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Guitar Cab Conversion
Guitar Cab Conversion
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Old 15th August 2006, 08:17 AM   #1
givethemrope is offline givethemrope  United States
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Default Guitar Cab Conversion

I have a peavy 5150 4 12 cab and really really want to convert it to (2) seperate 2 12s. any advice?
i have never done anything like this so any info would help.
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Old 15th August 2006, 11:23 AM   #2
Nigel Goodwin is offline Nigel Goodwin  England
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Join Date: Jul 2005
Location: North Derbyshire
It's mainly a question of impedances - a four by 12 can be wired in two basic ways:

1) series/parallel - this keeps the overall impedance the same as a single speaker, so four 8 ohm speakers give an 8 ohm cab.

2) parallel - this uses four 16 ohm speakers in parallel, giving a 4 ohm cab.

Most are series/parallel.

So splitting to two 2x12, assuming 8 ohm speakers, they can be wired as 4 ohm (in parallel) or 16 ohm (in series). If the speakers are 16 ohm?, you can only wire them in parallel, giving 8 ohms.
Nigel Goodwin
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Old 15th August 2006, 02:30 PM   #3
Albertb is offline Albertb  United Kingdom
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Join Date: Apr 2006
Location: Hants
"This said, as is the case with quite a number of "Plexi" era 100-watt heads, the negative feedback on the original 1969 circuit Marshall used for this reissue is taken off the actual speaker output itself. This means that the lower the impedance setting, the lower the damping factor - and the lower the damping factor is, the looser and more resonant the sound. Consequently, if you're using a 4 x 12 in. cabinet loaded with 16-ohm speakers that offers 16-ohm and 4-ohm mono inputs (eg: the Marshall 1960A, 1960B, 1960AV or 1960BV cabinets), the 4-ohm input will be looser and more resonant on the low end, while the 16-ohm input will be tighter and more controlled."

(Taken from an advert for a reissue Marshall 100W stack)

This illustrates another factor which might be worth thinking about in your case. It helps in musical instrument amplifiers to have a relatively poor, (low), damping factor at each speaker. The damping factor is a measure of how accurately the speaker cone movement and its resonances, (and hence the final sound), is controlled or brought to a halt by the amplifier when the signal changes or stops. Although this is the opposite of what is aimed for with a hifi amplifier, a low damping factor allows the speaker to ring a little more and can give a livelier sound more suited to some music. To illustrate this just remember what it sounded like when you played your beautifully lively Strat through your hifi setup! You would have heard that the sound is completely lifeless. (If not you have a remarkably crappy hifi system!) Hifi amps and speakers, (reasonable ones anyway), have very high damping factors!

The damping factor is the ratio of the impedance of the speaker to the source impedance it sees "when looking back into the cable" as it were. Hifi amps have impedances of a small fraction of an ohm giving very high damping factors with normal speakers. Solid state instrument amplifiers will have slightly lower values due to acceptable design compromises but they will still be high compared to valve instrument amps which, because of their output transformers, inherently cannot achieve very low values. (This is yet another plausible reason why solid state guitar amps differ from valve amps in tone).

Let's assume you have your 4 x 8ohm speakers to play with and for simplicity are using a solid state amp with a negligible output impedance. (The principle stays the same with a valve amp but the numbers change and become a little more complex). 2 of these in parallel will give 4ohms and those 4ohms in series will get back to 8ohms. 2 x 8ohms in series will give 16 ohms and these in parallel will again get back to 8ohms. So what is the difference? Well consider the damping factor in each of these cases:

1. An arrangement of 2 cabinets wired in series, with 2 x 8ohm speakers wired in parallel inside each cabinet.

This arrangement allows each speaker in the set to look back up the wire and see a source impedance dominated by the speakers in the other cabinet which will be 4ohms.

2. An arrangemeng of 2 cabinets wired in parallel, with 2 x 8ohm speakers wired in series inside each.

In this case each speaker sees an impedance dominated by its partner in its own cabinet which is 8ohms.

(In both cases the speaker's own impedance does not affect the situation).

That second impedance is significantly higher than the first, by a factor of 2, consequently the second will have a lower, (poorer), damping factor - which may well be what we want!

Incidentally, in each case the power fed to each speaker remains the same and is balanced equally among all 4.

Whether you would prefer that more controlled speaker response or not can only be checked by listening. It is no great problem to change the wiring and test. And of course it depends on the quality of the other components in the chain. But when people genuinely believe that changing a single passive component to one of the same value but of another make can radically change the tone of the instrument........ ;-)
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Old 16th August 2006, 04:03 AM   #4
Enzo is offline Enzo  United States
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Join Date: Sep 2004
Location: Lansing, Michigan
The feedback tap on the 5150 is always the 8 ohm winding.

If you want to use the same four speakers in two cabs instead of one, the amp won't know the difference if you wire for the same impedance.

PV 4x12 cans are pretty much 16 ohms if I recall. The four drivers are each 16 ohms.

To split them, you can either wire each in parallel for an 8 ohm cab, and then both of them represent a 4 ohm load to the amp. Or you could wire each 2x12 cab in series, so both cabs together make a 16 ohm load for the amp, but individually they would be 32 ohm cabs. Unusual to say the least. I assume you intend to use them either one or two at a time, so the first option makes the most sense.

Since the amp has output selector for 4/8/16 ohms, your new 4 ohm total for two cabs or 8 for a single cab would work out.

Unless you are into carpentry, you might look into a couple 2x12 cabs from Avatar. Not very expensive, and folks seem to like them. Stuff them with your own speakers.
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