ohms? watts? wha?

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FALKEN

FALKEN

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after playing combo amps for 10 years I finally bit the bullet and picked up a couple of heads on ebay. One of them is about 30-45 watts and the other is about 40-50 watts with a matching 2x12 cab. I want to be able to use the cab with both heads, and I also want to pick up a 2x12 open cab. I saw a nice one I like that is unloaded so I can pick my own speakers (which will be a whole nother adventure, I'm sure). So, I am totally confused about ohms and wattage and such. I am going for a vintage tone, whatever that means. I was hoping for a link to a primer on matching cabs with heads or someone who could spend a little time explaining it. I guess, how do you match ohms, in series and paralell, and how do you match watts, and how do I not damage these beautiful new heads ? Thanks in advance.
 
http://www.nzmusic.com/topic.cfm?i=11093&start=1

FALKEN said:
after playing combo amps for 10 years I finally bit the bullet and picked up a couple of heads on ebay. One of them is about 30-45 watts and the other is about 40-50 watts with a matching 2x12 cab. I want to be able to use the cab with both heads, and I also want to pick up a 2x12 open cab. I saw a nice one I like that is unloaded so I can pick my own speakers (which will be a whole nother adventure, I'm sure). So, I am totally confused about ohms and wattage and such. I am going for a vintage tone, whatever that means. I was hoping for a link to a primer on matching cabs with heads or someone who could spend a little time explaining it. I guess, how do you match ohms, in series and paralell, and how do you match watts, and how do I not damage these beautiful new heads ? Thanks in advance.
Click to expand...
 
The heads will tell you what Ohmage they are most efficient. You then try to match this with your speakers. Since you are looking at 2x12's, it'll be easy. If you wire the speakers in series, you add/double the ohms per speaker, if you wire in parallel then you halve the ohms per speaker. So if you have a head that says 8 Ohms, you can either get two 4 ohm speakers and wire them in series, or two 16 Ohm speakers and wire them in parallel.
 
http://www.bcae1.com/ (VERY thorough site)


http://www.marktaw.com/print/recording/Electronics/OhmsAmpsandSpeakers.html

vintage tone???one word.

jensen.they created vintage tone. :)

http://www.usspeaker.com/jensen index-1.htm

Ohm is a measurement of electrical resistance. The higher the ohms, the more resistance. Amps are rated at variable ohms because they can drive an eight ohm load, or four ohm load. You may have noticed that the watts increase as the ohms decrease. Some amps (usually power amps for PA use) are rated for 2, 4, and 8 ohm loads.
 
FALKEN said:
after playing combo amps for 10 years I finally bit the bullet and picked up a couple of heads on ebay. One of them is about 30-45 watts and the other is about 40-50 watts with a matching 2x12 cab. I want to be able to use the cab with both heads, and I also want to pick up a 2x12 open cab. I saw a nice one I like that is unloaded so I can pick my own speakers (which will be a whole nother adventure, I'm sure). So, I am totally confused about ohms and wattage and such. I am going for a vintage tone, whatever that means. I was hoping for a link to a primer on matching cabs with heads or someone who could spend a little time explaining it. I guess, how do you match ohms, in series and paralell, and how do you match watts, and how do I not damage these beautiful new heads ? Thanks in advance.
Click to expand...

A small red flag pops up.... you probably know this, but when you said, "I want to be able to use the cab with both heads", you don't mean that you will be sharing the cab between them, do you? Plugging two heads into a single cab would be a big no-no. I don't mean to insult you, but I have had guys ask me if they could do that.
 
ggunn said:
A small red flag pops up.... you probably know this, but when you said, "I want to be able to use the cab with both heads", you don't mean that you will be sharing the cab between them, do you? Plugging two heads into a single cab would be a big no-no. I don't mean to insult you, but I have had guys ask me if they could do that.
Click to expand...

LOL no! I mean I would like the option of using the cab with either head. as long as it wont blow it up.
 
If you load a 4x12 cab with 8 ohm speakers, you can wire them so the final output of the cab is 8 ohms. Series-parallel. You have to use a combination of both to keep it that way.

Now say you've got 2 speaker jacks on the back of your amp. Let's also say they're parallel. If you plug 2 8-ohm cabs in, it brings the amp down to 4 ohms. Less resistance. It draws more power from the amp. The amp gets louder.

Now if there isn't enough resistance from the speakers, your amp could blow up. Very few amps can handle a 2 ohm load. Usually 4 is about as low as you want to go.

Do you have a multimeter?


sl
 
I might have a multimeter. I got a cheap little device a little while back for testing cables. not sure if its the same thing. it has two "probes". so if the amp has an 8 ohm output, I can put two 4 ohm speakers in series (I guess this means there is one cable between the head and the first speaker and another between the first speaker and second speaker?), or I can put two 16 ohm speakers in paralell (both speakers feed off of the same line from the amp)? is this correct? is it easy to wire a cab?

also - am I correct in thinking I want 30 watt speakers?
 
It depends on how the jacks are wired. Parallel is positive to positive, negative to negative. Series is positive to negative. Usually amps and speaker cabs with multiple jacks are parallel.

It's easier to understand with diagrams.

http://www.usspeaker.com/speaker wiring-1.htm

If you can solder, it's easy to wire up your own cabinets. You can plug a speaker cable into any cabinet and measure the other end with a multimeter to make sure you're ok to go into the amp.

If that device with the probes has a dial on it that lets you measure for ohms, it should work.

As for wattage, I'm not sure. More power handling will be cleaner. Adding a cab (lowering resistance and pulling more power) should drive your amp harder, but distribute the power to more speakers. Two 25 watt, 4 ohm speakers wired in series should give you a 50 watt, 8 ohm cab. This should be safe to run with both of your heads.


sl
 
You might also consider having two wiring schemes within the one 2 x 12 cabinet, with each terninating in its own jack on the back of your speaker cabinet. This would give you 16 ohm or 4 ohm options with one scheme wired series and the other parallel (assuming you are using 8 ohm speakers).

For 'vintage' tone, there is the well known Celestion 'Vintage 30' which is a pretty robust unit that should stand up to the job wired either series or parallel (with the amps you've suggested). It posesses a very mid-rich tone that may or may not be what you're after, but it certainly does leans toward a more vintage tone, compared to modern units that have tighter bass and more pronounced highs - this is generally speaking, of course. The Vintage 30 definitely posesses a characteristc sound of its own.

Other (perhaps even more vintage) options might be found in Alnico magnet units, which would be more in line with a softer mid upper harmonic tone of playing, such as Blues. Again, just generalisations, but good starting points. Efficiency is another issue not mentioned, and the more efficient a speaker is, the less the amp has to do to get it to play loud. Efficiency ratings (and guitar speakers are all usually pretty efficient) is measured in dB/m, and the higher your dB value, the less power required by the amp to get it to play loud. 95dB/m - 99dB/m is the typical range, if I recall.
 
Oops - forgot the inclusion of a switch on the cabinet to select between 8 and 16 ohms.
 
For a 2X12 cab try Black Widow speakers, I love them, and I think they are cloce to indestructable with anything less than 1000 watts, however shoving boom stands through them will cause considerable damage. Oh yeah I forgot to mention, they are loud reguardless of how much power you run through them. Small power amps can drive them yet they can handle big amps too.
 
wow that was all really helpful stuff. I should mention that one head is a 1965 fender bassman with 2x12 cab and the other is a brand new new built-to-original-specs jtm45. Any other info based on this new information would be great. you guys are awesome. thanks again.
 
the jtm45 came. It looks nice. I want to try it out. I have two twins.

A fender twin reverb 65, and a musicman twin.

The fender twin has two speaker outputs, internal and external, and on the back of the amp it says "paralell speaker jacks; 4 ohms total". does this mean that one output would be 8 ohms? and I can use the speakers as though it were an 8 ohm cab?

The musicman twin has a switch for 4 or 8 ohms on the back. the speakers don't say how many ohms they are. The amp was "loaned" to me by my friend for the last few years. The setting is on 4 ohms but I am wondering if that is for when you use both outputs? this one says "speaker jacks are connected in series". Should I be running this on 8 ohms or 4?

The amps are almost identical.

Thanks.
 
FALKEN said:
the jtm45 came. It looks nice. I want to try it out. I have two twins.

A fender twin reverb 65, and a musicman twin.

The fender twin has two speaker outputs, internal and external, and on the back of the amp it says "paralell speaker jacks; 4 ohms total". does this mean that one output would be 8 ohms? and I can use the speakers as though it were an 8 ohm cab?
Click to expand...

No, what that means is that the total load on the two jacks, which are in parallel, cannot be less than 4 ohms, e.g., two 8 ohm cabinets. One 4 ohm cabinet also hits that minimum; with that you can't use the other jack at all.

The musicman twin has a switch for 4 or 8 ohms on the back. the speakers don't say how many ohms they are. The amp was "loaned" to me by my friend for the last few years. The setting is on 4 ohms but I am wondering if that is for when you use both outputs? this one says "speaker jacks are connected in series". Should I be running this on 8 ohms or 4?
Click to expand...

You should be running on the correct load, irrespective of which output jacks you are using. The "connected in series" means that you probably cannot use the second jack if nothing is plugged into the first, and the impedances add directly instead of reciprocally, i.e., two 8 ohm cabinets give you a 16 ohm load instead of a 4 ohm load as it does in a parallel configuration.

If you cannot match the amp impedance setting with the speaker impedance, the safer thing is to set the amp lower than the speaker(s) impedance, and NEVER load an amp with a lower impedance (less ohms, higher load) than its stated minimum.
 
thanks

looks like neither amp's speaker cabe will reach to the head so I have to try to borrow one anyway...but good stuff to know. now I know my amps a little better. I was able to deduce that both cabs are two 8 ohm speakers wired for a 4 ohm load.
 
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