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

[ashevans09]

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

[obbm]

[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.

[Delberthot]

If all the drivers are equal then there shouldn't be any difference sound wise between the 8 ohm and 4 ohm cabs.

I use a 4 ohm cab so that I can get the full potential out of the amp - others may have 2 8 ohm cabs to do this.

[Mr. Foxen]

Some valve amps don't go down to 4 ohms, if they do, an 8 ohm cab is still better, as you have the option of a second. The 'full potential' doesn't really apply to valve amps, as they aren't the same as solid state ones, where the output is dependant on load.

[Delberthot]

It really does help to read the whole question. I was in solid state amp mode.

my bad

[ironside1966]

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

There are many factors that determine how loud a amp is and a 100w is not twice as loud as 50w

Hope that helps

[Bill Fitzmaurice]

[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.

[bremen]

[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.

[fleabag]

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 ?

Edited January 30, 2009 by fleabag

[Bill Fitzmaurice]

[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.