Using external Spkr jacks on vintage fender amp

Marc Stone · From: Louisiana, USA

I've read that Fender amps supposedly were designed to handle up to a 50% impedance mismatch, but in the interest of avoiding that -
Want to run a 212 ex cab w a BF Vibrolux (2x10, 4 ohms). I have two nice 8 ohm 12s I want to load the cab with. If I wire the 2 12s in parallel, that should be 4 ohms correct? And then I could bypass the speakers in the amp, run straight into the cab and be matched, yes?
What if I wired the 12s in series for 16 ohms? Would that create 16 divided by 4 for a total 4 ohm load if I run the cab and the amp speakers together? Or am
I commiting amp suicide?
Thanks!

Dave Mudgett · From: Central Pennsylvania and Gallatin, Tennessee

Two 8-Ohm (nominal impedance) speakers in parallel gives 4 Ohms. That's what a Vibrolux Reverb wants, as long as it's the original output transformer. Doesn't matter if they're in the amp's cabinet or an external cabinet.

Two 8-Ohm speakers in series gives 16 Ohms. If you then ran that 16-Ohm load in parallel with the cabinet's 2x10" speakers' 4-Ohm load, the overall impedance would be 1/((1/4)+(1/16)) = 16/5 or about 3.2 Ohms. Not a big mismatch at all, but remember that most (around 80%) of the power will be dropped over the 4-Ohm load. The voltage drop across each will be the same, but the current through the 4-Ohm load will be 4 times higher than that through the 16-Ohm load.

Marc Stone · From: Louisiana, USA

Thx Dave. Forgive my ignorance, but do you mean the amp will lose 80% of its power and the components will be seeing four times the normal current? I'm not experienced with electronics, so I'm pretty sure I'm misunderstanding what you are trying to explain to me.
What would the practical impact of tha be on the output/performance of the amp?

Lane Gray · From: Topeka, KS

By "dropped" Dave means that's where the current gets turned into moving air. The 4 ohm array will get 80% of the output of the amp and the 16 ohm array will only see 20% of the output. Electricity, like water or people, will follow the path of least resistance, so most will go to the 4. Aim the 4Ohm array at the audience, and use the 16 as a monitor.
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Godfrey Arthur · From: 3rd Rock

Depending on which speaker has a higher sensitivity, that speaker(s) will appear louder. You could also experience frequency jumps during playing where the different resonances of each speaker will "blink" from speaker to speaker. Your treble might sound different through one of the speakers and then the bass will appear or disappear at random.

And if your 2x12 cabinet has certain resonances, (is a closed back especially) then you will also get that frequency "hopping" of certain frequencies ping-ponging between the Vibrolux and the 2x12 cab.

But if you're going to use just the 2x12 cab and the Vibrolux as a head, you will have more control over the sound.

Welcome to multiple speaker anomalies 101.
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Marc Stone · From: Louisiana, USA

Great thanks guys. No point in trying to drive a 212 with 20% of 35w, or opening myself up to weird sonic dropouts. I'm sure just running the VR as a head into a big ol Bandmaster 212 cab will give me all the extra girth I could wish for, when needed.

Lane Gray · From: Topeka, KS

Marc, it's not as linear as you're thinking. The overall sound will be more because you're pushing more air. But not a lot more. You'll also find that it'll probably sound better with more cones moving the air around.
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Godfrey Arthur · From: 3rd Rock

The amp must have enough power to drive your speakers. Use an amp that has 2 to 4 times the continuous power rating of your speakers, is a norm depending on the type of music you'll play and how large a venue.

The louder the music the higher wattage the amp must be.

A 35 watt amp driving more speakers will only clip the amp if the total number of speakers needs more than 35 watts especially if there are high peaks in the music.
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Dave St-Pierre · From: Ontario, Canada

I also have a BF Vibrolux Reverb, original trannies so a 4 ohm load from the original Jensons. I have a Peavy 410e cab, 4x10 wired down to a 4 ohm load (the stereo/mono switch has been disabled. If I want to use both the internal speakers and hook up the 4x10 Peavy, what ohm rating will the amp see? Will it hurt the amp?

