2 x 12 Speaker Cabinet wiring question

[thefruitfarmer]

Greetings All.

I have a couple of Celestion Speakers, G12-T80 from a Carslboro GLX 150C combo that has now died but I like the speakers, they really fill the room and sing when they get going. They are probably similar to these http://celestion.com/product/15/g12t75/

What I thought I could do is to have a 2 x12 cab out of it. However, as the Carlsboro is a stereo combo the speakers are 8 ohms each which now gives me too many options really as conventional wiring in either series or parallel would result in either a 16 ohm or 4 ohm cab, when I am thinking I really want an 8 ohm cab......

So...........

Do I really need to buy an empty cab? I could just leave the speakers in the combo and remove the defunct amp. The speakers sound fine in that box as it is.

Is it a good idea to wire it up in stereo? Then I would have two 8 ohm inputs which could work, I have seen several amps that can drive a pair of 8 ohm loads, I could get a stereo amp or have two different amps, one driving each speaker.

What do you all think?

I have had a bit of trouble finding a suitable back panel for stereo speaker inputs so if anyone has a link to a company that sells them then would be great.

[ore_terra]

Yes to all questions: of course you can take the amp out and keep the speakers only, and you can wire them independently and get 2 8ohm inputs.

Here a few jack plates

https://www.tubesandmore.com/products/jack-plate-plug-and-play-monostereo
https://www.amazon.com/Jack-plate-Mono-Stereo-Plug/dp/B00E1P4LGE
http://www.mojotone.com/amp-parts/Jack-Plates-and-Ferrules-for-Amplifiers/OEM-Stereo-Mono-Cabinet-4x12-Wired-Jack-Plate-Double-1-4_2
https://avatarspeakers.com/shop/parts/guitar-input/
http://www.allparts.uk.com/products/marshall-style-stereo-mono-cabinet-jack-plate
https://m.thomann.de/gb/marshall_buchsenanschlussplatte.htm?o=54&search=1486203594

The Petra Flowers

[Wonks]

What amp are you planning on driving the cab with, Fruity? A solid state mono amp should cope with a 4 ohm load, and a valve amp with 16.

Or you could always go bigger, find another couple of speakers and make yourself an 8 ohm 4x12" cab.

[Sam Spoons]

What amp are you planning to use with this? I'd guess all stereo power amps will drive a pair of 8 ohm speakers (and 4 ohm too). Only thing is most guitar amps are not stereo and (sorry if I'm stating the obvious, and for the benefit of any noobs) two speaker sockets doesn't make it stereo. Either way solid state amps should be happy with the resulting 4 ohm load but a valve amp needs to be correctly matched. As Wonks says shouldn't be a problem to work out something suitable though.

[thefruitfarmer]

Thanks for the replies, I will check out the links and see what they have got.

As for the amp I am wanting to use, I am probably going to go through a selection over the next few years.

What amp are you planning on driving the cab with, Fruity? A solid state mono amp should cope with a 4 ohm load, and a valve amp with 16.

Or you could always go bigger, find another couple of speakers and make yourself an 8 ohm 4x12" cab.

What I don't want is a big and heavy 4x12 lol.

I am looking at getting a Quilter 101 http://www.quilterlabs.com/index.php/products/mini-head-amps/101-mini-head which will drive two 8 ohm speakers, and would be happy at 4 ohm and 16 ohm too.

There is a way of "having your cake and eating it" here



This Engl Cab here has the facility to switch between two stereo 16 ohm inputs at 60 Watts each to one mono 8 ohm input at 120 Watts. I could have something similar and have the option of switching between two 8 ohm inputs and one 16 or 4 ohm input?

I am keen to keep the two 8 ohm inputs available as I might want a multi amp set up or a stereo amp at some point. The combo had a stereo chorus which I liked and might want to get back, with a stereo amp, eventually.

[thefruitfarmer]

Yes to all questions: of course you can take the amp out and keep the speakers only, and you can wire them independently and get 2 8ohm inputs.

