Frankenamp and impedance

[cellardweller]

I'm poor and I need more volume!

I have an Ampeg combo gutted (it's fried), and I am (hopefully) going to use it as an "extension" cab for a Crate, but I am going to disconnect the two speakers in the Crate (blown), and instead plug in the celestion currently in the gutted Ampeg.

The Celestion is an 8 ohm S12-150.

At the link below is all the info I can find on the crate speakers.

http://www.crateamps.com/pdf/manuals/GX120212+.pdf

(2) GX-212+ Custom Design 12”, 8 ohms

Does this mean that the total load is 8 ohms? or each individual speaker?

HELP!!!


Should I just disconnect one of the speakers in the Crate and wire the celestion in parallel with it?

 

[antichef]

Does it tell you the ohm rating on the back of the blown crate speakers?

Can you see whether they're wired in parallel (where each lead goes to each speaker) or series (where the current has to go in one speaker then to the other speaker and out the other speaker)?

 

[cellardweller]

Nope, I can only see the ohmage.

They are wired in parallel.

I called Crate, and the kid says that as best as he can tell, they are 8 ohm speakers.
So I should then just wire the celestion in parallel with whichever speaker sounds the better?

 

[Farview]

If it is a solid state amp, the impedance doesn't matter as long as it is above the minimum. The minimum would be either 4 ohms or 2 ohms.

If it is a tube amp, there would be a way to select the impedance on the amp.

 

[antichef]

I'm not sure I'm helping much, but two 8 ohm speakers wired in parallel make for 4 ohms of impedance (and in series, 16 ohms -- kind of intuitive, if you think about it).

I've heard the contrary about solid state amps - that you want to make sure they're matched, but maybe it was just the particular one(s) being discussed. I personally have run tube amps at mismatched impedance for short periods, but only on the low side (eg, running an 8 ohm output into a 4 ohm cab, but not a 16 ohm cab). Running the amp with no speakers is amp-i-cidal, of course.

In any event, you only risk blowing the output transformer and trashing the amp

The bestest way to go about this would be to measure the impedance of the two speakers as they are currently wired, assuming that didn't change because they were blown. I'm no expert here, but this website:

http://wolfstone.halloweenhost.com/Audio/spkimp_SpeakerImpedanceMatching.html#AmpTypes

says:

The best way to find out the impedance of a speaker is to read it off a label on the speaker. If the speaker is unlabeled, look for manufacturer's literature or spec sheets. If all else fails, you can attempt to measure speaker impedance with an ohm meter set for 20 ohms full-scale. The ohm meter will read slightly low:
3 to 3.5 ohms 4-ohm speaker
6 to 7.5 ohms 8-ohm speaker
13 to 15 ohms 16-ohm speaker

 

[Farview]

I'm not sure I'm helping much, but two 8 ohm speakers wired in parallel make for 4 ohms of impedance (and in series, 16 ohms -- kind of intuitive, if you think about it).

I've heard the contrary about solid state amps - that you want to make sure they're matched, but maybe it was just the particular one(s) being discussed. I personally have run tube amps at mismatched impedance for short periods, but only on the low side (eg, running an 8 ohm output into a 4 ohm cab, but not a 16 ohm cab). Running the amp with no speakers is amp-i-cidal, of course.

In any event, you only risk blowing the output transformer and trashing the amp

The bestest way to go about this would be to measure the impedance of the two speakers as they are currently wired, assuming that didn't change because they were blown. I'm no expert here, but this website:

http://wolfstone.halloweenhost.com/Audio/spkimp_SpeakerImpedanceMatching.html#AmpTypes

says:

You have it completely backwards!

Solid state amps don't have output transformers and they don't care about the impedance as long as it is over the minimum. This is why there is no impedance selector switch on solid state amps.

Tube amps do have output transformers and it does matter what impedance they run at. If you need to be mis-matched, you need to set the amp for a lower impedance than the cabinet. High impedance isn't a problem, low impedance is. I'm not even sure why you would need to, because if you have a 4, 8, or 16 ohm cabinet, the amp will have that selection.

Here is the thing, the lower the impedance, the closer you are to a direct short (0 ohms). You end up with too much current going through the transformer and it heats up and fails.

 

[antichef]

Here is the thing, the lower the impedance, the closer you are to a direct short (0 ohms). You end up with too much current going through the transformer and it heats up and fails.

makes sense. But I am sure that you don't want to run the tube amp with no speakers attached, either.

In any event, matching the speakers and the output is best

 

[cellardweller]

I ended up placing the amps back-to-back, unplugging one of the Crate's speakers, stretching the wires to the Ampeg's celestion.

It sounds a little better...

 

[Farview]

Take the celestion out of the ampeg and put it in the crate. It should be 4 screws holding each speaker in.