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Any particular benefit from using a 4ohm or an 8ohm cab?


ashevans09
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Hey there,

Being the sad and dozy sod that I am I've been wandering around various forums and came across the info that an all valve head puts out the same wattage at any ohmage. So, that considered, were you to run a cab with an all valve head, is there any particular benefit in going for an 8ohm cab over a 4 ohm cab?

Ta! ;)

Ash

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[quote name='ashevans09' post='394228' date='Jan 28 2009, 11:56 PM']Hey there,

Being the sad and dozy sod that I am I've been wandering around various forums and came across the info that an all valve head puts out the same wattage at any ohmage. So, that considered, were you to run a cab with an all valve head, is there any particular benefit in going for an 8ohm cab over a 4 ohm cab?

Ta! ;)

Ash[/quote]

The most important thing is that you select the output transformer tap to match the cabinet impedance.

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[quote name='ironside1966' post='394571' date='Jan 29 2009, 09:10 AM']As a rough guide most amps rate there power at 4 ohms, a 100w amp will deliver
100w into 4 ohms.
50w into 8 ohms.
25w into 16ohms[/quote]
With SS amps output current delivery versus impedance load at full power isn't linear. A 50w/8 ohm amp can be expected to deliver 70 watts into 4 ohms, 35 watts into 16 ohms. As for the notion of running as low a load as possible to 'get all the watts out of my amp', in most cases that's not a good idea. Most cabs can't make use of more than half their rated power before the drivers run out of excursion, and running any amp at minimum load impedance significantly raises the amount of heat generated in the amp, significantly lessening component life. As in all things, moderation.

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[quote name='Bill Fitzmaurice' post='394627' date='Jan 29 2009, 02:59 PM']With SS amps output current delivery versus impedance load at full power isn't linear. A 50w/8 ohm amp can be expected to deliver 70 watts into 4 ohms,[/quote]

Which means that the increase in volume going from 8 ohms to 4 ohms is 1.5dB - hardly audible.

[quote]As for the notion of running as low a load as possible to 'get all the watts out of my amp', in most cases that's not a good idea. Most cabs can't make use of more than half their rated power before the drivers run out of excursion, and running any amp at minimum load impedance significantly raises the amount of heat generated in the amp, significantly lessening component life. As in all things, moderation.[/quote]

indeed.

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[quote name='fleabag' post='395105' date='Jan 29 2009, 07:56 PM']Does that also work in reverse , Like a 50w/4ohm amp into an 8 ohm speaker would only lose 1.5db, The difference being Still barely audible ?[/quote] At low power where the amp isn't stressed you'll get 3dB, with a halving of impedance, though 3dB isn't all that much either. But at full power there's not a lot of advantage, so if your amp is inadequate into an 8 ohm load it will likely be inadequate into a 4 ohm load. Conversely if you have plenty of output into 4 ohms you'll have plenty in to 8 as well. All things considered 8 ohm cabs are better because if you ever need to use two you can.

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