Matching ohms of a head and cabinet?

Charlie4

Hello,

I was just wondering if you can help me out here. I'm in the process of selling an 8 ohm guitar cabinet and the possible buyer has an old ELK valve amp which seems
to have a 4 ohm input. I just want to know is this combination possible or will it damage the amp?
I couldn't find anything on the specific model he has - ELK ED 40.

Will appreciate any help/advice.

FenderBender

You will need to ensure that the amp is matched to the correct speaker impedance to prevent long term damage to the output valves and output transformer. If there are two or more speakers in the cabinet, open it and write down the rated impedance of the individual speakers. It may be possible to connect all or some of the speakers to present a 4 ohm load to the amp.

aja

I thought there was an unofficial rule that tube amps can handle an impedance that is at their rated amount or double, but never less..

I.e. the amp rated at 8ohm can use an 8ohm or 16ohm speaker impedance but not more than 16 or less than 8.

True or false?

FenderBender

Yes aja, there is an unofficial rule for this, however it is better to have a 2:1 low mismatch (ie speaker at half the amp's impedance).

An OT being able to withstand the 2:1 high mismatch (ie speaker at twice the amp's impedance) will depend a lot on the OT's quality and design. For a poorly designed (high leakage, poor coupling, not well insulated or potted) transformer, a 2:1 high mismatch may be marginal. Marshalls are apparently extremely sensitive to a 2:1 high mismatch. I don't know about the Elk.

It would be better to set up a 2:1 low mismatch, but 2 ohm guitar speakers are hard to find for this application.

High impedance (and open circuit) loads can cause punch through in OT windings due to the spike induced, caused by collapsing flux in the windings, when the load is of an excessively high impedance or open circuit even for brief moments.

aja

+1 karma for the informative post!

Thanks ?

Norman86

let me add my own opinion...

if your head is an 8 ohm head, then your cab needs to be 8 ohm, or higher (8, 16, 24, 32 ohms)

if its a 4 ohm head, then anything higher than 4 ohms is fine (4, 8, 12, 16 etc...)

you cannot go into lower impendences though, this causes the head or amp to overheat as it cannot handle the low load.
the lower the load, the closer you are to a dead short.

coming from a car audio background, where my one amp was capable of holding a 0.5 Ohm load, i know that ohmage can sometimes be confusing!

music is dynamic... just because you have a 20watt amp, doesnt mean you are using 20watts all the time on your 8ohm load.
it also doesnt mean that your load delivered by the head / amp is always 8 ohms!

different speakers also react differently to the given power, as at its resonating frequency, a speaker is at its highest ohmage, therefore needing the minimum mount of power to get the speaker at its loudest! but ONLY at the speakers resonating frequency!

but to end off, if you have 2 4ohm speakers wired in series, you get an 8ohm load.
if your amp / head can handle a 4 ohm load, then your amp will be fine, as it doesnt need to drive the speakers as hard!
if you were to wire the speakers in parallel, you would be presenting a 2ohm load to the amp, and that will cause it to overheat and get damaged.

so, keep your nominal impedance the same as the head, or higher!

ryanguit

Super dangerous posts okes!

Graeme is on it, read his posts and then ignore the rest. haha

solid state amps need equal or higher
Valve amps need equal. but can run at lower preferably where the the volume could be as low as half volume unlike transistor amps.
if you mismatch valve power amps with their loads you will always be messing with powertube life...

Valve amps: if you run at higher loads than recommended you Will damage the Tranny eventually, if not imediately!

Think of it like this:
Worst thing you can do to a valve amp? turn it on without a load... infinite load
worst thing you can do to a transistor amp (hifi car amps included)? short out the speaker terminals. too low a load.
Complete opposites!!! please use Google before giving advice okes...

Old Fender speaker jacks were wired up to short out if nothing was connected, because it was better than being open!!!!(still not ideal but rather blow output tubes than transformers)

AlanRatcliffe

ryanguit wrote: Super dangerous posts okes!

+10,000. Graeme and Ryan are on the money.

Just to reinforce what they have said - Lower is OK (in a pinch) with a valve amp, but NEVER higher - that doesn't use all the power coming out of the trannie and the remainder "feeds back" and can easily do major damage!

Norman86

Alan Ratcliffe wrote:
ryanguit wrote: Super dangerous posts okes!
+10,000. Graeme and Ryan are on the money.

Just to reinforce what they have said - Lower is OK (in a pinch) with a valve amp, but NEVER higher - that doesn't use all the power coming out of the trannie and the remainder "feeds back" and can easily do major damage!

