16 ohm in 8 ohm box? Resistor?

[eldungo]

:RoCkIn I have a British made Celestion V-30 just waiting to go into my Blues Jr. However, it's a 16 Ohm speaker. I've been told I simply need to place a 25w resistor across the speaker leads to make the transformer see 8 Ohms. I'd like a little more verification from the fine folks here at TGP before I pull the trigger on this. Anyone?

[jh45gun]

That works I did that once with a speaker and it worked fine. You have to get a 8 ohm resistor though that will handle some wattage so maybe some of the others can tell you more on that. I used a one of those old adjustable resistors that was wire wound ceramic with a slide adjustment that you moved it and measured until you got the ohms you needed. Not sure if they still make them or not?

[chrism]

:RoCkIn I have a British made Celestion V-30 just waiting to go into my Blues Jr. However, it's a 16 Ohm speaker. I've been told I simply need to place a 25w resistor across the speaker leads to make the transformer see 8 Ohms. I'd like a little more verification from the fine folks here at TGP before I pull the trigger on this. Anyone?

It should work fine but remember that you are losing half of the available output power in the resistor.http://www.thegearpage.net/board/images/icons/icon6.gif

[John Phillips]

That works I did that once with a speaker and it worked fine. You have to get a 8 ohm resistor
No, you need a 16-ohm resistor to make a total of 8 ohms with a 16-ohm speaker.

In practice even that's not right, because a speaker does not have the same impedance at all frequencies, and in fact rises quite high at the top end of the audio range - often up to three or four times the 'nominal' impedance. The average impedance of a 16-ohm speaker over the audio range is more like 20 or 25 ohms (depending on the speaker), so if you want to do this, I would use a 22-ohm resistor.

Even then, you will lose some top-end because at high frequencies the resistor will have a lower impedance than the speaker, and so will take a bit more of the power - but not too bad, if you're using a 22-ohm one.

It also needs to handle half the power of the amp (fully overdriven - which could be up to double the rated power) and you need to allow for it not being heatsinked, unless you're going to do that too, so I would use a minimum of a 25W and possibly a 50W.


Or, you could simply run the amp mismatched - it almost certainly won't harm it, most amps will handle a factor-of-two mismatch safely. You will lose a little power (25-30%), but not as bad as by using the resistor (50%).