Safe Impedance Mis-Match - Rectoverb Head

[cyber104]

Hi Guys:

Just looking for feedback.....

I'm old and hate lugging heavy gear around. I usually just take my Series 2 Rectoverb Head to the place where I practice. They have a variety of cabinets there that I can use but they are all 16 ohm.

On page 9 of the User Guide I found the following: "Sensitivity to speaker mismatching in regards to ohmage differences is low, hence no damage to the amplifier will occur. However, very low ohmage loads will cause the power tubes to wear faster"

And on page 17: "When running a higher resistance ( for example: 8 ohm output into 16 ohm cabinet ), a slightly different feel and response will be eminent. A slight mismatch can provide a darker smoother tone with a little less output and attack. This response is a result of the amplifier running a bit cooler."

And finally on page 19: A diagram showing the 8 ohm output running to a 16 ohm cabinet.

Since I don't have a 16 ohm output it sounds like I'm fine running one of these 16 ohm cabinets from my 8 ohm output.

Does anyone think this IS a problem?

Thanks,
Chris

 

[domct203]

A 16 ohm cab in the 8 ohm speaker jack is fine. It is always safe to have the cab's impedence greater than the amp's OT tap.

-- OR --

If there are two 16-ohm cabs you could always just hook them up in parallel for 8-ohms total.

Dom

 

[Resonant Alien]

Agreed. As long as the speaker impedance is greater than the amp output impedance, you are fine. Never run it the other way around (speaker impedance less than the amp output impedance) - you could/probably will fry your OT. Think of it like having a 8 gallon bucket of water (your amp) and you want to transfer the water to a different bucket (your speakers/cabinet) - you can pour your 8 gallons of water into a 16 gallon bucket, but you cannot pour 8 gallons of water into a 4 gallon bucket.

 

[94Tremoverb]

That's not actually true. Most tube amps are safe with a half to double mismatch, and in fact a *low* mismatch (amp driving lower than the correct impedance) is safer for the *amp* (including the OT), although it is harder on the tubes. Even a dead short is not a major risk for a short period, which is why old Fender amps have a switch in the speaker jack to do exactly that if no speaker is plugged in - because it's safer than no load at all.

A high mismatch (amp driving higher than the correct impedance) is also safe up to twice the matching impedance, but *not* higher - ie don't run a 4-ohm amp into a 16-ohm cab. If you do this you run the risk of arcing in either the tubes or the OT and this *is* a risk to the OT, just like running with no load only a bit less severe. There is no minimum period for risk this way, either - damage can be immediate if you crank the amp up.

This applies to *tube* amps. Solid-state amps are the other way round and it's correct that they should not be run into lower than the minimum impedance, or you can expect to blow the output transistors very quickly.

But for the original question, yes it's safe to run an 8-ohm amp output into a 16-ohm load. Especially with a Mesa amp, which are less sensitive to impedance mismatches than some other amps. (I would not try this with a modern Marshall, for example.)

 

[VanWreckto]

Yes it's safe.
8 ohm amp into 16 ohm cab works fine.
I've even added an 8 ohm Mesa cab into the amps 4 ohm jack while running a 16 cab into the 8 jack.