impedance matching

firefly08

Is it a big deal with a valve amp if it's plugged into a speaker with only half the required impedance?

I was testing one in a shop recently and I noticed the guy had set it up with an 8 ohm speaker cab, but the output on the amp is 16 ohms. It seemed to work fine. I was under the impression that if the amp was damaged from this, it would be immediately apparent. Is it possible there could be long term damage that I wouldn't notice straight away? Because if I buy it I'll probably have to go for this display model, which I know has been used with the wrong speaker at least once, at full volume (but it's only 5 watts mind you).

Squidgy Black

You should only use a tube amp with a speaker higher or equal to it's impedance, or else it can put great strain on the transformers. It'll eventually cause the transformers to overheat and blow.

firefly08

You should only use a tube amp with a speaker higher or equal to it's impedance, or else it can put great strain on the transformers. It'll eventually cause the transformers to overheat and blow.

Thanks. that's what I was thinking - but what I'm trying to find out is, has the life of this amp been shortened, or it's quality reduced in some way, even though nothing blew in it?

Paolo_M

firefly08 wrote: »

Thanks. that's what I was thinking - but what I'm trying to find out is, has the life of this amp been shortened, or it's quality reduced in some way, even though nothing blew in it?

Short answer: probably not.

The rule of thumb is always match where possible.
However there are a tonne of variables at play.

This thing of "higher impedance is ok but lower is not" is a myth and completely incorrect, though I have seen it written many times. Lower can be equally as damaging. One causes over-current in the transformer the other causes over-voltage, which leads us to the next variable.

The volume you play at effects whether a mismatch will be damaging. Current and voltage increase with more volume in the output transformer. Power is a product of current and voltage. The louder you play with a mis-match the more likely it is that you'll cause damage. The output transformer has max voltage and current limits. Exceed them and it will fail.

The final significant variable is transformer quality.
Marshall traditional specify their transformers "just enough".
It's part of their sound as the transformer saturates and distorts at high volume and that is part of the Marshall sound. The problem with this is that there is little margin for error. Couple that with people playing their Marshalls flat out and you get a lot of transformer failures.
Most other high end manufacturers (MESA et al) uses well speced transformers and generally have a clean power amp so there is a large margin for error. So much so that MESA advertise that try impedance mismatches for the tonal variations associated, you don't void your warranty by doing that.
Fender sold many of their combo mismatched as standard.


If you don't fancy remembering all of this (and there's more involved) then just match the impedance.

Miklos

If you're playing a 16 ohm amp into an 8ohm amp it's only a matter of time before you fry it, especially if you try crank it.

Low into high, it'll fly.
High into low, it'll blow!

Paolo_M

NickWray wrote: »

If you're playing a 16 ohm amp into an 8ohm amp it's only a matter of time before you fry it, especially if you try crank it.

Low into high, it'll fly.
High into low, it'll blow!

You shouldn't play an amp into an amp.
See my earlier post, this is a dangerously misleading myth, it is not correct.

Keyzer

Paolo_M wrote: »

You shouldn't play an amp into an amp.
See my earlier post, this is a dangerously misleading myth, it is not correct.

I think he made a typo...

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Paolo_M wrote: »

You shouldn't play an amp into an amp.

correct me if im worng, but i believe thats how The Kinks got their overdrive tone!

Miklos

Keyzer wrote: »

I think he made a typo...

Indeed. I meant amp into speaker..

Keyzer

While we're on the topic of impedance, which I know nothing about, can someone give me some advice.

Say I have a 22 Watt amp and I want to hook up an attenuator - how does it work?

Amp = http://www.thomann.de/ie/fender_65_deluxe_reverb.htm
Attentuator = https://taweber.powweb.com/weber/minimass.htm

Will the attenuator above match the amp?

firefly08

Thanks all. Incidentally, I asked the same question on the Marshall forum, and I was told my the Marshall guy that

The class 5 runs in class A and is not prone to damage from load mismatching.

I will not be playing it with mismatched impedance anyway, just in case.

Paolo_M

Keyzer wrote: »

I think he made a typo...

NickWray wrote: »

Indeed. I meant amp into speaker..

Hence the wink and smile in my post.

Kara Weak Spoonful wrote: »

correct me if im worng, but i believe thats how The Kinks got their overdrive tone!

Didn't they stick needles through their speakers as well, or something like that?

Squidgy Black

Paolo_M wrote: »

Hence the wink and smile in my post.



Didn't they stick needles through their speakers as well, or something like that?

Yeah Hendrix experimented with that at one stage too I believe, torn speakers.

Van Halen used two Marshall plexi's as his tone, slaved one into another, super bloody loud though

Miklos

[QUOTE=stetyrrell;76495414
Van Halen used two Marshall plexi's as his tone, slaved one into another, super bloody loud though[/QUOTE]


...and look what happened to him!