the importance matching impedance with tube amps

[mlannoo]

This thought just occurred to me the other day and it surprised me that I've never heard anyone else pose this question ... I normally work on amps that are 50watts or smaller so I never questioned this.

We all know the importance of matching our tube amplifier's impedance to our speaker load's impedance. But does the wattage of the amp relative to the wattage of the cabinet play a role if you're just powering it up with a load attached and not playing it?

I'm about to service a 100 watt Laney head that has a switchable impedance selector (4/8/16) ... but i realized that the only cabinet i have here at home to connect to this head is an 80 watt/16ohm 2x10 extension cab. It doesn't seem safe to power the amp up connected to a cab with a maximum wattage that is smaller than the amp - even though the impedance can be matched at 16ohms.

any thoughts? thanks for looking.

[Earl]

I'd say that's a correct assumption! You could add an L-Pad to eat up some of the power, and keep the cab safe.

You can do the math yourself, or use the L-Pad calculator at Ted Weber's site.

[tubeswell]

No problem at all. The only time your cab will probably suffer is when you've cranked the amp volume. The cab doesn't experience the 'wattage' unless its actually doing the work. When it just sitting idle, all its providing is some load resistance for the tubes.

[mlannoo]

No problem at all. The only time your cab wil probably suffer is when you've cranked the amp volume.

Really? I try to imagine an extreme example ... like hooking a 100 watt head up to an 8 inch / 8ohm / 15 watt speaker ... sure the impedance is correctly matched, it just doesn't seem safe to use it as a load ... even if the volume never goes up.

[tubeswell]

The 'load' is provided by the resistance of the speaker - in this case 16 Ohms.

The wattage is a rating for how much power the speaker can handle.

Power (watts) is an expression of the rate at which work is done. In electrical terms power = amps x volts. An idle OT secondary has no voltage swing moving in it, therefore power will be 0V x amps = 0W. If the amp is idling, the speaker won't be moving. The voice coil just sits there inert surrounded by the magnetic field.

In many cases, people hook up less powerful speakers to more powerful amps to get more speaker responsiveness. This is infact the way I run my PA foldback. using a 320W per channel power amp driving a 200W rated speaker cab on each side and 2/3 on the vol dial. You just can't run the amp to its full power that way without burning the voice coil(s).

[d95err]

In many cases, people hook up less powerful speakers to more powerful amps to get more speaker responsiveness. This is infact the way I run my PA foldback. using a 320W per channel power amp driving a 200W rated speaker cab on each side and 2/3 on the vol dial. You just can't run the amp to its full power that way without burning the voice coil(s).

Assuming the volume control is logarithmic, 2/3 on the dial would be much less than 2/3 of the power, so you'd be fairly safe I guess.

Personally, I would never run an underrated speaker to an amp. Even if *you* know what you're doing, one of these days someone more ignorant will turn it up when you're not watching and the speaker will blow. It's an accident just waiting to happen.

[Enzo]

But he is in his own shop. I don't hesitate to connect an 800 watt amp to my 100 watt speaker cap just to see if it makes sound. I don't have to turn it up. No one is at the controls but me. I assume he is in control in his shop. It is his shop speaker, he's not installing the underrated speaker in a combo.

A 100 watt amp doesn't sit there making 100 watts all the time, any more than a 150 mile per hour car goes that fast all the time. If you max the amp and crank it, those 100 watts might be more than your 80 watt speaker could handle, but turn it down half way and the 50 watts would be easy for the 80 watt speaker. CLose enough to see if the amp is working. At least working in all ways except absolute max power output.

Yes, I would connect the 100 watt amp to the 15 watt speaker for low volume testing. Until the amp is turned up to produce more than the 15 watts, the 15 watt speaker is in no danger.

After all, I really can drive a 150mph Corvette through the middle of town at 35.

[mlannoo]

that's a great way of looking at it! that makes complete sense. thanks!

[Enzo]

Just don't go over the speed limit, or the amp police will write a ticket.

[slowfingers]

I play my 100watt Marshall through a 15watt Celestion Alnico Blue at home for the last three years with no problem, I just dont turn it up to loud.
slow.

[Earl]

Just my personal perspective, but if some amp is in "the shop", it's most likely got some problem. Potentially, a problem that could cause the amp to spew forth 'stuff' into whatever it's hookes to.

Another analogy could be - being in a boat without a life preserver is safe...until the boat sinks .. :-)

[Enzo]

No one is suggesting carelesness.

If you just returned from a fishing outing and realized your jacket was still on the boat, would you put on a life jacket to walk on the boat to retrieve the jacket?

[Old Tele man]

...typically, power loss at half-Z and twice-Z is only down about 12% from its optimum value at Z.

...however, beyond those points, power falls-off much faster with < Z/2 than with higher >2*Z values.

[tubeswell]

...typically, power loss at half-Z and twice-Z is only down about 12% from its optimum value at Z.

...however, beyond those points, power falls-off much faster with < Z/2 than with higher >2*Z values.

Much obliged Old Tele man, that's a handy thing to know. Cheers

[6267]

Reminds me of the time we hooked a 60w sand amp up to a 2" speaker. It made the best screechy hair metal guitar sounds–it never blew! However, some sort of strange liquid started leaking out of the voice coil and it got a bit raspy after about 5 minutes. You'll generally know you're pushing a speaker long before it blows unless you hit it with some mean transient and the volume control at the same time or something.