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Impedance etc

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[quote name='jonthebass' post='453355' date='Apr 3 2009, 01:53 PM']Question bump[/quote]

I wouldn't risk using an impendence that doesn't appear as an option on a valve amp. I guess guess that choosing either 4 or 8 ohms could be close enough, but if it doesn't blow up immediately you could still be shortening the life of the valves in the output transformer. Due to the lack of forgiveness in valve technology, and given the vast array of speakers available with the right impendence for your amp, I wouldn't bother with a 6 ohm cab.

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[quote name='jonthebass' post='453069' date='Apr 3 2009, 10:42 AM']Hello Folks,
A question for you those of you with a wealth of amp knowledge:

On my Mesa Boogie Bass 400+ (all valve amp) I have 3 pairs of speaker outputs - 2x 2 Ohm, 2 x 4 Ohm and 2 x 8 Ohm. All fair and good.
I was looking at Alex's 'Big One' speaker box and noticed that it is a 6 Ohm unit.
What speaker output would I need to use or is it not worth entertaining such an idea with a valve amp?

All and any help much appreciated,
Cheers,
Jon.[/quote]

It will be happy on the 8 ohm tap, and probably on the 4. The impedance is only nominal anyway, it varies with frequency anyway. Lots of old guitar cabs use 15 ohm speakers, and the taps on my old Carlsbro are 3.75, 7.5 and 15, so there is wiggle room. That sort of efficient cab and a 400w valve amp will be ludicrously loud. I'd still go for the Vintage though.

In fact, the Mesa guitar heads manual suggests 'safe mismatches' so they must use robust output transformers, I think its the Marshalls that are delicate and gave the reputation of all valve amps being so. Sviet interceptors continued using valve radios as transistor ones would cook in event of a nuclear explosion, and associated EMP, but a valve one would be fine, so they are tough.

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Quick rule of thumb guide for all you people interested in Ohms...

This guide assumes that the speakers are wired in parallel.

Two 16 ohm speakers = 8 ohms
Two 8 ohm speakers = 4 ohms
Three 8 ohm speakers = 2.66 ohms ( use 2 OHM setting )
Four 8 ohm speakers = 2 ohms ( use 2 OHM setting)
Two 4 ohm speakers = 2 ohms ( use 2 OHM setting )
One 4 & one 8 ohm speaker = 2.66 ohms ( use 2 OHM setting )
One 5.33 & one 8 ohm speaker = 3.2 ohms ( use 2 OHM setting )
One 2.66 & one 8 ohm speaker = 2 ohms ( use 2 OHM setting )
Two 2 ohms speakers = 1 ohm
Three 4 ohms speakers = 1.33 ohms
Two 4 & one 8ohm speakers = 1.6 ohms

Edited by WHUFC BASS

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[quote name='WHUFC BASS' post='533560' date='Jul 6 2009, 09:50 AM']Quick rule of thumb guide for all you people interested in Ohms...

Two 16 ohm speakers = 8 ohms
Two 8 ohm speakers = 4 ohms
Three 8 ohm speakers = 2.66 ohms ( use 2 OHM setting )
Four 8 ohm speakers = 2 ohms ( use 2 OHM setting)
Two 4 ohm speakers = 2 ohms ( use 2 OHM setting )
One 4 & one 8 ohm speaker = 2.66 ohms ( use 2 OHM setting )
One 5.33 & one 8 ohm speaker = 3.2 ohms ( use 2 OHM setting )
One 2.66 & one 8 ohm speaker = 2 ohms ( use 2 OHM setting )
Two 2 ohms speakers = 1 ohm
Three 4 ohms speakers = 1.33 ohms
Two 4 & one 8ohm speakers = 1.6 ohms[/quote]

Is this assuming that the speakers are wired in parallel?

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I've been reading through all these posts for a while now and think I understand it (just), but would like to post my proposed setup to double check I haven't missed something.


Basically, I currently run a Sessionette 100watt combo (4x10) with a 2x10 Behringer Ultrabass BB210 cab. The cab is rated 600watts @ 8ohms. The Sessionette has an 8ohm port for the cab on the back.

However, as the Sessionette is only 100watts, it starts to show signs of weakness when the volumes are all cranked up (which is almost all the time to keep up with the rest of the band).

So, am I right in thinking, I will be ok running for example an Ashdown MAG 600H EVO II Head, with the Behringer Cab.

The ashdown is 575watts RMS at 4ohms, and the cab is 600watts and 8ohms. Additionally, is it possible that I could get more volume running the Behringer cab with a higher wattage head than running the Sessionette 100 to it's limit (curent setup).


Would it be possible to easily cause damage with this setup or do I have the wrong end of the stick?


Thanks


Roob

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[quote name='Roob' post='541079' date='Jul 15 2009, 10:45 AM']I've been reading through all these posts for a while now and think I understand it (just), but would like to post my proposed setup to double check I haven't missed something.


Basically, I currently run a Sessionette 100watt combo (4x10) with a 2x10 Behringer Ultrabass BB210 cab. The cab is rated 600watts @ 8ohms. The Sessionette has an 8ohm port for the cab on the back.