Here a few jack plates

https://www.tubesandmore.com/products/jack-plate-plug-and-play-monostereo
https://www.amazon.com/Jack-plate-Mono-Stereo-Plug/dp/B00E1P4LGE
http://www.mojotone.com/amp-parts/Jack-Plates-and-Ferrules-for-Amplifiers/OEM-Stereo-Mono-Cabinet-4x12-Wired-Jack-Plate-Double-1-4_2
https://avatarspeakers.com/shop/parts/guitar-input/
http://www.allparts.uk.com/products/marshall-style-stereo-mono-cabinet-jack-plate
https://m.thomann.de/gb/marshall_buchsenanschlussplatte.htm?o=54&search=1486203594

Why thanks...

Now I have actually bothered to follow those links there are jack plates that give all the options:

one 16 ohm mono input

one 4 ohm mono input

and two 8 ohm inputs

Everything covered!

[Wonks]

Or you could build two 1x12" cabs. Stereo effects do work better when the cabs are further apart, especially when gigging without the cabs miked up. Otherwise the area where the stereo effects can be heard is quite small.

But if you don't want to do that and use the existing combo enclosure, then one of those plates that ore-terra linked to will do the job.

There is obviously a small risk that for those with a switch, at some point in the future, someone could unintentionally play with the switch and leave it in the wrong position and you then plug-in as normal with bad results for the amplifier(s) - especially if you are using it as a stereo set-up and the switch is set to mono.

Likewise, the plug-only plate versions have the easy ability for a jack to be inserted into the wrong sockets. The descriptive labels on them leave much to be desired for clarity. Also, you are reliant on switched jack sockets to work each time a jack is inserted. We know that unused jack sockets (e.g. on unused serial FX loops) can oxidise over time and cause signal problems. An annoyance on a line-level signal but potentially tube amp-damaging.

If I was using a valve amp, I'd certainly prefer to have a proper switch designed with a guaranteed minimum number of operations. Solid state amps are generally forgiving of open and short circuits, but not valve amps.

[Sam Spoons]

The Quilter amp you mention appears to have a mono, solid state power section. The two sockets will be happy with a combined load of down to 4 ohms. Quilter don't give any meaningful specifications regarding power output but you can be pretty sure it won't be as loud into 16 ohms as it will into 4 ohms.

I'd wire the cab using two sockets, one with switch contacts. Wire socket 1 through the switch contacts on socket 1 so that if you plug into S1 you get both speakers in parallel and if you plug into S2 you get two speakers separately.

[thefruitfarmer]

There is obviously a small risk that for those with a switch, at some point in the future, someone could unintentionally play with the switch and leave it in the wrong position and you then plug-in as normal with bad results for the amplifier(s) - especially if you are using it as a stereo set-up and the switch is set to mono.

............................

If I was using a valve amp, I'd certainly prefer to have a proper switch designed with a guaranteed minimum number of operations. Solid state amps are generally forgiving of open and short circuits, but not valve amps.

Fair points Wabbit, I do believe that if it is possible for a mistake to be made that someone at some point will inevitably make it.

I was over a mates yesterday and when he bought his 2nd hand amp and cab the seller put tape over the input sockets on the cab that were not to be used in the current set up, just in case the ohmage and wattage explanation got lost on the way home.

Some of those jack plates have a rocker switch, which could be knocked by accident and some of the others have a recessed side to side switch so I will probably go for the recessed version.

[thefruitfarmer]

I'd wire the cab using two sockets, one with switch contacts. Wire socket 1 through the switch contacts on socket 1 so that if you plug into S1 you get both speakers in parallel and if you plug into S2 you get two speakers separately.

That sounds like a pretty fool proof solution.

[Sam Spoons]

Yup, even if you plug it in wrong you'll still have at least one speaker connected. Most valve amps will survive running into 8 ohms on their 4 ohm setting without suffering serious harm (definitely not recommended though especially running loud). Short or open circuit OTOH could easily fry the OP tranny. Solid state amps are usually pretty good at looking after themselves.

[Wonks]

I thought it was the other way round Sam, as I thought valve amps don't like a higher load than the amp is set to - open circuit being the worst example - but can normally take driving into a lower load.

Isn't there some easy mnemonic to remember which way round it is?

[Sam Spoons]

You may be right Wonks, I guess one step either way would not usually cause a problem (with the usual disclaimer) as long as you weren't caning it, Sabbath levels and all bets are off.

