rules for ohm ratings....

[goodbyebluesky]

Whats the general rule of thumb on whats acceptable speaker wise to hook up to an 8 ohm output of a guitar amp/head??

I have a 2x12 open back combo whose power amp has fizzled, and would love to put my Epi Valve Special thru it, but unlike the Valve Junior I only have one cab output and it says "8 ohm minimum". So that means there has to be an 8 ohm load on the amp, or what? It'll blow the amp?

Is it safe to assume that together these 2 speakers will provide an 8 ohm load? Like if they are 4 ohm speakers..... 4 + 4 = 8 = I'm good to go right?

I have a handy little experimenter's box rigged up with a 1/4" switchcraft input that splits the signal to speaker wire with the right ends to snap onto the speakers. I made it a long time ago but never used it.

 

[crazydoc]

Whats the general rule of thumb on whats acceptable speaker wise to hook up to an 8 ohm output of a guitar amp/head??

I have a 2x12 open back combo whose power amp has fizzled, and would love to put my Epi Valve Special thru it, but unlike the Valve Junior I only have one cab output and it says "8 ohm minimum". So that means there has to be an 8 ohm load on the amp, or what? It'll blow the amp?

Yes, it might, with any load less than 8 ohms.

Is it safe to assume that together these 2 speakers will provide an 8 ohm load? Like if they are 4 ohm speakers..... 4 + 4 = 8 = I'm good to go right?

I think it's safe to assume that if you wire these speakers in series to the output, you will have a minimum of 8 ohms of impedance (it's unlikely the speakers are less than 4 ohms each.) However, if they are each 8 ohm speakers, you will have a 16 ohm load. This won't harm the amp, but will reduce the output power by half, which may be no big deal to you. It may also change the distortion characteristics if you are trying to overdrive the amp.

If you wire any speakers in parallel that are less than 16 ohms each, you will present less than 8 ohms to the amp, which could (but not necessarily will) destroy it.

 

[goodbyebluesky]

Hmmm.... just did some cursory reading on wiring in series and parallel on www.termpro.com.

I peeked at the back of the amp, and it appeared to be in series. So I unplugged the positive and negative coming from the pre/power amp and attached the positive and negative of my little rigged up box to it in same fashion. Fire it up....

I was expecting it to lose half its volume but it didn't seem like it. I'll have to wait til I get up tomorrow afternoon to be able to crank it up. Its possible that it actually did lose half of its output but its deceiving me because the Epiphone has a closed back, and the 212 is open back? It sounded a bit more full.

As long as nothing blew up.... everything is dandy for another day.

 

[Dani Pace]

Remember, output power and perceived volume are NOT the same thing. Doubling the resistance does NOT mean cutting the volume in half. Perceived volume is largely determined by speaker efficiency.

 

[JCM900]

2, 4ohm speakers wired in parallel will give you an ohm load of 2ohm.
2, 4ohm speakers wired in series will give you an ohm load of 8ohm.

 

[goodbyebluesky]

Yea I was able to crank it up today after getting up. I don't perceive any volume loss actually, and it sounds fuller to my ears.
I'm guessing these are 4 ohm speakers so its running an 8 ohm load just like before with the stock 10" speaker.

Thanks for the help guys.

 

[crazydoc]

Glad it worked out. It may be that the new speakers are more efficient than the original one (as noted by Dani Pace above), so you could be getting a greater SPL (sound pressure level = volume)) out of the new ones even at 16 ohms, than you got out of the old one at 8 ohms.

 

[ggunn]

...if they are each 8 ohm speakers, you will have a 16 ohm load. This won't harm the amp, but will reduce the output power by half, which may be no big deal to you.

True for solid state amps but not necessarily for tube amps.