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FRANZONI
Double Platinum Member
Ireland
3543 Posts |
 Posted - 04/08/2008 : 10:53:12
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Hi all
i've been trying for a while now to lower the bands stage volume and by and large it's working because i've been using a 15 watt selmer combo but in some of the bigger venues i would like to use my marshall and 2x12..the question is this...my cab at the moment is loaded with two 8 ohm celestion V30 speakers,wired in parallel if i was to rewire it for series the amp should see a 16 ohm load on the cab...?? at the moment it would see a 4 ohm load...i have always understood that amps run loudest at 4 ohms so you can probably see what i'm getting at here...would rewiring it to series make a big difference and would it affect the sound.i usually run it clean(marshall superbass head using it for guitar)...i'm also planning on cutting a slot out of the back panel of the cab to make it open back as well as i normally use open back combos.... any advice welcome...thanks...... |
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Goran
Double Platinum Member
Sweden
2203 Posts |
Posted - 04/08/2008 : 11:25:15
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Well, in the real world the issues you write about don�t make that big difference, the most important factor to consider is the speakers sensivity (or what it�s called, if you put 1 watt in how many do you get out). For example Celestion Vintage 30 has about 100 db �sensivity� whereas an old Jensen has maybe 92 db, that makes a hell of volume differences if you�re using the same amp and settings.
I�ve searhed the web for information, but�.. I highly recommend Dave Hunters book of Tube amps, on this question and everything you want to know about tube amps. Great book!!!!
http://www.amazon.com/Guitar-Amp-Handbook-Understanding-Amplifiers/dp/087930863X
I found this, interesting:
http://guitarplayer.wordpress.com/category/how-to-choose-the-right-guitar-speaker-cabs/
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Edited by - Goran on 04/08/2008 11:26:47 |
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DeFrag
Moderator
USA
3409 Posts |
Posted - 04/08/2008 : 15:48:47
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So, the higher the sensitivity of a given speaker, the louder it will be compared to one of a lower sensitivity?
Does a set of speakers in parallel sound different than a set wired in series? |
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Laurie
Double Platinum Member
Canada
4854 Posts |
Posted - 04/08/2008 : 16:26:25
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quote:
Originally posted by DeFrag
Does a set of speakers in parallel sound different than a set wired in series?
Does the impedance change? ie. is it real-world - the same speakers wired in parallel (4 ohms) then series (16 ohms)? Or is it a theoretical question - impedance is the same either configuration?
If theoretical, there will be no difference.
If real world, then "it depends"... assuming the amp can have the output impedance changed from 4 to 16 ohms, then the sound will depend on a number of factors, in my experience mainly on the damping factor of the amp at the different impedances.
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FRANZONI
Double Platinum Member
Ireland
3543 Posts |
Posted - 04/08/2008 : 19:04:01
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| i was going on the data on what i've seen on power amps....i.e 200 watts@4 ohms...130 watts@8 ohms etc....so my theory was if i wired the cab to take 16 ohms and the changed the setting on the back of the marshall it might take some of the 'sting' out of it volume wise...i take on board gorans point about speaker sensitivity...i also happen to have a couple of 25 watt greenbacks i could put in with a SPL of 96db rather than the V30's 100db and i as i have posted before..i have put T.A.D tonebones in this amp..basically they are adapters for the power valves that let you put EL84's in rather than the regular EL34's..this brings the power amp section down to 32 watts...the manufacturer reckons that in the pentode versions i have in a marshall amp,about 8 watts per valve...all i'm afraid with the greenbacks is,as far as i can remember they broke up fairly quickly soundwise and a clean sound is important to me....so am i way of course with this theory and the end result sound wise and volume wise will be the same as now...... |
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Laurie
Double Platinum Member
Canada
4854 Posts |
Posted - 04/08/2008 : 19:20:44
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quote:
Originally posted by FRANZONI
i was going on the data on what i've seen on power amps....i.e 200 watts@4 ohms...130 watts@8 ohms etc....so my theory was if i wired the cab to take 16 ohms and the changed the setting on the back of the marshall it might take some of the 'sting' out of it volume wise...i take on board gorans point about speaker sensitivity...i also happen to have a couple of 25 watt greenbacks i could put in with a SPL of 96db rather than the V30's 100db and i as i have posted before..i have put T.A.D tonebones in this amp..basically they are adapters for the power valves that let you put EL84's in rather than the regular EL34's..this brings the power amp section down to 32 watts...the manufacturer reckons that in the pentode versions i have in a marshall amp,about 8 watts per valve...all i'm afraid with the greenbacks is,as far as i can remember they broke up fairly quickly soundwise and a clean sound is important to me....so am i way of course with this theory and the end result sound wise and volume wise will be the same as now......