However, as the Sessionette is only 100watts, it starts to show signs of weakness when the volumes are all cranked up (which is almost all the time to keep up with the rest of the band).

So, am I right in thinking, I will be ok running for example an Ashdown MAG 600H EVO II Head, with the Behringer Cab.

The ashdown is 575watts RMS at 4ohms, and the cab is 600watts and 8ohms. Additionally, is it possible that I could get more volume running the Behringer cab with a higher wattage head than running the Sessionette 100 to it's limit (curent setup).


Would it be possible to easily cause damage with this setup or do I have the wrong end of the stick?


Thanks


Roob[/quote]

Well you could do with a more powerful head to give you more headroom, but it terms of sheer volume going to the Ashdown with the 2x10 is a bit more problematic. The MAG 600 will knock out about 325W or so into an 8 ohm load, so just over 3 times the power of your Sessionette (assuming it's rated at 100W into 4 ohms). That's about 4.5dB increase in volume.

The thing is, by losing the 4x10 speakers in the combo you'll also experience a similar drop in volume as you won't be shifting as much air (if the speakers are all of similar efficiency). You might not get an increase in volume, but you won't be straining the amp as much.

If your Behringer is rated at 600W RMS (and they're quite known for using dodgy peak/music power ratings) then you'll be fine with the MAG.

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[quote name='Roob' post='541079' date='Jul 15 2009, 10:45 AM']I've been reading through all these posts for a while now and think I understand it (just), but would like to post my proposed setup to double check I haven't missed something.


Basically, I currently run a Sessionette 100watt combo (4x10) with a 2x10 Behringer Ultrabass BB210 cab. The cab is rated 600watts @ 8ohms. The Sessionette has an 8ohm port for the cab on the back.

However, as the Sessionette is only 100watts, it starts to show signs of weakness when the volumes are all cranked up (which is almost all the time to keep up with the rest of the band).

So, am I right in thinking, I will be ok running for example an Ashdown MAG 600H EVO II Head, with the Behringer Cab.

The ashdown is 575watts RMS at 4ohms, and the cab is 600watts and 8ohms. Additionally, is it possible that I could get more volume running the Behringer cab with a higher wattage head than running the Sessionette 100 to it's limit (curent setup).


Would it be possible to easily cause damage with this setup or do I have the wrong end of the stick?


Thanks


Roob[/quote]


Bacially, you aren't going to get more volvuem from a cheap 2x10 than you are from a cheap 4x10. Can you tun the ashdown into the 4x10 somehow? Or, run both, the Ashdown has a tuner out I think so you can use it to feed the other amp, stick the 2x10 on top of the 4x10 standing tall so its pointing at your ears.

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Hmm, interesting points, bit of a loss and a gain then.

In theory, it would be great to use the 4x10 speakers in the sessionette as it's own cab without the amp (assuming they will take the Mag 600), however i'm not sure how easy it would be to implement that.

Perhaps a cheap used Behringer BB115 to compliment the BB210 would work well?

Mr.Foxen - Could you describe a bit more what you mean by running the ashdown into the sessionette? Would running the ashdown head into the sessionette amp work? I.e. are you saying it just takes the signal and then lets the sessionette run as loud as i've set it, so it works independantly from the Behringer, which will be taking it's signal and power from the Ashdown?


Cheapest option is the best atm :)

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Quite honestly I think you'd be better off ditching the combo. If it's showing signs of distress a high volume chances are it's the speakers complaining, especially if you're boosting the bass.

You can pick up a MAG 300 (scarcely quieter than the 600W model) and 4x10 for not a lot of cash used. In fact - [url="http://basschat.co.uk/index.php?showtopic=54505"]http://basschat.co.uk/index.php?showtopic=54505[/url]

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[quote name='Roob' post='541165' date='Jul 15 2009, 12:05 PM']Hmm, interesting points, bit of a loss and a gain then.

In theory, it would be great to use the 4x10 speakers in the sessionette as it's own cab without the amp (assuming they will take the Mag 600), however i'm not sure how easy it would be to implement that.

Perhaps a cheap used Behringer BB115 to compliment the BB210 would work well?

Mr.Foxen - Could you describe a bit more what you mean by running the ashdown into the sessionette? Would running the ashdown head into the sessionette amp work? I.e. are you saying it just takes the signal and then lets the sessionette run as loud as i've set it, so it works independantly from the Behringer, which will be taking it's signal and power from the Ashdown?


Cheapest option is the best atm :)[/quote]

Basically, you plug the Ashdown head into the behringer cab as one rig, then run a lead from the tuner out of the Ashdown (supposing is same as my ashdown) into the input of the Sessionette. Might need to eq fiddle. Score a big cheap 15 and run the ashdown into that with the treble right down, and turn the bass right down on the sessionette would be ideal way for this.

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Ok, thanks for the advice guys, did some thinking and will probably start saving the pennys for a good 2x10 or 1x15 combo with plenty of watts to play with (to replace the Sessionette), so I can plug my Behringer cab in.