Godfrey Arthur · From: 3rd Rock

Dave Grafe · From: Hudson River Valley NY

Godfrey Arthur · From: 3rd Rock

Dave Grafe · From: Hudson River Valley NY

What did I not understand? Please explain the context I missed, I included every word you said, is there more? Here's the deal: You made several statements that are not factual or scientifically supported, and in the interest of communal best knowledge I have endeavored to set facts straight for those present who have no way of knowing any better. It is in no way personal but in these statements you were simply mistaken.

As regards your link, Crown sells power amps to audiophiles and PA users, to entities focused on the absence of all signal clipping, and their marketing department is on their toes in that regard. However, we are talking musical instrument applications here, specifically Fender amps and their extension speaker jacks, in a world where clipping is an essential element of art itself, and where putting a 35 watt speaker into your 100 watt amp is counter-indicated.

I don't know about your straw man, and I have no need of mucking anywhere. So very sorry you feel compelled to go there Crying or Very sad

Godfrey Arthur · From: 3rd Rock

Godfrey Arthur · From: 3rd Rock

I realize speaker amp relationship questions can get confusing. Confused

And why not, it's a combination of complex systems.

There are many views and opinions on what is a rule and what is an opinion. Huh?

One thing I've learned is since you have an amp system and a speaker system, mating these two is a balancing act.

You need to use your ears and your brain to survive.

Having said that there is always an exception.**

According to the "experts" using speakers that are rated for way more than the amp capacity will not likely hurt the speakers.

And then, if using speakers that are 3 to 4 times the capacity of the amp, the speakers will not be damaged no matter how hard the amp is clipped.**

Will it hurt the amp? In reality this is never a cut and dried answer. There are always the exceptions to the rule.

Will stressing the amp cause it to fail by using speakers that don't flinch because they are rated at a higher wattage than the amp? Maybe.

But here is where it gets interesting.

A speaker has a voice coil and because of the amp signal (voltage), generates heat in the voice coil. Now in order for the speaker to cool itself, the amp must push the speaker cone enough for the air to rush around the coil to cool it. The speaker is a motor.

This cooling function can happen if a "clean" sine wave signal (amp not too overdriven) is being fed the speaker. The excursion of the cone will be enough to allow air to cool the coil.

If a "clipped" square wave (amp pushed way too hard) is being fed the speaker, (and is double the voltage at this point) the speaker's excursion will not be enough to cool the coil. The coil is not moving enough which then causes voice coil overheating. Has to do with power dissipation. Funny how that happens. The speaker is still moving in most cases but under a square wave clipped signal from a not dialed-in too thoroughly rig, not enough to cool the coil.

Depending on the frequency of the signal, a 200 watt amp can push a speaker a large percentage more to a safe cooling operation than a 100 watt amp that is clipping. The extra cone excursion will allow air around the voice coil.

If the speakers are rated the same capacity as the amp can cleanly dish out, if you run the amp for extended periods with a clipped signal, damage MAY result to your speakers.

White line is the dreaded clipped square wave; amp pushed way too hard. The yellow line is the sine wave "clean" signal the amp can safely deliver.

If the speakers are rated for the same capacity as the amp can cleanly produce, running the speakers with a square wave signal will cause damage to the speakers.

At A, B, D, E, F, H, the voltage is changing allowing the voice coil to move to pull in fresh cool air. At points C, G, the coil may be moving a little but not enough to cool properly. During the clipped portion of the waveform, current is still flowing through the voice coil. But more heat is being generated than is proportionally dissipated.

Failure to quote Breaking Benjamin

Now if we're not provided with enough information, we can't arrive at a good enough answer.

So we hop around with vague generalities.

How many watts is the external speaker/cab in question?

Do you like to play distorted or are you Mr. Clean Tone with maximum headroom?

This will figure much in configuring the type rig you would need.

Without proper info the answer is not complete.

Now the exception** is always there.

SRV playing with a 40 watt Super Reverb always managed to blow his Electro-Voice 10"s that had massive wattage capacity... so his tech had spare speakers on hand.

So it comes down to this, unless you can afford a roadie or an amp guru at your beck and call to carry your gear loaded with extra heavy high wattage speakers or get you an extra fresh amp while he fixes your blown amp, you need to kick your ear/brain factor into high gear.

Jack Black?

Brad Pitt??

DiCaprio???