Hi Franzoni! I was responding to DeFrags post rather than answering your original question.
It is guaranteed that if you wire the speakers in series - 16 ohms - and you do not change you amp output transformer tap (leave it at 4 ohms) that the volume will be lowered.
Volume is logarithmically related to power, and multiplying the speaker impedance by a factor of four while leaving the drive voltage the same (not changing the tap on the amp - as long as your amp can take it!) will change the power to the speakers like this:
P = (V^2)/R
so increasing R by 4 times reduces the power to a quarter what it was before (it's a bit more complex than this in real life, but this is close). One quarter the power is (if i recall corectly) 6dB down, so you will definitiely hear it.
Don't try it unless you know your amp can take it though... increasing the load impedance on tube amps can sometimes cause problems.
If you change the impedance on the amp, the power out of the speakers will stay the same - the impedance change re-matches the amp and speaker. Maximum power transfer occurs when the source (amp) and load (speaker) have the same impedance.
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Edited by - Laurie on 04/08/2008 19:23:22 |
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FRANZONI
Double Platinum Member
Ireland
3543 Posts |
Posted - 04/08/2008 : 21:29:40
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R = Resistance...? so what i'm doing is increasing the resistance by 4 by wiring them in series and leaving the amp on 4 ohms...? i think i read before that pushing 4 ohms into a 16 ohm load is ok but not the other way around...?? i'm sorry for all the questions lauries and guys but i'm just a guitar player...i don't have any background in electronics....i'm afraid i'm a bit of a dabbler when it comes to this stuff... i was also playing with the idea of getting a couple of square feet of perspex and putting it in front of the speakers to attenuate them and cutting the slot out of the back panel of the cab as well as we mike all the gear anyway on stage including the drums..... |
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FRANZONI
Double Platinum Member
Ireland
3543 Posts |
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Laurie
Double Platinum Member
Canada
4854 Posts |
Posted - 04/08/2008 : 22:50:24
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quote:
Originally posted by FRANZONI
just found this......
http://www.marshallforum.com/marshall-amps/452-newbie-power-tube-pulling.html
according to some of these guys i should set the head to 8 ohms if i'm gonna rewire the cab to 16 ohms......
That sounds right - cab wired in series will be 16 ohms, set the amp so it doesn't have a problem with the 16 ohms (if they say 8 ohms, they would know best). It will drop the volume by 3dB with the amp at 8 and the cab at 16. You won't really notice 3dB, so probably not worth the effort.
Sorry if I was vague before!!
I have a couple of thoughts on the perspex idea.
1) if it's too close to the speakers, it may damage them physically by changing their acoustic environment too much (speakers are not meant to 'drive into a wall").
2) Even if the speakers are OK, too much back-emf might be generated by the compression effects between the speakers and the perspex.
In short - it's not something I'd do with my amp if the perspex was going to be closer than a couple of feet away.
If trying to lower the volume but keep the tone, it might be worth considering an attenuator?
http://www.bossarea.com/forum/topic.asp?TOPIC_ID=4128
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Edited by - Laurie on 04/08/2008 22:51:57 |
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FRANZONI
Double Platinum Member
Ireland
3543 Posts |
Posted - 04/09/2008 : 08:56:04
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thanks lauries for the replies..... the perspex idea i got from a guy called joe bonamassa who uses it to lower the stage volume ..it would be a couple of feet off the front of the speakers....i have a master volume musicman rated at 130 watts and it's nowhere near as loud of as sensitive as the marshall..i can see now why hendrix and clapton we able to play places like the royal albert hall without being miked..... ..... |
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Laurie
Double Platinum Member
Canada
4854 Posts |
Posted - 04/09/2008 : 15:39:06
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No worries! The drummer in my old band used to use a perspex shield and it worked fine.
Stage volume is always a delicate thing in my experience. So much time and energy goes into practising and gear purchase and setting-up that we just "want to be heard", eh?