Superb thread, some good info here.

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So wait- I have a head that is:
700W @ 2 ohms, 475W @ 4 ohms, or 275W @ 8 ohms

a 410 that is:
4 ohms. 350 Watts RMS continuous

and a 115 that is:
4 ohms. 400 Watts RMS continuous

is this good/bad? am i using this efficiently? when talking to my guitarist, should i say i have a "475W rig"?

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[quote name='M4L666' post='549461' date='Jul 23 2009, 10:19 PM']is this good/bad? am i using this efficiently? when talking to my guitarist, should i say i have a "475W rig"?[/quote]
No, you have a 700W rig.

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I have a question that I don't think has been covered!!!

I am running a MarkBass SA450, which runs 300W into 8OHMS and 500W into 4OHMS

Its well know that you want your speakers to be able handle more than your amp can give to allow a little headroom... but if you have two speakers.... do you add the Wattage together, or treat them as separate...

I want to use some portable cabs for on the go gigging.

I am planning on getting my hands on some MarkBass Traveler cabs in the near future two of these will run at 400W, 8OHMS each. Therefore my amp will be putting a 4 OHMS load into them (8ohm + 8ohm = 4ohm), meaning 500W...

So Am I likely to cause damage to the speakers, or is the combined wattage of 800W enough to take the 500W @ 4 ohm...

cheers

Edited by pantherairsoft

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Yeah, you add the power handling of each cab. The power from the amp will be split equally between the two (assuming both cabs have the same impedance, which they do), so each cab will be receiving 250w from your SA450.

It's worth bearing in mind that the 400w of the cabs refers to their thermal power rating, or how much power they can take electrically. How much they can handle mechanically can vary greatly depending on the speakers used. If you're feeding them with heavily compressed sounds and using a lot of bass boost they could run out of excursion even though they exceed the amp's rated power.

Just use your ears - if you hear them complaining turn the amp down or reduce the bass level.

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[quote name='stevie' post='549473' date='Jul 23 2009, 09:29 PM']No, you have a 700W rig.[/quote]
don't say that it'll just encourage him to get a bigger rig!

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[quote name='Musky' post='645088' date='Nov 4 2009, 11:31 AM']It's worth bearing in mind that the 400w of the cabs refers to their thermal power rating, or how much power they can take electrically. How much they can handle mechanically can vary greatly depending on the speakers used.[/quote]

Exactly! See Finbar's dead Schroeder driver to for an example of mechanical failure.

[quote name='Musky' post='645088' date='Nov 4 2009, 11:31 AM']If you're feeding them with heavily compressed sounds and using a lot of bass boost they could run out of excursion even though they exceed the amp's rated power.[/quote]

Bass boost yes, compression no. The more compression the easier a time the speaker has mechanically (but the worse a time it has thermally).

[quote name='Musky' post='645088' date='Nov 4 2009, 11:31 AM']Just use your ears - if you hear them complaining turn the amp down or reduce the bass level.[/quote]

And that's all you need to know! (And if you're going for dirty sounds so you can't distinguish noises of complaint, then err on the side of caution by using more speakerage).

Alex

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Cheers for the words of advice. I do use lots of effects, but not too much overdrive and distortion... and I never 'push' my amp... As of yet I have never had my gain or volume past 50%. I like a nice clean, smooth tone.

Cheers again for the guidance!

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My Marshall DBS 200w has 2 speaker outputs on the back,i have a 2x10+1x15 @ 4 ohms cab,and a 1x15 @ 8 ohms cab.
Excuse my numptyness,but what affect will this have on the amps output ?
On the rear of the amp it states,minimum 4 ohms.

Edited by hillbilly deluxe

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[quote name='hillbilly deluxe' post='680109' date='Dec 10 2009, 12:07 PM']My Marshall DBS 200w has 2 speaker outputs on the back,i have a 2x10+1x15 @ 4 ohms cab,and a 1x15 @ 8 ohms cab.T
Excuse my numptyness,but what affect will this have on the amps output ?
On the rear of the amp it states,minimum 4 ohms.[/quote]
Nothing good - the combined impedance of the 2x10/1x15 will be 2.6666 Ohms (which is less than the 4 Ohms minimum impedance your amp is capable of driving) so, DON'T DO IT!!! :)

Here's the science bit: Impedances in parallel are not simply additive, in fact the formula is 1/R1 + 1/R2 + ... 1/Rx = 1/Rt (being the total)

Therefore your set-up is 1/8 + 1/4 = 3/8, making your Rt 8/3, or 2.66666 Ohms

Hope this explains things.

Ian

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my power amp has the following specs.

Specifications
4Ω stereo: 2 x 300Wrms
8Ω stereo: 2 x 150Wrms
8Ω bridged: 1 x 400Wrms
Max power: 800Wmax @ 8Ω

I run it into 2 cabs rated 500 @ 8ohms, so i get 150 watts per cab meaning 300 watts? is this correct?

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