Ohm mismatches

Kebabkid

This one always confuses me and posts/sites say it's generally OK for lower output into lower or higher rated cabs than the source and they're just won't be as efficient or they'll be a loss of volume. So, what about for an amp with a 16 Ohm ext.speaker into an 8 Ohm cab?

My main concern is if this will cause any damage but if not, what can I expect?

Thanks

jasonbone75

This way round is bad juju. If I interpret your post correctly, you have a speaker extension output expecting to see a load of 16Ohm and you are suggesting to connect to a cabinet presenting an 8Ohm load then this is not good. You are roughly doubling the current flow through the output transformer. This assumes you are referring to a valve amp with output transformer.

ecc83

This is actually the 'safest' form of load 'mismatch' Yes, the 8R load will demand more current but valves are pretty self limiting and in any case., a seriously high current will only be demanded for very high volume playing.

THE most important factor in any load mismatch* is that it matters little (to valves) at lower powers.

Going the other way, an 8 Ohm tap driving a 16 Ohm speaker is much more dangerous to valves, OP transformers and pockets! Such a situation causes much higher voltage spikes from the valves but once again, keep things at sane volumes, 1/2 power say, and any decent amp will be fine.

*Transistor amps go the other way. Never connect a load LOWER than the book says and NEVER connect any kind of transformer to a transistor OP stage.

Dave.

Kebabkid

ecc83 said:

This is actually the 'safest' form of load 'mismatch' Yes, the 8R load will demand more current but valves are pretty self limiting and in any case., a seriously high current will only be demanded for very high volume playing.

THE most important factor in any load mismatch* is that it matters little (to valves) at lower powers.

Going the other way, an 8 Ohm tap driving a 16 Ohm speaker is much more dangerous to valves, OP transformers and pockets! Such a situation causes much higher voltage spikes from the valves but once again, keep things at sane volumes, 1/2 power say, and any decent amp will be fine.

*Transistor amps go the other way. Never connect a load LOWER than the book says and NEVER connect any kind of transformer to a transistor OP stage.

Dave.

thanks guys for the info and see 2 different opinions but I will try it at low volume and see what gives.

It's a 1x12 valve amp combo with a 16 Ohm ext.speaker out going into a Mesa Boogie Cab at 8ohms? On that note, why don't Mesa Boogie offer Ohm output options ?

jasonbone75

I would go with Dave's post! He has forgotten more about amps than I know 

Kebabkid

jasonbone75 said:

I would go with Dave's post! He has forgotten more about amps than I know 

Thanks

LionAquaLooper

I found these useful:

Do you run the amp higher in an impedance mismatch? | The Gear Page

https://www.thegearpage.net/board/index.php?threads/124559&highlight=speaker+impedence

https://www.thegearpage.net/board/index.php?threads/67069&highlight=speaker+impedence

mart

Doesn't it depend on whether the amp is valve or solid-state?

slacker

Kebabkid said:

ecc83 said:

This is actually the 'safest' form of load 'mismatch' Yes, the 8R load will demand more current but valves are pretty self limiting and in any case., a seriously high current will only be demanded for very high volume playing.

THE most important factor in any load mismatch* is that it matters little (to valves) at lower powers.

Going the other way, an 8 Ohm tap driving a 16 Ohm speaker is much more dangerous to valves, OP transformers and pockets! Such a situation causes much higher voltage spikes from the valves but once again, keep things at sane volumes, 1/2 power say, and any decent amp will be fine.

*Transistor amps go the other way. Never connect a load LOWER than the book says and NEVER connect any kind of transformer to a transistor OP stage.

Dave.

thanks guys for the info and see 2 different opinions but I will try it at low volume and see what gives.

It's a 1x12 valve amp combo with a 16 Ohm ext.speaker out going into a Mesa Boogie Cab at 8ohms? On that note, why don't Mesa Boogie offer Ohm output options ?

How does the ext speaker pair with the combo speaker? That might make 5.3 ohms.

fretmeister

mart said:

Doesn't it depend on whether the amp is valve or solid-state?

Yes. As per ecc82's post above.

p90fool

fretmeister said:

mart said:

Doesn't it depend on whether the amp is valve or solid-state?

Yes. As per ecc82's post above.

You've just turned poor old Dave into a 12AU7!

fretmeister

p90fool said:

fretmeister said:

mart said:

Doesn't it depend on whether the amp is valve or solid-state?

Yes. As per ecc82's post above.

You've just turned poor old Dave into a 12AU7!

ha hahahaha!

Kebabkid

Thanks everyone

ecc83

p90fool said:

fretmeister said:

mart said:

Doesn't it depend on whether the amp is valve or solid-state?

Yes. As per ecc82's post above.

You've just turned poor old Dave into a 12AU7!

Don't mind! Makes an excellent cathode follower. Let's see if we can nail this fekkin' load business down once and for all?

In fact it does not matter in some ways whether we are talking valves or transistors. It is the TRANSFORMER that causes the problem but you never see transistor amps with OPTs (there has been some).

Take the extreme case where there is no load on the amplifier i.e the load is 'infinite'. Now that gets transformed UP to the valve anodes as "infinity times the step up ratio" (yes, I know, bllx mathematics!) In other words even a tiny signal current through the valves will produce an infinite voltage.

Can't of course, inter-winding capacitance will for one thing keep it down but you can still get a couple of kV+ on the anodes! (before everything goes bang!)

If instead of infinity i.e an O/C you use a higher impedance load than the amp is set* for you are in danger of higher voltages and valve flash over.

A lower load will just make the valves work a bit harder and you might blow an HT fuse (if the amp has one of course!) If the amp is well designed, especially the OPTraff then it will stand a limited amount of both forms of abuse. I never had to replace an OPT in any Blackstar amplifier save a very early HT-5 which just decided to go short to frame.

Transistors: No transformer so the above scenario cannot occur but transistors do not limit current the way valves do so, load them at too low a value= more current=buggered! Of course, designers can and do take steps to make amps bombproof but it is expensive to do really well and in any case, amps sound total ***t when protection circuits kick in.

Bs again...their sstate amps use some pretty aggressive but clever control devices which actually mean you cannot determine the sine wave power of the amps but they are nonetheless 'king loud! (NB that is old knowledge, might have changed?)

*This is not 'set in stone' for any particular valve. Look at a power valve data sheet and you will see that the "optimum load" is different for different circuit configurations, anode currents, bias types etc, "Optimum load" really means a balance between good power delivery at a respectable distortion level. Gitists are not much fussed about the latter!

Dave.