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zentropa
Gold Member
USA
837 Posts |
Posted - 04/09/2008 : 15:59:53
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note: i have no scientific backing for this, just personal experience with my own amps/pa's and doing live sound a few times... a lot of this is based upon my ability to cut, how much push the cabs had, etc.
i've always found that amps are the "weakest" when outputting 4 ohms and driving a 16 ohm load.
i've always found that my amps were the "strongest" when outputting 8 ohms and driving an 8 ohm load.
strongest to weakest
8 ohm into 8 ohm
8 ohm into 16 ohm
4 ohm into 4 ohm
4 ohm into 8 ohm
4 ohm into 16 ohm
with the "weaker" setups i would often found the tone i wanted but with much less speaker push and often i had trouble cutting if playing with someone with a ballsy amp.
not sure if what i am posting agrees with the physics of the situation, but this was just based upon my experiences. a lot of em were from an era where my amp head could output at 4 or 8 and i had 2 2x12" cabs that could be switched from 8/16 loads. i experimented with a lot of parallel vs. series setups, etc.
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Laurie
Double Platinum Member
Canada
4854 Posts |
Posted - 04/09/2008 : 16:20:21
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The only thing that disagrees with theory is putting 8 ohm into 16 ohm before 4 ohm into 4 ohm.
What's probably happening at 4 into 4 is to do with emf. Speaker control is dependent on emf and the higher the impedance the higher the emf in the circuit - the low impedance of 4 ohms means the speakers aren't "as controlled" as they are at higher impedance - not being pushed to maximum excursion - so don't sound as loud?
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DeFrag
Moderator
USA
3409 Posts |
Posted - 04/09/2008 : 16:30:23
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| I'm liking Joe Bonamassa these days.. |
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zentropa
Gold Member
USA
837 Posts |
Posted - 04/09/2008 : 16:35:44
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laurie:
was probably my amp. it sucked. couldn't cut to save my life.
last show i used it at i got blown off the stage by another guitarist using... a fender deluxe 112+... 90 watt 1x12 solid state combo.
at the start of the show i was running a stereo 2x12 setup w/ dual 100 watt 4 ohm into 4 ohm loads (stereo 2x12's). by the end of the show i was running bridged 200 watt 8ohm into 8ohm (1 2x12). never was able to cut but managed to blow out a speaker trying :)
i went out and bought a twin reverb 2 weeks later.
another non-scientific but based on my personal experiences way of reducing volume... would be to beef up the speaker wattage. e.g. instead of 60 watt v30's, go for something more like 100+ watts each. will probably affect the tone a bit but will also reduce volume and push. downside is it would increase the cab weight and will probably suck some tone. |
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Dr. Bob
Moderator
Australia
6593 Posts |
Posted - 04/09/2008 : 17:38:42
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Hi Guys
I have stayed out of this discussion until now.
I think you have all missed the fact, that Franzoni is using valve guitar amplifiers.
If I read all my valve theory correctly, & in simple terms,
valve amps really don't like running into mismatched loads.
The mismatch will be reflected back through the output transformer, & back into the valves of the output stage.
Creating possible instability, & worst case failure of the output stage of the amplifier.
Sure it might not happen right away, but you will be putting undue stresses, on your amp.
Because of the high voltages in valve amplifiers, it's very easy to get flash-over, of the high voltage, & a possible series of cascading faults, ---- & very quickly.
A lot of valve amps will burn out an output transformer, if the speakers are disconnected or blow & go open circuit.
Hence the reason that most valve amps have multiple output impedance windings-taps on the transformer.
To keep the output stage running the way the designer intended.
Running higher impedance loads on transistor amplifiers is generally OK,
you will get a decrease in in apparent volume, as the output stage now has to run into a higher resistance/impedance.
I think this is what Franzoni was talking about in his posts.
Or has been lead to believe.
But the theory does not transpose itself, directly into the valve world.
If you really want to drop the output level, you could put a voltage dropping resistor network, in series with the B+ (high voltage) that is on the plates of the output tubes.
This is what Mesa do on their earlier amps to get lower power.
like on the mark series, they have a 50 or 60W high power mode, & also a 15W low power mode, by dropping the B+ Voltage to the output tubes.
My "Fender 75" valve amp, also has this arrangement, & a 15W low power mode.
But remember you are talking about some serious wattage resistors,
one of the resistors in the "Fender 75" is rated at 30 Watts, & it does get hot  when it's in circuit.
I hope this has made some sense, but more likely it's just kicked up a lot more gig dust.
Franzoni, if you lived closer, I would be happy to do the low power mod for you.
But Dublin is a bit of a walk from my house Oz.
And besides, my soldering iron would get cold by the time I got there..
Regards Dr. Bob
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wiring speaker cabs...