Im sorry, but i do not agree with you guys and think you guys can learn something about the actual speakers being used.

how can you say never higher impedance?

a head will have an output of say, 8 ohms.
lets say this is a marshall head, as i know they have 8 and 16ohm outputs.

This does NOT automatically mean that your 8 ohm head is always giving its power out at 8 ohms, NOR does it mean you are always using the maximum amount of power from your amp.

HERE IS WHY:

You have a cab, fitted with an eminence speaker(s).
i would say celestion, but celestion do not provide proper specs for their speakers, so in this instance, they suck.
eminence on the other hand provide a very nifty pdf found here for my explanation:

http://eminence.com/pdf/deltademon.pdf

theres a few things you should notice:

A speaker ALWAYS has 2 impedances, Nominal AND DC resistance.
Nominal resistance is what we normally work on, but the DC resistance is the lowest impedance the speaker will show to the amp / head.
On this specific speaker, the nominal impedance is 8ohms, perfect for the amp output of 8ohms!
The DC resistance however is 6.9 ohms. The amp can handle that though, due to the way amps are manufactured!
So yes Alan, i will agree with you there with lower is okay in a pinch. But only because an 8 ohm speaker is not actually 8ohms.
If it was lower, as in 3.8ohm (as what it likely could be for a 4ohm speaker, then we'd have a problem!)

the HIGHEST impedance shown to the amp, will ALWAYS be where the speaker resonates. (fs)
the closer you get to the speakers Fs, the higher the impedance becomes, regardless of the power.
However, the higher the frequency played by the speaker, the higher the impedance becomes as well.
Lower Frequencies require lower impedances and more power than higher frequencies!

This graph, as taken from that manual, shows exactly that.

Pe is the THERMAL HANDLING CAPABILITY.
a speaker with a Pe if 100W RMS, can handle 100W of power.

an amp on the other hand, can only produce so much power, unless driven into clipping, where it produces more because of square waves.
you can always use a speaker with a higher thermal handling than the amp can provide, but preferably never lower.

The rest of the T/S (Thielle / Small) Parameters, give you the box specifications, the size and type of cabinet that will best suit that speaker.

It comes down to:

an amp is 8ohm, get an 8ohm speaker. 16 ohm, 2 8 ohm speakers in series, or an actual 16ohm speaker.
AS LONG AS YOUR NOMINAL IMPEDANCE IS CORRECT, the amp and speakers will be fine.

but just because the amp is 8 ohm, DOES NOT MEAN the amp is always giving out 8 ohms, due to music being DYNAMIC, and as such, not always requiring the same amount of power.
as mentioned earlier in the post, lower Frequencies require lower impedances and more power than higher frequencies!

One more thing... if an amp is 8 ohms, and you need a speaker half of that impedance, you need to DOUBLE the impedance.
IN RESISTANCE, half of 8ohms is 16 ohms. Read that again, 16 ohms!
IF you then use a 4 ohm speaker, thinking you have halved the impedance, you are sorely mistaken, as then you really have doubled the impedance and doubled the load on the amplifier.

now please tell me my post is dangerous again. :?
The only thing dangerous is not knowing how speakers work of how they affect amplifiers.

FenderBender

Norman86 wrote: unless driven into clipping, where it produces more because of square waves.

This is true with transistor amps, but valve amps are a different kettle-o-fishTM. A power valve driven into overdrive produces higher order harmonics that give a valve amp it's unique tonal characteristic. Valve amps are designed to work in the "clipping" zone, unlike transistor amps. Not that this is really important here. It just shows that valves and transistors do not behave the same way. The nature of valves and the output coupling transformer between the valves and the speaker makes things VERY different.

Norman86 wrote: IN RESISTANCE, half of 8ohms is 16 ohms. Read that again, 16 ohms!

Er.....now that's dangerous!

Norman86 wrote: now please tell me my post is dangerous again. :?

It is dangerous Norman ?

Norman86 wrote: The only thing dangerous is not knowing how speakers work of how they affect amplifiers.

or the fact that you know very little about valve amps ?

Norman86

A power valve driven into overdrive produces higher order harmonics that give a valve amp it's unique tonal characteristic. Valve amps are designed to work in the "clipping" zone, unlike transistor amps.

I know that... it is still clipping though.

the amp still relies on how the speaker behaves. different speakers affect the amp differently.
one thing is still the same though, regardless of the speaker, different frequncies require differing amounts of power, because of the impedance shown to the amp.

edit:

IN RESISTANCE, half of 8ohms is 16 ohms. Read that again, 16 ohms!

sorry, lost track of thought there:

Half of 8ohms is 4 ohms. when you want to double the resistance, it is 16 ohms! My mistake!

