Amp Guru question.

[Trouble]

I understand that you should not run an amp below it's minimum ohm load. So the question is, if I have an amp that is rated at an 8ohm min. and I run a 8 ohm speaker and a 16ohm speaker, would that be a problem? Wired in parallel that would be 12 ohms correct?

[DetroitBlues]

Calling KBP810! Calling KBP810! Come in KBP810!

[ScottF]

Here's an example... The old blackface fenders from the 60's had an extra speaker out. "Normally" it was an 8 ohm primary internal speaker and if you plugged in another speaker cab (hopefully an 8 ohm), you'd lower the total impedance to 4. No problem. That was a stock Fender thing. So, all those Output Trannies from yesteryear, most are still ticking along. Normally stepping up or down one "notch" isn't that big an issue. Might sound like ass though. So, having an 8, running a 16 ohm load or a 4 ohm load is reasonable. Running a 16 ohm load on a 4 ohm tap is a bit too much a mismatch IMHO.

 

To answer your question more directly, it *shouldn't* hurt the OT, but mismatching the impedance will not be good for the power tubes. The real question will be how it sounds.

[DavesNotHere]

I understand that you should not run an amp below it's minimum ohm load. So the question is, if I have an amp that is rated at an 8ohm min. and I run a 8 ohm speaker and a 16ohm speaker, would that be a problem? Wired in parallel that would be 12 ohms correct?

Resistance in series adds simply R1 + R2 =Rt

Resistance in parallel adds 1/R1 + 1/R2 = 1/Rt

So your speaker total would be 1/16 + 1/8 = 5.33 ohms

[kbp810]

Good info thus far; to add a few more thoughts to the matter:

 

The ohm rating labeled on a speaker is it's expected average impedance (resistance against AC), which while under use can vary as the frequency traveling through it varies. If you read the resistance of a speaker with your multimeter (resistance against DC), you will often find it to read within a few ohms of it's labeled rating (for example, checking an 8 ohm speaker with your multimeter may show it to read at 5.2 ohms) - so basically the whole point of that is that the ohm rating of the speaker is not an exact science; there is a fair tolerance level, and you are essentially just trying to get it into the ballpark.

 

How speaker load impacts the amp:

In a nutshell, the Output transformer sees the load of the speakers and reflects it back on to the power tubes as a multiple based on the windings of the transformer. Let's say an output transformer has an 8ohm output secondary (for the speaker), and it's primary from the power tubes is 2.2k (275:1 ratio) - that is your "ballpark" rating for the expected balanced load. If you connected a 4 ohm speaker to that, your reflected load back on the power tubes is also cut in half, which now becomes 1.1k - in that scenario, you would be running the tubes (and the output transform) hot. Conversley, if you went the opposite direction and connected a 16ohm load you would double the reflected primary resistance, 4.4k, and you would be running the tubes and the output transformer cold. I used even numbers to try and simplify that, but remember from above that it's actually going to vary slightly.

 

(note that an output transformer with multiple secondaries, for example 4, 8, and 16, means that the winding ratio for each output varies to keep the primary load static, respectively)

 

Running the tubes hot will likely sound awesome... but will result in very short tube life. Running the tubes cold, usually doesn't sound all to good.

 

Either can cause damage, but going lower (running the tubes and OT hot) is much more likely to do so... generally you can go one step in either direction without causing damage (or at least any "immediate" damage), going two steps in either direction is a bigger risk.

[DetroitBlues]

And the Reverend KBP810 has spoken, thus we are now all edgeamacated.

[tbonesullivan]

I recall hearing that if you have a much higher load you risk flyback voltage causing arcing. I've always been taught to match impedances as best you can.

 

Also, mixing a 8 ohm and a 16 ohm load gives you 5.33 ohms for the total system in parallel. However since the speakers have uneven impedances, you will see more of the power going to the path of least resistance, the 8 ohm speaker. I am not familiar at all with the design of the fender amps OT. It may be that when you plug in the external speaker it switches to a different tap on the out put transformer, one for a 4 ohm load.

 

I'm used to using Carvin amps, which have an impedance switch and two output jacks, on both the combos, and on the amp heads. When I first got a Marshall amp I was confused as heck because suddenly I just had a bunch of jacks, and no switch, the same with the mesa boogie. I guess it does make it more idiot proof, but I'd rather not have to worry that i'm plugging into the wrong jack.

[Trouble]

Thanks all, I'm going to run 2 16ohm speakers in parallel so should be ok. Still have an 8ohm Celestion with no home though.