Cab Impedence vs Amp Impedence

"All hail Pete Traynor!"

Stratin2traynor said:

I just built a 2x12 cab for my Valve Junior. It is 16 ohm. Works great with the Valve Jr. I would like to try it with my Traynor which has a 8 ohm plug for an extension cab. From what I understand (very limited understanding of course) I should be able to hook it up to the Traynor without a problem because the "ohm" is lower on the amp than the cab. Is that right?? Anyone, anyone?? I want to try it out but, I want to be sure I won't blow up my amp!

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Nope! But don't worry, you won't blow anything up. Tube amps are pretty forgiving.

The "ohms" are supposed to match. That is, the amp should be set to the same ohms as the cabinet. It may have an impedance switch or it may have different output jacks for 4,8 or 16 ohms.

You're right that your cab is 16 ohms, which is higher than for what the Traynor extension jack is designed. What will happen is that if your Traynor is a combo with its own built in speaker the load will be shared between the speaker in the combo and your extension cab. When these loads are different the lower load will hog the power.

So you might not find that 2x12 to sound very loud! The combined load will be only a bit less than the 8 ohms of the internal speaker so the amp itself won't be stressed. As I had said, tube amps are very forgiving of speaker mismatches.

Go ahead and try it! If the volume isn't high enough see if there's a way to unplug the internal speaker and run only the extension cab.

You can wire up a dpdt switch in your cab to switch the speakers from series to parallel. That will let you run 4 ohms or 16 ohms. Running 4 ohms will mean the 2x12 will hog the power and that might be a solution.

Of course, your 2x12's must be 8 ohm each to get 16 ohms. So there's no way to get 8 ohms from that combination. BUT! BUILD ANOTHER ONE!:banana:

2 cabs of 16 ohms daisy chained will give you 8 ohms. With 4x12s and the internal speaker even small amps can break leases!:tongue:

Jims said:

I have it on good authority that the real problems come from open (incredibly high impedance) and shorted (practically no impedance) loads. A mismatch of a few ohms isn't a big deal, you'll just experience a loss in volume.

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It depends on the amp. Tube amps will tolerate a short, assuming you don't try to wail at full power for a prolonged period of time. They hate and fear an open load. It allows huge voltage swings to build up in the output transformer. Often these voltages are high enough to start shorting between wire turns and windings. Not good!

Solid state amps are kinda the opposite. A short can kill them but they can run with no load all day and never care! Techs can and do work on SS amps with no speaker plugged in all the time. You see, a solid state amp doesn't really have an optimum impedance for a speaker load. It simply pumps more power as the load is lowered. When the load gets too low (like with a short!) it will pump so much power that it burns things out. An amp rated at 100 watts into 4 ohms might be putting out 65 or something into 16 ohms. Into 2 ohms it might try to put out 200 watts. I pulled these numbers out of my butt but you get the idea.

Often on a solid state amp you will see a warning on the back telling you not to run below a minimum load. "Danger Will Robinson! Do not operate into less than 4 ohms!" The designer had calculated that at 4 ohms the amp would safely supply a rated power level. Below that things will heat up big time. With a short the transistors will try to deliver every bit of power available from the power supply. You might think a fuse would blow to protect them but designers have had a problem with that ever since transistors came into use. Transistors blow far faster than fuses!

Just FYI.:zzz: