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Old 29th August 2007, 10:47 PM   #1
tareqx2 is offline tareqx2  United States
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Default Marshall 4x12 cab

Ok, so this is my first cab project. I read a couple of the posts to help me but i'm really confused. What is confusing to me is the impedence. I don't get it. So basically i have a choice to buy either 8 or 16 ohm speakers. I don't have a head, but i do intend to buy one (it will be a marshall). So first, is there any good site (or person) that can thoroughly explain impedence to me? Second, what kind of choices of amp heads do i have when choosing 8 ohm or 16 ohm? I'm really confused. Thanks for any help.
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Old 30th August 2007, 02:12 AM   #2
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Don't worry, it's all really rather simple. This site should provide a good explaination of impedence. As for which heads will match your speakers, most heads have an impedence selector switch to match to a cabinet. Simply select the desired impedence, and you're ready to rock. Also remember that you can connect speakers in series or parallel, which will yield different impedences (16 ohms and 4 ohms for 2 8-ohm speakers, respectively). Speakers behave just like resistors in this aspect.
I hope I have been able to help you in some way.

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Old 30th August 2007, 04:16 AM   #3
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What kind of advantages/disadvantages appear when deciding 16 or 8 ohm? That site helped out a little, but it is still a little confusing. After hearing that you can select the impedence on the head, that kind of relieved me. One question, when connecting speakers in parallel, how do you calculate impedance? it didn't explain it well enough.
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Old 30th August 2007, 05:34 AM   #4
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Quote:
Originally posted by tareqx2
What kind of advantages/disadvantages appear when deciding 16 or 8 ohm? That site helped out a little, but it is still a little confusing. After hearing that you can select the impedence on the head, that kind of relieved me. One question, when connecting speakers in parallel, how do you calculate impedance? it didn't explain it well enough.
In series resistances add, and in paralell you calculate by 1/(1/R1 + 1/R2 +... etc). Simply, two identical resistances in parallel are half the individual resistances.

With 4 speakers, you can either
a: wire two 8R in series, the other two in series, then parallel them to get 8R. 8R +8R = 16R, Same for the other pair. Both pairs in parallel gives a net of 8R.
b: Wire all four in parallel for 2R. Not recommended
c: Four 16R speakers in parallel for a net of 4R.
d: same as a: with 16R drivers to give a net of 16R.

I'd go with option a: myself.
Another option would be c: with two seperate cabs as per suggestions below. That way you can use one cap or two depending on the venue and get an 8R (1 cab) or 4R (2 cabs) and use the appropriate tap dependig on what you use.

A tube head has an output transformer in it to match the low impedance load of the drivers to the high impedance that the tubes want to see. If you connect the speaker to the correct tap the amp will give it's max power potential.

Here's a link to the old EVM12L (a superb guitar driver) datasheet which shows how to wire in series and parallel.
http://archives.telex.com/archives/E...-12L%20EDS.pdf

Some other points.
Build them as 2 2x12 cabs. It'll add about 2kg to the total weight, but it makes them a hell of a lot easier to move. You can also stack them vertically on a small stage. An added benefit of this is no horizontal phase cancellation between the drivers and better projection into the room. It also puts some of the drivers near ear level which IME means you'll tend to play at more sane levels. Four twelves can move a hell of a lot of air. This doesn't look 'traditional' so is a negative to some I've suggested it to in the past, but it works better.

Use good quality ply and brace the boxes well. Cover them in spray on pickup truck bed liner. It's as tough as hell and not that expensive by the accounts of bass DIYers I know in the US. Will also neatly cover any slight imperfections in woodwork and is hard to mark it up. Tolex looks like dirt after it's been gigged for a while and carpet pickups up smells.

Use Neutrik Speakon connectors on the cab(s). Tough reliable and aren't apt to get kicked out by someone on stage or go noisy like 1/4" jacks do. Premade leads are cheap and with lots of options for the amp end depending on what you need there. They're the industry standard now in PA.

Buy a cheap furniture dolly and nylon cinch straps to move them. Save your back. Two smaller boxes fit better into a lot more vehicles than a 4x12 which will be an awkward shape at the best of times.

Unless you're Hetfield and actually playing huge arenas you don't need a big head. It won't get much grit at realistic volumes. Try smaller heads, esp if you're playing trad bar gigs where 15 or 30W will likely be enough in reality and sound better too as you'll be capable of pushing it into distortion at a sensible volume.

Unless you really have to have a factory Marshall, build a clone if you like to DIY. You can also tailor the sound to what you want and add other features. I built a 100W head (now runs re-biassed with one set of output tubes for 35W) for a friend, and it ended up with 3 tube channels in it, all footswitchable and all very different sounding. I did it as a love job and it didn't cost that much in parts. The extra pre channels would probably cost $US50 add 1kg to the weight. Now he has a cab that he can play any gig with because of it's flexibility. For smaller bars, you could also just take one 2x12 and a milk crate to put it on.

I also just handed a harp amp to a friend to test. It's an EL84 based design of about 15W with two EVM12L drivers and it's just too loud. I added a triode switch so it's 3W with a different tone (softer, sweeter). You don't get these options on most commercial amps.

Turn it down. I'm a bassist, it's my job to tell you that.
It's shockingly rare to find a guitarist that plays at sensible volume that the rest of the band doesn't have to play even louder to compensate for. In most small/medium venues this can make it unpleasant for the audience too. Loud is fine, but lots won't enjoy painful loud, especially the women and if they don't stay, either will the guys and the owner won't book you again.

Sorry if I'm ranting, it's only meant as advice and I hope I'm not preaching to the converted. But I don't see the need for a guitar 4x12.
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Old 30th August 2007, 03:59 PM   #5
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Thanks for all the info Brett, i do agree with the 2x12 idea. Its just that for my first project i was really focused on a 4x12. It doesn't have to do so much about the sound rather than the fact that i can say that i made it. My idea is to build a Marshall 1960ax cabinet (found here) the info on connecting speakers was very helpful. I am going to go with your choice a. Thanks for the schematic as well. In the future i will be looking into making a 2x12 most likely. for now i am going with this though. Thank you for your help.
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Old 30th August 2007, 06:07 PM   #6
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Post some pix when it's done. All the best.
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Old 30th August 2007, 11:13 PM   #7
tareqx2 is offline tareqx2  United States
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Hey, how come it says that if you use the Marshall 1960ax as a half stack with a 100w head that it is recommended to use a power break? What is a power break? Another question. would the head i buy for the amp have to be 100w or could it be like a 50w tube head? kinda confused me because the Marshall vintage-modern head is 50w tube and the cab for it is 100w.
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Old 30th August 2007, 11:23 PM   #8
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Quote:
Originally posted by tareqx2
Hey, how come it says that if you use the Marshall 1960ax as a half stack with a 100w head that it is recommended to use a power break? What is a power break? Another question. would the head i buy for the amp have to be 100w or could it be like a 50w tube head? kinda confused me because the Marshall vintage-modern head is 50w tube and the cab for it is 100w.
As I said earlier, I doubt you'll ever need 100W on stage for guitar. Go with the 50W (but you wont even need that much) as it'll break up earlier at more sane volumes.

A power brake is basically a big variable resistor the soaks up a lot of the wattage put out by the amp so that you can get the clipping / overdrive / distortion from the power stage without being so loud you can hear it in the next postcode. Use a smaller head, therefore no need for a power brake. Marshall just want to sell you more stuff.

Also, it sounds as though you don't understand the difference between amplifier and speaker power ratings. I'll explain later if you need it clarified.
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Old 30th August 2007, 11:26 PM   #9
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Clarification would be great. As i said it is my first amp project, i play guitars, don't make them
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Old 31st August 2007, 12:32 AM   #10
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Quote:
Originally posted by tareqx2
Clarification would be great. As i said it is my first amp project, i play guitars, don't make them
OK. Speaker rated power is how much the speaker itself can handle before damage. Lets say you plugged an electric organ into an amp (any amp would do) which is rated at greater than 100W and pressed and held down some of the keys on the organ. Whilst doing so, you measured 100W being delivered into the speaker. You place a brick on the keys and walk away, with 100W going into the cab. If it's rating is accurate, eventually, there'll be damage to the speakers of some kind. And everybody for miles around will hate you. The 100W rating of the speaker is how much power it can absorb for a period of time before there is damage. It's not about how much it needs to make a decent sound in terms of loudness or tone. The speaker only absorbs how much power it's given by the amp.

I used the organ as it's capable of generating a tone at a constant level, unlike a guitar. This is important as guitars give out a much less constant volume as you shift chords etc and strike the strings at different pressures so only on the peaks will you potentially be giving the speaker full power and even then it depends on whether you turn it up enough to do that. With a guitar amp, part of it's 'tone' is the distortion it can generate when it's over driven, or giving more than it's rated power, eg 100W in the case of the Marshall. The trouble is, guitar speakers are very efficient, especially 4 x 12" so they need very little power to go very loud. As you're only using a small fraction of the amp's power, even strumming hard, powerchording etc you're not going to get the overdrive, because the amp may only be putting out a few watts. Hence my earlier recommendations for a smaller amp and to be able to split the 4x12 into two 2x12 so you can get the tone of the amp breakup at different volumes.

Using a power brake is a bit like having a powerful car that you love the sound of up at high revs, but you can't use them around town, so you drag a very heavy trailer around so the engine has to work harder to get those nice sounding high RPM's. Same thing with the amp that you want to run at 100W to get the tone, but you deafen everybody around doing it, so you need a way to get the amp to make the power, and divert some of it off to the powerbrake to get the volume lower.

Marshall stacks and 100W amps are from an era when decent PA's were far less common, and nowhere near as good as even cheap stuff today. Back then all the guitar sound that the audience heard came from the stack. Now it's often from the PA, and even if the audience is getting it from the stack on stage, ie no PA support, then with OH&S and noise rues etc you simply can't use it to it's capacity. They're still made because of tradition and people want them because they think they're "it". And of course Marshall are happy to keep making them and making money off them if the punters will pony up for one, whether they need it or not.

Once you've built the cab, try to borrow some amps of different powers and try them, and then you'll find out what you need.
My housemate plays in a really loud metal band. He's got a 100W Sovtek head and 2 4x10. Even at the loudest venue around here (Marquee) he uses one of the 4x10 and barely has the amp turned up much at all and gets all his distortion from a pedal. Last gig he used a Crate powerblock and swears he's never humping the heavy tube amp out again.
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