gonzoromirez Posted February 14, 2010 Share Posted February 14, 2010 (edited) Hi all, just before i ask this question, I have read the sticky topics on impedance and from what i can gather, i already know the answer but just want to double check, I'm not being lazy honest! I have borrowed a Ampeg SVT classic head to try out, its a tube amp so it has an impendance selector which lets you select 4 or 2ohms. The cab i have is an ashdown 410T rated at 8ohms on its own. Will I damage the head by trying to use it with this cab? Also i gather that if i ran 2 ashdown cabs rated at 8ohm in a daisy chain then i would be ok as long as i had 4ohm selected on the amp? is there any way of using my cab with the amp without having to buy or borrow another cab? cheers guys Edited February 14, 2010 by gonzoromirez Quote Link to comment Share on other sites More sharing options...
Mr. Foxen Posted February 14, 2010 Share Posted February 14, 2010 That amp probably won't be happy with that cab. You could rewire the cab to be 2 ohm probably, if you were handy like that. Best getting another cab though. Quote Link to comment Share on other sites More sharing options...
gonzoromirez Posted February 14, 2010 Author Share Posted February 14, 2010 I'd be willing to give that a bash, would need some kind of tutorial though, know of anything on here? thanks Quote Link to comment Share on other sites More sharing options...
Mr. Foxen Posted February 14, 2010 Share Posted February 14, 2010 Bear in mind it will make the cab wierd, in that no-one has 2 ohm cabs. Make sure you tippex over the 8 ohm bit on the back. People blindly believe that even with a meter showing otherwise. Its parallel wiring you want. Kind of works if you think of it like fractions of an inch. The 8 ohm speakers are 1/8 of an inch. Stick 4 in a row and you have 1/2 an inch. 2 ohm. Get a resistance meter to check, it will probably come up not exactly 2. Quote Link to comment Share on other sites More sharing options...
thebassman Posted February 14, 2010 Share Posted February 14, 2010 [quote name='gonzoromirez' post='744866' date='Feb 14 2010, 03:12 PM']Hi all, just before i ask this question, I have read the sticky topics on impedance and from what i can gather, i already know the answer but just want to double check, I'm not being lazy honest! I have borrowed a Ampeg SVT classic head to try out, its a tube amp so it has an impendance selector which lets you select 4 or 2ohms. The cab i have is an ashdown 410T rated at 8ohms on its own. Will I damage the head by trying to use it with this cab? Also i gather that if i ran 2 ashdown cabs rated at 8ohm in a daisy chain then i would be ok as long as i had 4ohm selected on the amp? is there any way of using my cab with the amp without having to buy or borrow another cab? cheers guys[/quote] I think these guys are kidding you. The amp will run perfect with an 8ohm cab. The only effect will be the amp will output half the stated power. If you double the ohms you half the output power. You only need watch out for matching cabs with lower ohm rating than the amp can output. Cheers thebassman Quote Link to comment Share on other sites More sharing options...
gonzoromirez Posted February 14, 2010 Author Share Posted February 14, 2010 thats good to know thanks. Although i did read somewhere that with tube amps this can be harmful? but i guess you can't believe everything you read on the tinternet! Quote Link to comment Share on other sites More sharing options...
obbm Posted February 14, 2010 Share Posted February 14, 2010 [quote name='thebassman' post='745028' date='Feb 14 2010, 05:45 PM']I think these guys are kidding you. The amp will run perfect with an 8ohm cab. The only effect will be the amp will output half the stated power. If you double the ohms you half the output power. You only need watch out for matching cabs with lower ohm rating than the amp can output. Cheers thebassman[/quote] Sorry but that only applies to solid-state amps, it doesn't apply to valve amps. In a valve amp the anode load of the output valves is determined directly by the impedance of the speaker load. This is why they have different output transformer taps in order to ensure that the valves always see the correct load. If you have an impedance missmatch the valves will have an incorrect load, either draw too much or too less current and not work at their designed section of the load curve. Quote Link to comment Share on other sites More sharing options...
skankdelvar Posted February 14, 2010 Share Posted February 14, 2010 (edited) [quote name='gonzoromirez' post='745061' date='Feb 14 2010, 06:08 PM']thats good to know thanks. Although i did read somewhere that with tube amps this can be harmful? but i guess you can't believe everything you read on the tinternet![/quote] Ah, but you can. You were right first time. It [i]can[/i] be harmful. Mismatching impedances is much more of an issue with tube amps. As you've borrowed this £1400 amp from someone else it makes perfect sense to treat it with care - as is your intention. Frankly, you're better off running the amp into the appropriately rated cab if you want to avoid potential hassle - and experience the joy of an Ampeg in full flight. Edited February 14, 2010 by skankdelvar Quote Link to comment Share on other sites More sharing options...
AlanP2008 Posted February 15, 2010 Share Posted February 15, 2010 (edited) [quote name='Mr. Foxen' post='744891' date='Feb 14 2010, 03:48 PM']Bear in mind it will make the cab wierd, in that no-one has 2 ohm cabs. Make sure you tippex over the 8 ohm bit on the back. People blindly believe that even with a meter showing otherwise. Get a resistance meter to check, it will probably come up not exactly 2.[/quote] Agree... EXCEPT that the diagram bottom right is labelled wrongly - they are actually shown series-parallel there, and if you use 4x 8ohm speakers like that, it will result in an 8 ohm cab (and it is probably how the 4x10 cab is currently wirted)... If they were really all actually in series, the result would be 32 ohms... Wiring as per bottom left is full parallel, and will get you 2ohms... And correct... DON'T mismatch the output of a valve amp. If the load impedance is higher than that set on the amp, it can result in high voltages in the output stage, potentially causing flash-over of output valves and/or output transformer. If you really must mis-match it it, it is better to have a lower impedance load than that set on the amp (although this isn't exactly good either, and can cause over-dissipation in the output valves, shortening their life (albeit likely not catastrophically)) Alan Edited February 15, 2010 by AlanP2008 Quote Link to comment Share on other sites More sharing options...
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