Quick question about impedance

[ATSbassman]

Got a 8 Ohm cab but my valve head is 4 Ohms

What does this mean when running them both together? Am I a risk or blowing something up? Running at less power?

Sorry if this has been ask before (Most likely has hahaha) but I am hoping someone can explain it all to me in relatively easy terms, thanks

[Happy Jack]

No problem.

It's the other way around that's a problem, i.e. running an 8-Ohm head into a 4-Ohm cab.

Running a 4-Ohm head into an 8-Ohm cab simply means that your head won't be putting out all the power it's capable of. If (like me) you're fond of headroom, you won't even notice.

[spacey]

Yes it will be fine.
Some say you will only get half power, but this does not mean half volume.
Watts are not volume, 200w is not twice as loud as 100w, it is only a couple of DB more.
What it does mean is the amp wont be running near the limit, as anyone who has had a trace eliiot turn in to a smoke machine knows.

[chris_b]

Most commercially available amps have a 4 ohm minimum.

That means you can run 1 x 8 ohm cab, 2 x 8 ohm cabs ( 2 x 8 ohms = 4 ohms in this case) or 1 x 4 ohm cab.

That's all.


Also;
A 500 watt amp running at 4 ohms will put out full power. If it's running 1 4 ohm cab it will be putting out a max of 500 watts, if it's running 1 8 ohm cab it will be putting out a max of approx 300 watts.

You would hardly notice the difference between 500 watts and 300 watts but you will notice the difference bewteen runing 1 cab and 2 cabs. More speakers sound bigger, louder and IMO better.

Edited January 12, 2014 by chris_b

[dincz]

I think the posters so far have missed the fact that it's valve amp.

A high load impedance is not a problem for a solid state amp (usually), but with a valve amp it could be a problem. Nasty things (like high voltage spikes) can happen in the output transformer when running into a higher than ideal load impedance. In this case it's not a serious mismatch and you might get away with it, but available power output will be reduced.

[chris_b]

You're right.... missed that.

For valve amps you have to match the cab to the amp or you will risk damaging the biasing and/or valves.

You need an 8 ohm cab on the end of the speaker lead if the amp is set to 8 ohm output. My last valve amp had 1 8 ohm outputs and 2 4 ohm outputs. I guess which way you hook up the cabs is dependent on how the outputs are configured.

Which amp? Do you have the user manual?

Edited January 12, 2014 by chris_b

[Dad3353]

[quote name='ATSbassman' timestamp='1389525421' post='2334984']
Got a 8 Ohm cab but my valve head is 4 Ohms

What does this mean when running them both together? Am I a risk or blowing something up? Running at less power?

Sorry if this has been ask before (Most likely has hahaha) but I am hoping someone can explain it all to me in relatively easy terms, thanks
[/quote]

Most valve amps need to match the speaker impedance to the output transformer. This is important, and a mis-match could provoke expensive damage to the amp. Most amps have, for this, a means of switching, or separate speaker sockets, in order to obtain this match. Does your amp not have any other possibility than 4 ohms..? That would be odd.
There is an exception worth mentioning. Many 100w amps have two pairs of o/p valves, and may be run in a reduced power mode, typically by removing one pair of valves (some amps can be switched for this to happen...). In this case, the o/p transformer 'sees' a different load; to compensate, the 4 ohm setting would be correct for a 8 ohm cab. This only applies if this 'half-power' mode is being used. Is this the case with your amp..?

Edited January 12, 2014 by Dad3353

[0175westwood29]

[quote name='chris_b' timestamp='1389527344' post='2335027']
You're right.... missed that.

For valve amps you have to match the cab to the amp or you will risk damaging the biasing and/or valves.

You need an 8 ohm cab on the end of the speaker lead if the amp is set to 8 ohm output. My last valve amp had 1 8 ohm outputs and 2 4 ohm outputs. I guess which way you hook up the cabs is dependent on how the outputs are configured.

Which amp? Do you have the user manual?
[/quote]

what chris said!

andy

[ATSbassman]

I'm running a Mesa Boogie Basis M2000 into a Ashdown ABM 410T

Attached is a photo from the manual which may be able to give some idea, to me it reads it will be okay but only use 300w

Edited January 12, 2014 by ATSbassman

[obbm]

M2000 is not a valve amp. It has a valve pre-amp and MOSFET (solid state) power stage. It'll be fine with 8-ohms though you'll get a reduced maximum power output. It most situations I doubt that you'll notice the difference from a similar 4-ohm cab.

[BassTractor]

[quote name='obbm' timestamp='1389535963' post='2335174']
M2000 is not a valve amp. It has a valve pre-amp and MOSFET (solid state) power stage. It'll be fine with 8-ohms though you'll get a reduced maximum power output. It most situations I doubt that you'll notice the difference from a similar 4-ohm cab.
[/quote]

This of course is very true, but it may stop the thread.
To keep the thread running to and fro, I think it's best if we adopt the notion that the Ashdown ABM 410T is a [b]valve cab[/b]!

(Sorry, ATSbassman, I couldn't resist.
OBBM is right, but I just found it funny how the thread developed, and the initial, wrong, answers became correct after all.)