Either way it's always a risk running a valve amp into the wrong impedance (I have done it a few times over the years, when the "numpty factor" kicked in, and suffered no lasting effects but I'm usually pretty careful. "phew, got away with it" being the thought rather than "it'll be alright").

[Wonks]

Ef37a normally speaks with thunder and lightning about going even one step the wrong way, but it may depend on the sizing of the output transformer and how loud the amp is etc. as to whether any damage occurs.

Checking round on the web, it's definitely driving a higher impedance load than the transformer is set for that will do the most damage. Driving the next impedance setting down can be done, but will drive the valves harder and shorten their life a bit. Driving into a higher impedance is less work for the valves but can cause very high 'flyback' voltages from the transformer that are also bad for the valves and also for any wiring insulation or capacitors that aren't rated for those voltages, So these high voltages can then damage the insulation on the transformer windings. They start to short and then everything starts going wrong in the power section.

Modern amps should be capable of taking a step up or step down in setting without doing permanent damage except for shortening the power amp valve, but vintage amps normally had transformers tightly specified to the amp requirements (i.e. lowest cost) with little spare capacity. And with most early amps being combos, they generally didn't have to worry about extension cabs at the time so it didn't matter much.

[ef37a]

Ef37a normally speaks with thunder and lightning about going even one step the wrong way,.

Well yes, a bit! As with many things electronic, it depends. As you rightly say Wonks, driving the next impedance up from the amp setting (an 8 R load on a 4R tap say) is almost always safe in almost all circumstance EXCEPT if you are driving the beans off a big amp and especially a fixed biased amp as against cathode biased. "Big" I would define as more than 15wattts (but then lower power amps TEND to be cheaper and with cheaper components..'Pends, see?)

Then, some amps are load optimized at about 8 Ohms and can drive any common speaker load. "Our" very own HT-1 can handle 4-16 Ohms quite happily but it is of course very low power. Loudspeakers are not in any case a resistive load, why we speak of "impedance". The load increases with frequency (Z is usually specified at 400Hz) and even properly loaded amplifiers can fail due to excessive voltage spikes. Some designs incorporate flyback protection diodes in the OP circuit. B's use a different solution (NOT foolproof!) and I had zero OPT failures, even a 100W that was driven had and long enough into buggerall load (OC) just melted a couple of EL34s and blew fuses.

You can of course always put a resistor across the load equal TO the load to bring 8R say back to 4. The resistor should have a power rating equal to the speaker. Wasteful and a bit "wrong"? For sure but if you intend to drive the bits off an amp it should have the correct load. For bedroom levels a 1:2 mismatch is fine but manufacturers (well their lawyers actually!) will tell you nay!

Dave.

[Hugh Robjohns]

...it's definitely driving a higher impedance load than the transformer is set for that will do the most damage. ...

As you rightly say Wonks, driving the next impedance up from the amp setting (an 8 R load on a 4R tap say) is almost always safe in almost all circumstance

Er... aren't you saying completely opposite things to each other here?

Any chance of getting your ducks lined up in the same row for the benefit of anyone that's now horribly confused?

H

[ef37a]

...it's definitely driving a higher impedance load than the transformer is set for that will do the most damage. ...

As you rightly say Wonks, driving the next impedance up from the amp setting (an 8 R load on a 4R tap say) is almost always safe in almost all circumstance

Er... aren't you saying completely opposite things to each other here?

Any chance of getting your ducks lined up in the same row for the benefit of anyone that's now horribly confused?

H

No Hugh. Wonks and I agree that one setting up of LOAD resistance to AMP setting, e.g. 8R load, 4R setting as I said, IS safe in ALMOST all circumstances. The exception, where we STILL agree, is when a "big" amp, >15W say, is driven very hard and into prolonged clipping, a situation that is fairly common with guitar amps.

I also said (and Mr W might have as well?) that there is not even a free lunch with a valve amp when it IS correctly loaded. The fact is some amplifiers, especially older models can be a bit fragile! Failure is always a possibility even in the best circumstances.

A very good proportion of my time was spent driving amps to and beyond their limits to ensure reliability. Ultimately a valve or/and a fuse blew. Guitar amp mnfctrs know that their product will be abuse far more than almost any other piece of electronics and design and test accordingly. Or should do!

Dave.

