Question about speakers/ohms

Ryan Gimpert · From: Detroit, Michigan, USA

I have a D130 recone (8 ohm) that I have been saving.

I have the chance to pick up a 66 Bandmaster head, and I was hoping that I will be able to use it with this speaker. I believe the Bandmaster puts out 4 ohms.

If I am intent on using a 15, am I best to find one that is 4 ohms or will my D130 work?

If so, what are some suggestions?

Thank you and please pardon my ignorance!

Robert Parent · From: Gillette, WY

Ryan,

An amplifier with a 4 ohm output will work with an 8 ohm speaker just fine. You will not have the same amount of power available but nothing will be harmed.

One should not use an amplifier rated at a higher impedance with a speaker of a lower impedance rating. (ie... an 8 ohm amp with a 4 ohm speaker).

Hope this helps...
Robert

Dave Mudgett · From: Central Pennsylvania and Gallatin, Tennessee

This has been discussed a lot - e.g.,

http://bb.steelguitarforum.com/viewtopic.php?t=170054

http://bb.steelguitarforum.com/viewtopic.php?p=1561893

http://bb.steelguitarforum.com/viewtopic.php?t=185395

Generally, it is considered "OK" with an old Fender amp with a stock output transformer in good condition to run the speaker impedance mismatched up to 100% too low, but I don't think it's a good idea to go 100% too high. That rule, "It's OK to run the impedance too high, but not too low" is true for most solid-state amps, but NOT most tube amps.

Look at the first linked thread above, which excerpts some information from Ted Weber about why this is the case - the issue is 'flyback voltage', which is a second-order effect voltage generated at the speaker as the voice coil moves through the magnetic field, and is sent back up to the transformer. Other things being equal, this voltage will be higher in a voice coil with more turns, i.e., higher impedance. In typical tube operation, the output transformer is a step-down transformer, which means that the turns-ratio is > 1, and in fact generally quite large, and changes a large voltage swing at the output tubes into a much smaller voltage at the speaker - but pushing a much larger current, which is what drives the speaker. But the reverse happens with the flyback voltage - it is generated at the speaker, and is stepped up into a much larger voltage at the primary of the output transformer - here, the inverse turns ratio of the transformer determines how much the voltage is stepped up. The upshot is that if the transformer is set for a lower impedance load than the speaker is actually rated, the flyback voltage is multiplied higher than it should be. These flyback voltages can be pretty high, and if high enough can cause arcing in the OT primary, and thus destroy it over time. It is obviously a good idea to minimize the flyback voltage, and thus why the OT should not have its turns-ratio set for a lower impedance speaker than is in the circuit.

Rule of thumb:

Solid-state amp: OK to run at any speaker with impedance down to the minimum impedance rating. Amp typically runs cooler with higher-impedance speakers.

Tube amp: Ideally, run with the rated speaker impedance for best power transfer and performance, but if you need to run mismatched, better to run a speaker with a bit too low an impedance than too high. Personally, I would never run more than 100% mismatch, and then only if I was sure the OT was very robust indeed. I just don't generally do it, YMMV.

My take.

Clete Ritta · From: San Antonio, Texas

Ryan,

I have a 66 Bandmaster (pictured in my avatar). Its a wonderful amp!
I usually pair it to the original cab loaded with two vintage 8 ohm P12Ns, so the speakers load is 4 ohms.
These arent hi watt speakers, so Im always careful not to overload it with too much loud distortion, though it sure is tempting! Razz

It sounds great using a K-130 4 ohm JBL 15" as well.
Like Dave said, it will work with an 8 ohm load, but definitely do not go any higher than that!
I know this from personal experience (blown tubes, luckily that was it).

Clete

Ed Phipps · From: Chino Valley, AZ

To Dave Mudgett,

Great article Dave. I have never thought of the back EMF as flyback voltage but that's exactly what it is.
I always remember Thevenins Theorem about total power transfer only being possible when the impedances are equal. It's a great theory but not quite possible with varying impedances as we have in speech or music.
Merry Christmas and all my best to you all in PA.
Ed

Dave Mudgett · From: Central Pennsylvania and Gallatin, Tennessee

Thanks Ed. Yup, perfect impedance matching for maximum power transfer isn't achieved in practice. But I still think it's a bad idea to have the OT turns ratio higher than necessary - you lose power transfer (and possibly fidelity) with any type of mismatch, but if you're gonna go for more mismatch, better to make the speaker impedance too small than too high. BTW - this is backwards from the situation with the typical solid-state amp, which is safer to run with a higher-than-rated speaker impedance.

I know guitar players who run too low a speaker impedance specifically to cut power and fidelity in a tube amp. Reasonable at a certain level, but it does run the tubes down more quickly, which makes no sense to me at NOS tube prices these days. If I want less power and fidelity, I use a smaller amp run where it was designed. My main guitar amp these days is a Princeton Reverb. The bigger ones mostly sit and languish these days.

Ryan - why not use the 8 Ohm D130F with a smaller 8 Ohm speaker, say a 10" or 12", in parallel? That's what I do if I use my Dual Showman Reverb head - the 15" 8 Ohm speaker (a JBL K-130) is in an old Dual Showman bottom that was split into two 1-15" cabs by a friend many years ago, and I have a small lightweight cab housing a somewhat lighter duty 12" 8 Ohm speaker. I tend to angle that small cab up at me as a monitor. Lots of ways to do things.

Forum Index > Electronics		Next oldest topic :: Next newest topic

All times are GMT - 8 Hours	Jump to: