mattjones_81 Posted June 28, 2012 Share Posted June 28, 2012 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 Quote Link to comment Share on other sites More sharing options...
icastle Posted June 28, 2012 Share Posted June 28, 2012 Nope. There's no wiring permutation that'll give you 4Ω from four 8Ω speakers. Quote Link to comment Share on other sites More sharing options...
mattjones_81 Posted June 28, 2012 Author Share Posted June 28, 2012 I know - i don't need 4 ohms. If I can switch between 8 and 2 ohms I can run the soundcity at 8 ohms and the marshall at 2 ohms. My question is if i use a 4 or 16 ohm switch on my cab will it give me the equivalent 2 or 8 ohms that i need? Quote Link to comment Share on other sites More sharing options...
xgsjx Posted June 28, 2012 Share Posted June 28, 2012 It should do. You're basically switching the wiring of them from series to parallel. Quote Link to comment Share on other sites More sharing options...
icastle Posted June 28, 2012 Share Posted June 28, 2012 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. Quote Link to comment Share on other sites More sharing options...
0175westwood29 Posted June 29, 2012 Share Posted June 29, 2012 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 Link to comment Share on other sites More sharing options...
0175westwood29 Posted June 29, 2012 Share Posted June 29, 2012 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 Link to comment Share on other sites More sharing options...
Musky Posted June 29, 2012 Share Posted June 29, 2012 [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! Quote Link to comment Share on other sites More sharing options...
mattjones_81 Posted June 29, 2012 Author Share Posted June 29, 2012 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! Quote Link to comment Share on other sites More sharing options...
obbm Posted June 29, 2012 Share Posted June 29, 2012 You can make it 2-ohm/8-ohm switchable using a high-current 4-pole change-over toggle switch. Quote Link to comment Share on other sites More sharing options...
mattjones_81 Posted June 29, 2012 Author Share Posted June 29, 2012 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! Quote Link to comment Share on other sites More sharing options...
xgsjx Posted June 29, 2012 Share Posted June 29, 2012 You need one of these!... Quote Link to comment Share on other sites More sharing options...
obbm Posted June 29, 2012 Share Posted June 29, 2012 A typically suitable switch is [url="http://uk.farnell.com/eaton/7694k4/switch-toggle-4pdt-15a-250v/dp/1383063"]http://uk.farnell.co...250v/dp/1383063[/url] Wired as diagram. Quote Link to comment Share on other sites More sharing options...
Beer of the Bass Posted June 29, 2012 Share Posted June 29, 2012 (edited) 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 June 29, 2012 by Beer of the Bass Quote Link to comment Share on other sites More sharing options...
mattjones_81 Posted June 29, 2012 Author Share Posted June 29, 2012 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! Quote Link to comment Share on other sites More sharing options...
BILL POSTERS Posted June 29, 2012 Share Posted June 29, 2012 I'd be more worried about mistakenly using the cab with the switch in the wrong position or forgetting to change it over. Quote Link to comment Share on other sites More sharing options...
0175westwood29 Posted June 29, 2012 Share Posted June 29, 2012 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? Andy Quote Link to comment Share on other sites More sharing options...
fretmeister Posted June 29, 2012 Share Posted June 29, 2012 Sell the cab. Buy a Marshall VBC 412 (4ohms only). Use it for everything. They are fecking marvellous. Quote Link to comment Share on other sites More sharing options...
Dad3353 Posted June 30, 2012 Share Posted June 30, 2012 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... Quote Link to comment Share on other sites More sharing options...
icastle Posted June 30, 2012 Share Posted June 30, 2012 [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... Quote Link to comment Share on other sites More sharing options...
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