AlanRatcliffe

now please tell me my post is dangerous again. :?

OK! Your post is dangerous. ? I'm no EE, but I've seen a few blown trannies in my time - especially on Marshall amps, which seem very sensitive to high mismatches.

how can you say never higher impedance?

Because a valve amp's output is transformer coupled to the speakers - and this is the crux of the biscuit. Lower impedances and the valves still see the relatively constant input impedance of the transformer's primary and do not try output more power and overheat the way a solid state power stage does. At higher impedance loads the speakers do not use all the power produced and the transformer starts storing the excess in the magnetic core. Eventually the core discharges like an inductor, generating voltage spikes which can punch through the transformer's insulation and short the windings. There's a good technical explanation in the http://www.geofex.com/ FAQ.

A speaker ALWAYS has 2 impedances, Nominal AND DC resistance.

Sorry, but resistance is not impedance. Resistance is DC, impedance is AC and is frequency dependant (so is measured at 1KHz). AS AC devices, DC resistance doesn't really have a practical meaning in speaker performance, but is often listed in specs as its far easier for the layman to measure than impedance.

Granted, power output varies and the impedance of the speaker load changes with frequency, but as you pointed out the important thing is to keep the impedance at the same nominal values. That's assumed when talking impedances in audio.

SvR65

I'm glad I read this - very informative for someone who doesn't really know much about ohms, transformers etc!!

(To give an idea - my knowledge of transformers doesn't stretch further than the movie!) Haha! ?

TomCat

Just to stick my oar in here too...... ?

Alan and Graeme are correct.......The fact that my SE amp totaled its OT and OP valve by mistakenly not plugging in my cab is testimony to that....OK that was an open CCT but the same rule applies to higher impedance's with valve amps. The OT has nowhere to dissipate it's power so it stores up in the core and then...... :'(....and good OT's are expensive.......

EDIT: BTW the issue with impedance and resonance of a loudspeakers is very valid in that the impedance is a lot higher at resonance.....one point to remember that a high impedance peak in a frequency response is very different to an overall mismatched impedance.

One hits resonance occasionally during normal program operation and the mismatch is overcome when the program material changes and the loudspeaker dissipates the stored up energy. This can also happen in transistor amplifiers where the stored energy is either dissipated as heat or absorbed by the power supply in some modern amplifier designs, hence some amplifiers handle reactive loads better than others.

The term back EMF is often referred to under these conditions.

aja

I know a guy who works with radios and juke boxes. He regularly runs them with no speakers or anything attached whatsoever ? ???

I asked him about it and he said the notion that running tube amps with no speakers is dangerous is rubbish :-\

I don't believe him but it doesn't explain why he doesn't run into problems :-\

FenderBender

aja wrote: it doesn't explain why he doesn't run into problems :-\

He has been lucky. Maybe the OTs are well built or overengineered and can cater with the back EMF (thanks TomCat!). Either way, he is taking a chance.

makepeace

hmmm.. i wonder if there is a way to build a switchable dummy load into a head..

plugging in a speaker will route the signal through the speaker and when a speaker is not plugged in, the signal goes into the dummy load.

would save a hell of a lot of amps from demise due to carelessness..

anyone care to point out whats wrong here ^ ?

FenderBender

Actually....this would work well. A nice fat 100W aluminium encased resistor across the speaker socket would extend the life of the output valves vs the old Fender method of shorting the output when a jack is removed.

Valve amps are cool with resistive loads. I use resistors in the tube tamer.

makepeace

and walah! problem solved?

http://www.amazon.com/100W-Non-Inductive-Dummy-Load-Resistor/dp/B0002KRDGC

they're cheap as well.

if someone felt like being really creative.. they could design an electronic circuit that identifies the impedance of the speaker attached and chooses a resistor to pair it with (in series or parallel) which will present the correct load to the head. one less thing to worry about.

oh yeah Graeme you do the power attenuators right?

TomCat

Graeme Parfett wrote: Actually....this would work well. A nice fat 100W aluminium encased resistor across the speaker socket would extend the life of the output valves vs the old Fender method of shorting the output when a jack is removed.

Valve amps are cool with resistive loads. I use resistors in the tube tamer.

I've actually been considering this very mod to my 8w SE to prevent another accidental OT meltdown....... 2 x 10W 15 ohm resistors in parallel should do it..... 8)