A question about ohms.

[ledzepp29]

My amp (see signiture) has an impedance switch for 8 or 16 and when I got it it was on 16 and it sounds fine so I'm guessing thats what it is supposed to be? I was looking at marshall 1x12 cabs and it said they were 8 ohms? why is my 1x12 combo 16 ohms (assumingly) and the 1x12 cab 8 ohms? and if I got one of these cabs to attach to my combo what would I put the impedance switch on??? I don't know how to determine impedance, obviously. o ya and another random question is, can an amp that came stock with el84s be replaced by el34s, like my marshall, sometime I want to pickup some el34s and A/B them with the el84s......o ya, theres all that biasing stuff...o well
thanks

 

[Artie]

Re: A question about ohms.

Think of it like this - when you look at the SD pups specs chart, you'll see that all the pups have different DC resistances. This has mostly to do with how much wire is wound on the coil. A speaker is the same way. Its main "motor", (the voice coil), is just a certain number of turns of wire wound on a paper or aluminum form. A manufacturer can make it anything they want. In the world of audio, 4, 8, and 16 ohms is typical. So you could easily have one 12" thats 8 ohms, and another thats 16 ohms.

The easiest way to tell the difference is, they should say so right on the speaker somewhere. If they don't, you can get in the ballpark by making an "ohms" measurement with a simple meter. Ususally, a speaker's DC resistance will be slightly less than its AC impedance - so, an 8-ohm speaker will probably read around 6 to 6.5 ohms, and a 16-ohm speaker will probably read around 14 to 16 ohms.

Sorry, can't help on the tube question.

 

[aleclee]

Re: A question about ohms.

EL34s and EL84s are not interchangeable. There are some adapters (e.g., THD YellowJackets) that will let you use EL84s in place of 34s but I don't think there's anything that'll let you use 34s in place of 84s.

 

[ledzepp29]

Re: A question about ohms.

alright I just checked the handbook and it said that the amps internal speaker is 16 ohms like I figured, and it said that if you drive a 4x12 cab and not the internal speaker it will run at 16 ohms but if you run a 4x12 cab along with the internal then it will run at 8 ohms, I understand that now even though I won't ever use it through a 4x12. But I am still confused on one thing, the marshall 1912 1x12 extension cab can only run on 8 ohms, and my amps internal is a 16 ohms but if the internal speaker is 16 ohms and the 1x12 cab is 8 ohms what will I do???? can I just switvh the speaker to a 16 ohm or is the whole thing wired for 8 ohms??? help me out

 

[ledzepp29]

Re: A question about ohms.

I plan on putting a V30 in my amp now and whenever i get a cab I want to put either another V30 in it or a celestion g12h 30 watt. I think an internal V30 and a g12h 30 in the cab would sound awesome, the V30 would dominate the sound and the g12h would flavor it a little. or maybe even a greenback in the cab and the sound would be even more diverse. o the possibilities.....

 

[Artie]

Re: A question about ohms.

Ok, here's the thing: Your amp can drive either an 8-ohm, or a 16-ohm load. Whichever you use, you must set the rear switch accordingly.

So, if you run only the internal speaker, (16 ohms), you set the switch for 16. If you run only an external Marshall 1960 4x12 cab, (also 16 ohms), you would set the switch for 16. If you run the internal and external at the same time, you'ld set the switch for 8 ohms.

The formula for figuring speaker load is like this:

So, if you have an 8-ohm speaker and a 16-ohm speaker connected at the same time, you'll have a total resistance of 5.333 ohms. So, that wouldn't be a good idea.

 

[ledzepp29]

Re: A question about ohms.

ok I see now. I guess I will have to get a custom cab, I was looking at a site called j design cabs and They make a 1x12 that will suite the size of my amp and you can put whatever kind of tolex, grille, speaker, ohmage and hardware you want. I will see if they can put some white piping along the grille to completely match, then all i gotta do is throw on a marshall logo and I'm set. better get savin

 

[ledzepp29]

Re: A question about ohms.

o ya one last question could i use a celestion g12h 30 watt speaker internally for my combo which is 40 tube watts rms??? Would it blow? The reason I ask is the g12h 30w is by far my favorite speaker according to sound clips and reveiws and descriptions.

 

[aleclee]

Re: A question about ohms.

So, if you have an 8-ohm speaker and a 16-ohm speaker connected at the same time, you'll have a total resistance of 5.333 ohms. So, that wouldn't be a good idea.

There shouldn't be a problem with running this load at that 8 ohm setting. It might not sound as good as an 8 ohm load (the amp definitely won't put out max power with this mismatch) but most tube amps won't suffer any permanent damage with up to a 2x impedance mismatch. When mismatching, it's generally safer to have the cab be lower than than the amp's setting.

 

[aleclee]

Re: A question about ohms.

o ya one last question could i use a celestion g12h 30 watt speaker internally for my combo which is 40 tube watts rms??? Would it blow? The reason I ask is the g12h 30w is by far my favorite speaker according to sound clips and reveiws and descriptions.

Using a 30W speaker is definitely a risk with a 40W amp (especially a tube amp). With a tube amp, if you lose your one and only speaker while the amp is outputting max power, there's a good chance that it'll damage the output transformer. If it's a SS amp, the risk is only to the speaker since SS amps can safely run with no load on the power section.