surfguy13 Posted February 23, 2013 Share Posted February 23, 2013 Hi guys I have a pair of fender PA columns with 4 x 8" speakers in each cab. The cabs have seperate 32 ohm and 8 ohm jacks on the back. I want to hook up both cabs to an old mid 60s Selmer treble and bass head if possible but I'm not too sure about impedance issues and also how to hook them up. I intend to use them mainly for guitar but was surprised when I tried a bass through the head and one of the cabs last night to find that 4x8" speakers sounded very good indeed. So....possibly bass as well. Could I, for example, take a single cable from the 8 ohm speaker out jack on the head and split it? Would this still give me 8 ohms to each cab? I assume I could also use a splitter box? I can't think of any other way to do it and am still unsure about impedance. The speakers sound absolutely amazing and I'd like to use both if possible. Any ideas on how to do this would be warmly appreciated! Quote Link to comment Share on other sites More sharing options...
BassTractor Posted February 23, 2013 Share Posted February 23, 2013 (edited) Supposing both 408 cabs are fully functional and roughly equal, then one can use a splitted cable [i]in principle[/i]. However, if you do split the cable, you'll get a 4 Ohm cab or a 16 Ohm cab as seen by the head, depending on which cab jacks you use. [b]EDIT : THE FOLLOWING IS ABOUT SOLID STATE HEADS. IT NEVER CROSSED MY MIND THAT SELMER DID VALVE AMPS. SORRY![/b] The head will thrive with a 16 Ohm cab, as it will have to emit less current, the lazy bastard. As to 4 Ohm, the head might tolerate this, but I don't think it's very likely that it will indeed. If it did, Selmer would possibly have put some 4 Ohm outputs on it. If the Selmer is powerful enough (a test will tell much more than a number), then using the 32 Ohm jacks is secure and will work. Splitterboxes are many things: Some are just electrical connections, in which case it's virtually the same as splitting a cable. Some have electronics inside to regulate the load with. If you can find one of those with good quality, then it would be able to change the 4 Ohm load into an 8 Ohm load. There is a theoretical risk there that I wouldn't bother too much about. Connect it, and let your ears decide. FWIW, I've connected everything to everything in my days. Sometimes it works, sometimes you suddenly are a great composer of avant garde music, and sometimes you're saved by a fire rescue lady on top of a ladder. It's all good. best, bert Edited February 23, 2013 by BassTractor Quote Link to comment Share on other sites More sharing options...
Bill Fitzmaurice Posted February 23, 2013 Share Posted February 23, 2013 [quote name='surfguy13' timestamp='1361619521' post='1988316'] Could I, for example, take a single cable from the 8 ohm speaker out jack on the head and split it? Would this still give me 8 ohms to each cab? [/quote]You have it backwards. The cabs load the amp, not the other way around. If you Y the output jack to each cab, plugging into the 8 ohm jacks, that will place a 4 ohm load on the amp. Doing so to the 32 ohm jacks will place a 16 ohm load on the amp. If the amp output is rated for 8 ohm operation which to use depends on the amp. If valve use the 4 ohm option, if SS use the 16 ohm. Quote Link to comment Share on other sites More sharing options...
Musky Posted February 23, 2013 Share Posted February 23, 2013 (edited) Unlike SS amps valve amps should never be used with cabs of a higher impedance than the amp is rated for. I'm not overly familiar with the T&B, but it'll have either a selector switch or separate outputs for a given impedance. Since you have the option of two impedances with each cab (32 and 8 ohms) you should bear in mind that hooking up two cabs of the same impedance will usually halve the total load on the amp. Again, unlike SS amps valve amps will produce the same power into any load it is designed for, so if the T&B has a 16 ohm output, plugging into both the 32 ohm inputs on the cabs will be safe. Don't use the amp with just a single cab plugged into the 32 ohm input though! Edit: Just googled the output impedances on the T&B and it seems they're 15 and 7.5 ohms. Which presents something of a conundrum. Does any one know if the gap between the more standard 8 and 16 ohms will make a difference? Presumably not as the speaker loads are just nominal? Edited February 23, 2013 by Musky Quote Link to comment Share on other sites More sharing options...
surfguy13 Posted February 23, 2013 Author Share Posted February 23, 2013 Incredibly helpful replies, thank you!!! I think I am starting to understand. I didn't realise that hooking up two cabs will effectively halve the impedance....if I'm honest I thought it would double it!!! Yes, the Selmer has two outputs on the rear panel....a 7.5 and a 15 as Musky mentioned. It is a 50 watt valve head, not SS. It has SS rectification though if that makes a difference? So....am I right in thinking that if I take a lead from the 16 ohm 'out' on the amp and then somehow split the lead with a jack going to each 16 ohm socket on the cabs I'll be OK? If this is correct what would be the best way of splitting the signal to each cab? I assume a simple connector block is out of the question? If there is a dedicated product I could use to split the signal from the amp that would be great although I'm happy to try anything!!!! Thanks guys.... Quote Link to comment Share on other sites More sharing options...
Bill Fitzmaurice Posted February 23, 2013 Share Posted February 23, 2013 [quote name='surfguy13' timestamp='1361635195' post='1988607'] Yes, the Selmer has two outputs on the rear panel....a 7.5 and a 15 as Musky mentioned. It is a 50 watt valve head, not SS. It has SS rectification though if that makes a difference? So....am I right in thinking that if I take a lead from the 16 ohm 'out' on the amp and then somehow split the lead with a jack going to each 16 ohm socket on the cabs I'll be OK? [/quote]Or you could use the 4 ohm load option on the 7.5 ohm tap. Six of one, half a dozen of the other, the result will be the same. Back in the day when the original Kustom PA was introduced they inexplicably only had one output jack for the two columns. The supplied cable was two sixteen foot lengths, coupled in a single jack that plugged into the head. It worked, but it was hardly convenient. Quote Link to comment Share on other sites More sharing options...
BassTractor Posted February 23, 2013 Share Posted February 23, 2013 Just to be sure: my initial reply was wrong. It never crossed my mind that the head could be a valve amp. Sorry! Like others have written: do not connect a valve head's 8 Ohm output to a 16 Ohm cab load. b, b Quote Link to comment Share on other sites More sharing options...
surfguy13 Posted February 24, 2013 Author Share Posted February 24, 2013 No problem at all!!! I should have made it clear that the Selmer was a valve head, not SS. I'm a guitarist that plays bass, not a bass player, and sometime forget this!! I REALLY appreciate the advice regardless, I always get really confused with impedance, no idea why, it's not [i]that [/i]complicated! [quote name='BassTractor' timestamp='1361649072' post='1988871'] Just to be sure: my initial reply was wrong. It never crossed my mind that the head could be a valve amp. Sorry! Like others have written: do not connect a valve head's 8 Ohm output to a 16 Ohm cab load. b, b [/quote] Quote Link to comment Share on other sites More sharing options...
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