[Sam Spoons]

Now I'm confused Dave, t'was me wot said

Most valve amps will survive running into 8 ohms on their 4 ohm setting without suffering serious harm

(I did add that it most definitely wasn't recommended) and Wonks who said

I thought it was the other way round Sam, as I thought valve amps don't like a higher load than the amp is set to - open circuit being the worst example - but can normally take driving into a lower load.

?

[Hugh Robjohns]

No Hugh. Wonks and I agree that one setting up of LOAD resistance to AMP setting, e.g. 8R load, 4R setting as I said, IS safe...

I really don't think that's what Wonks was saying:

...driving a higher impedance load than the transformer is set for ... will do the most damage. ...

This seems to be directly contradictory to me... I don't know who is right, but I think we ought to settle on a consistent story!

H

[Sam Inglis]

I was told by my amp technician that I could safely drive my Leslie (which I think is a 3.5 Ohm load) from a valve amp set to 7.5 Ohms. Tried it and it seemed fine, though the Leslie can't take much level so I didn't have the volume turned up very high.

[ef37a]

Ok chaps! Let's see if I can nail this.

Valve output stages:
Safest situation, amp transformer tapping "matches" rated speaker impedance (Z) but DO note that this does not mean total safety, driving an amp very hard can cause arcing and valve/OPT damage. VERY rare with good designs but if you have a £5000 vintage Fender beware!

Sit'2) OP transformer set to 4 Ohms speaker Z = 8Ohms. Fairly safe EXCEPT if you gig it at high and hence clipping levels. Might be fine, I like to think "ours" would but naughty.

Sit'3) OP transformer set to 4 Ohms speaker Z= 16 Ohms. Not a good idea. If you just wanted to function test an amp, for hum say, fine or very low level practice but any kind of gig level would be dangerous.

Sit'4) OP transformer set to 8 R speaker Z 4R. Pretty immune from voltage spikes but the currents in the valves might get so high they blow an HT fuse (N.B. not ALL valve amps have HT fuses!) . In any case this will shorten valve life.

Transistorized OP stages:
Never load LESS resistance than that stated in the handbook and this is an absolute, not as above subject to qualification. For most modern SState amps that load will be 4 Ohms minimum but some very old designs might not like anything lower than 8 Ohms. There are some really big transistor amps that can safely drive 2 Ohm loads and a very few that will cope with as low as 1 Ohm.
It is rather commonly stated that SS amps can run with no load, i.e. OC? Not recommended, they can go instable or "squegg" and burn out the Zobel network and that can lead to a cascade fault. 100 Ohms or so keeps things sweet. Another no-no, do not connect transformers to SS amps unless you ask the maker first.

<Errors and omissions excepted> Are we all cool now?

(Note to Hugh. I am of course only talking about guitar amps, valved and SState. These are pretty rudimentary and I know the sort of stuff you deal with is very well protected)

Meant to add, and I hope this will not add to the confusion! It is the use of an OUTPUT transformer that causes the overvolts trouble with valves. If transistor amp had an OP traff they would be even more vulnerable. Perhaps why such SS amps are only ever seen in special situations, e.g. PA systems where loads can be tightly specified? I know not of a SS guitar amp with a transformer?

Dave.

[Drew Stephenson]

I have nothing useful to add to the technical discussion but it is fun to see Hugh confused for once

[thefruitfarmer]

Cool, am slightly less confused about the ohmage issue now, I think I will stick to using the ohmage dictated by the speaker cab to match that of the amp, which should always be okay.

Something I used to know, but have now forgotten, is how to wire the pair of speakers up in series or parallel, and how each type of wiring affects the total load and wattage.

I have just ordered a Marshall style back plate from Thommans, which will probably come with instructions. However, if someone has a link to a good "how to wire up a pair of speakers" diagram that would be great.

[Hugh Robjohns]

...if someone has a link to a good "how to wire up a pair of speakers" diagram that would be great.

How's this:

[thefruitfarmer]

...if someone has a link to a good "how to wire up a pair of speakers" diagram that would be great.

How's this:

That's beautiful.....Thanks

How does it affect the wattage that the cab would be able to accept, when wired in series or parallel?

I remember that one way it would double the wattage but I can't remember whether it is parallel or series that does that.