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Switching impedance on 4x10 cabinet


mattjones_81
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Probably a daft question but just want to double check.

I have a Marshall 4x10 cabinet that uses 4 X 8 ohms speakers wired to give a total of 8 ohm impedance.

This is fine for my soundcity B120 which has an 8 ohm tap. The problem is I have recently acquired a marshall VBA 400 which can only run at 2 or 4 ohms. I realise my cab can be rewired to give a total load of 2 ohms but my soundcity can only go down to 4 and I would like to be able to use both heads with this cab without having to rewire each time.

My question is this - i know lots of cabs (marshalls etc) have switchable inputs for either 4 or 16 ohms - (or 8 ohms stereo). Could I install one of these switch plates into my cab (e.g something like this - http://www.ebay.co.uk/itm/Marshall-Cabinet-Jack-Socket-1960A-B-/390341143048?pt=UK_Guitar_Accessories&hash=item5ae2252a08#ht_500wt_951) and instead of 4 or 16 ohms it would be the 8 and 2 ohms I need?

Don't want to blow up any output transformers!

Thanks

Matt

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Not easily.

Your existing cab is wired in series [b]and[/b] parallel (so 2x 8Ω drivers in series to give you 16Ω and that pair wired in parallel to the other pair to bring you back to 8Ω).

Your wiring would have to break the series wiring and wire all four 8Ω speakers in parallel.

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[quote name='0175westwood29' timestamp='1340950644' post='1711825']
Is the vba not just 2 ohms load as the lowest? Ie if you select 4 ohms and plug the 8ohm cab in it should run fine no?
[/quote]
It's a valve amp, so doing that would release the magic smoke!

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So here's my maths!! If the speakers in a 1960a cab are 16 ohm then

2x16Ω drivers in series to give you 32Ω and that pair wired in parallel to the other pair to bring you back to 16Ω
4x16Ω drivers in parallel to give 4Ω total

so using 8 ohms speakers would give:

2x8Ω drivers in series to give you 16Ω and that pair wired in parallel to the other pair to bring you back to 8Ω - Use Soundcity
4x8Ω drivers in parallel to give 2Ω total - Use Marshall

Does anyone know if the switch will change the wiring from parallel to series parallel then?

Maybe I should just buy it - wire it up and check the impedances with a multimeter? Argh! Why didn't i pay more attention to physics at school! Help!

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obbm - tell me more? What sorcery is this you speak of? How would I procure such a device? And how should it be wired? (All this electric gubbins really isn't my thing as you can probably tell!) If you could give me a complete fool proof idiots guide I would be most grateful!

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If you've got a crossover and tweeter, It'll complicate matters as crossovers are designed to work with a specific impedance. If you connect woofers wired at 2 ohms to a crossover designed to see 8 ohms, the results may be all over the place, so you may have to sacrifice your tweeter and run the 10"s full range if you want switchable impedance.

Edited by Beer of the Bass
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Nope - no crossovers or tweeters involved so makes things easier - although whilst very appreciative I didn't really understand the wiring diagram OBBM sent through - as previously mentioned I am rubbish at these things!

Also worried that the switch OBBM suggested may get snapped off as I load / unload the cab out of the back of my van so am leaning towards the recessed jack plate I put a link to - just hope it works in the way i hope!

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Good evening, Matt...

I would concur with fret; you're going through considerable handiwork to get a 'bastard' result, when the real solution is to acquire a cab which fits both heads. The 'I forgot to switch over' element will be critical, seeing the amps you're using; you will only have to be mistaken once. The cost of a cab will be slight compared to the potential damage to the amp.
Bite the bullet, sell (with regrets...) your current cab to fund a unique, well-adapted cab fit for purpose.
Dave's switching will work fine, and is technically correct, but if (as I suspect...) you are human (could be wrong, has been known...), human error becomes the important factor. For my part, I wouldn't take the risk.
Just my tuppence worth...

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[quote name='0175westwood29' timestamp='1341008478' post='1712927']
Oh yeh ive been ss for to long now i suppose could always get an 8 ohm dummy load? And plug that in aswell as the cabinet?
[/quote]

Not really a practical option.
You're not going to find an 8Ω resistor with a high enough wattage - and even if you could, 400W is, as near as damnit, half a bar of an electric fire...

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