I should set up my rig to get the most out of my amp.

I'm only gettin 50 watts of power at most right?

My guess is that you're probably getting 70-75 watts going up to 8 ohms instead of the minimum (4). The manual will probably give you wxact figures.

Cheers

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ben adrian
Now Doing Amp Repair in San Francisco
ben@feedbackloop.net
the gear

Ok, I think I follow, but just to check

Marshall 8004 solid state power amp...2x40 watts/8 ohms per side

Marshall 1912 cab, 150 watts mono/8 ohms

can I run only one side of the 8004, or is there any chance of damage?

Thanks

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Jackson RR5
Moser V
Dean BlackGold V
ESP LTD M-350

Line 6 HD147+FBV
Marshall 1960A


...yes, knave, I am in a power metal band

For solid state, you can run just half of the power amp and it's totally safe.

For you, right now, the only functional setup is half of the power amp going into the cab. You could rewire your cabinet in stereo with two 16 ohm sides and run bothe sides fo the power amp to what would be the two new sides of the cab.

Cheers!

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ben adrian
Now Doing Amp Repair in San Francisco
ben@feedbackloop.net
the gear

thanks very much mate, I have the 2x12 but I use that with the 9005 set up, its tube and needs to run stereo...

thanks again man, awesome thread!

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Jackson RR5
Moser V
Dean BlackGold V
ESP LTD M-350

Line 6 HD147+FBV
Marshall 1960A


...yes, knave, I am in a power metal band

uhm, I've just been playing the 8004 through the 1x12, and I'm getting some CRAZY volume changes...any ideas?

__________________
Jackson RR5
Moser V
Dean BlackGold V
ESP LTD M-350

Line 6 HD147+FBV
Marshall 1960A


...yes, knave, I am in a power metal band

I have many ideas.

1. Short in the cable between preamp and power amp. (try a different cable)

2. Short in the speaker cable. (try a different cable)

3. Short in the internal wiring of the cabinet. (try a different cabint)

4. Jack going bad on any of the pieces of gear. (this is more hard to diagnose. it involves substituting pieces of the gear; preamp, power amp, cab, etc)

5. Power amp problem (try the other channel)

This problem doesn't necessarily relate to what we were just talking about. Basically, start swaping out pieces of gear until the problem stops. Once it does stop, you've found the culprit, or how the signal gets to the culprit.

Cheers!

__________________
ben adrian
Now Doing Amp Repair in San Francisco
ben@feedbackloop.net
the gear

thanks man, I thought it would be something being loose, its that kind of change, if you know what I mean

__________________
Jackson RR5
Moser V
Dean BlackGold V
ESP LTD M-350

Line 6 HD147+FBV
Marshall 1960A


...yes, knave, I am in a power metal band

Someone please correct me if I have this wrong.

I have a Fender Pro Reverb combo which has an 8 ohm rating. I'm building a 2x12 extension cab to use with this amp. I will need to get (2) 4 ohm speakers and wire them in series for it to be compatible with my Fender amp. Is this the best way to do this?

edit: Actually, I think I'll go with 2 16 Ohm speakers and wire them parallel. I want to use Celestion V30's and I can't find them readily available in a 4 Ohm configuration. This should work ok, correct? Thanks.

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Either way will work.

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Very helpful thread.

I've never been entirely sure that my setup is correct. I have a Hughes & Kettner Tube 100. There are two 4 ohm speaker jacks on the back. The manual says they're "dual jacks wired in parallel" and should be used for "speaker cabinets with an overall impedance of 4 ohms, e.g. two 8-ohm cabinets." Also, underneath the jacks themselves it says "2x8 or 1x4 ohms".

So, my question is, if I have an 8 ohm stereo cabinet, then these are the jacks I should use, right? I.e., I should run two speaker cables from the amp, one from each 4 ohm jack, to the two inputs on the stereo cabinet? (There's also a single 8-16 ohm jack on the amp.)

Thanks in advance to anybody who can help.

Just to clarify...

My Traynor YBA-1A requires an 8 ohm load. Is it safe to wire up two Eminence Wizards (16 ohms each) in parallel to get the required 8 ohms?

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Only if the wizard rolls natural 20s.

the real answer is yeas, that's fine.

Cheers

__________________
ben adrian
Now Doing Amp Repair in San Francisco
ben@feedbackloop.net
the gear

Oh yeah!

HAPPY 420 EVERYONE!

And thank you sir.

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