No!

Angus Young's amp guru on the road with AC/DC.

Note:

All the above did not address the tone, the grail of amp/speaker combinations.

Only amp and speaker implosion. And not even speakers failing from the adhesive coming unglued under stress.

How your external speaker/cab will sound with your rig and your playing, is another ball of wax that needs addressing.
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Ezekiel 33:7

Tony Prior · From: Charlotte NC

in practicality, very few of us, if any at all, have ever fried a Fender Output Transformer. The theory says we can and maybe will should we mismatch the speaker load, the practicality says different.

Fender circuit designs want us to use the primary speaker out for ONE speaker and the ext out for the 2nd. Of course you don't have to do that , you can run multiple speakers off the primary jack, especially if it's a "head" driving a couple of cabs daisy chained.

I have personally connected and witnessed all sorts of speaker conglomerations over the years with Fender amps, 4 ohm OT's 8 Ohm Ot's, it didn't matter, we just plugged em' in.

Is the theory above correct,? of course it is.

When I was in electronics school (late 60's) I ran a 12VDC to AC converter in my car and hooked up my Sony Reel to Reel tape deck in my car. My Professor spent half an hour in class telling everyone why it would not work. He was adamant. So we took a field trip ( to the parking lot) where I turned it all on and we listened to CHICAGO on my tape deck. Our Professor just shook his head and said ' "it's not supposed to work " .
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Dave Mudgett · From: Central Pennsylvania and Gallatin, Tennessee

To clarify my original reply for Marc - I said that with a nominal 4-Ohm and 16-Ohm load in parallel, the 4-Ohm load would get (nominally) 80% of the power and the 16-Ohm load would get 20% of the power. I said nothing about loudness. That will, of course, depend on many factors, including the frequency spectrum of the signal being amplified, and the frequency response and sensitivity of the speakers. Dave G. is absolutely correct that the complex impedance curve of the speakers varies with frequency and the matchup between the musical signal frequencies and that speaker frequency response is a big factor.

I also intimated that I don't think using a 3.2-Ohm nominal impedance is a big issue in an amp expecting a 4-Ohm nominal impedance. In general, it is better with a tube amp (which has an output transformer) to use a slightly-lower-than-rated-impedance load than a slightly-higher-than-rated-impedance load. This is discussed a lot of places, including on this forum. The issue is that a higher-impedance load tends to create a higher "flyback" voltage from the speaker back to the output transformer. If this gets high enough, it can cause arcing and subsequent melting of the insulation in the transformer windings.

I think most later Fender output transformers are over-designed and can handle a certain amount of this, but I wouldn't push it. The higher the mismatch and harder one pushes an amp, the more likely something like this is. I have had output transformers go on old Fenders - these were being run with the correct load impedance, I just played them hard for years and years and they finally went. And IMHO, output transformers from certain makes are more sensitive than those hardy old Fenders.

The other thing is that, technically, maximum power transfer occurs when the complex impedance of the amp and the complex conjugate of the load impedance are the same. It is essentially impossible to exactly match these complex impedances in practice. But the closer to matched, the better power transfer. Some people prefer to run unmatched. There are sometimes valid reasons to do this. Myself, I generally prefer to run amp and load as closely matched as possible. But again - I don't think running a 16-Ohm cab in parallel with a Vibrolux's 4-Ohm internal speaker combo is a big issue.

I don't really want to wade into the argument that ensued. But I basically see it from Dave's point of view - I would generally prefer a higher-efficiency speaker in a lower-powered amp. Using a good high-efficiency speaker can bring a lower-powered amp alive, to my tastes. Of course, measuring speaker "efficiency" or "sensitivity" is not necessarily straightforward. Some manufacturers measure sensitivity at one frequency (1 KHz), which doesn't really tell the full story. I like Eminence's approach: http://www.eminence.com/support/understanding-loudspeaker-data/

Paul Honeycutt · From: Colorado, USA

All I can tell you is that back in the sixties we plugged different speakers and amps together in every combination you can imagine and never blew anything up. Deluxe Reverb's into Bassman cabs, Bassman's running two 2 x 12" cabs. Twin's with a single 15" cab along with the 12's. You name it, we probably did it.

And some of the combinations actually sounded good!

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