8 ohm-- to-- 4 ohm

[Curtis Alford]

Donny,
I am neither willy nor nilly. I had been running equipment for sound with my band as well as sound for other bands. We have NEVER IN 52 years lost a peice of gear from misuse. Please state your basis for this attemp to flame me.

[Curtis Alford]

Jim,
I know several amp techs that don't repair a lot of amps correctly.
I did not state it was a Peavey resistor, I stated it was the same type of resistor used by Peavey in their amps.
I did not reccomend any resolution for the question. I stated what I have used when necessary. I did not reccomend this as a fix for life.
I find you are always up front to dispute other peoples opinions and resolutions for verious problems.
You may be a first rate tech and maybe not I have no person knowledge.
If you don't try it and prove it is not workable don't flame me for what I have done.

[Curtis Alford]

Dummy Loads built by Weber, do we all agree they a resistor units? Some are fixed valvues, some are variable. Resistors do generate some heat. Excessive heat is not a normal condition.
I did not intend to up set any one but, a resistor is a resistor. The type of resistor is important. From what I can gather from the weber website Jim referenced. The 25watt pictured is the same type I am using. I have this when used, across the speaker terminal. With a LTD400S amp at approximately 50% power it is hardly above room temp. I am not finding any excessive heating. This amp maxed out put is about 28V @ 4ohm. I do use a 8ohm speaker with no problem.
The power amp problem is, example, rated for 2 ohms, 4 ohms,and 8 ohm. We are having most problems in the mid rated loads.We do not daisy chain speakers, One cabinet per channel, mains and monitors. Power conditioner did not help either. Runniing 75% of power is where most problems occur. This is not an every gig happening. Cords are checked every gig.
Any constructive idea. This is only power amps nr combos.

[Jim Sliff]

Curtis, you say it's the same resistor Peavey uses while talking about dummy loads...but now backpedal and say you don't know what it's for. If you don't know what it's for what does it have to do with a dummy load?

You claim to be running at "50%" power. Would you please explain how you are determining your "50%" figure? Because if it's halfway-up on the volume control....oh, forget it. If you're using a 'scope you can tell...if not, as I said - forget it.

Dummy Loads built by Weber, do we all agree they a resistor units?

No.

They make MORE than one type - and the MASS is NOT A RESISTOR. It's a speaker coil...an inductive load. It is constantly in flux like a speaker, because it is (in simple English ) a noiseless speaker. It is absolutely nothing like wiring a static resistor into a circuit.

Go do some reading - preferably amplification books. Study dummy loads. Call or email techs and ask them what THEY use - and WHY.

BTW, "flaming" is meant to offend, aggravate, annoy and such without merit. You're not being flamed; you're being exposed as someone with no practical background in electronics who is giving bad advice that can cost people money. You've been given links and resources and offered nothing but your "luck" in return.

And BTW - yes, I gave you a link to a dummy load. Because if you REALLY want to use one (even though, as stated, there are FAR better systems available) you might want to use one with enough power handling to not turn someone's amp into burnt toast.

You don't believe me, you don't believe Donny...so I suggest you go to www.tedweber.com, go to the amp forum, post details about your "10 watt dummy load" and see if those reactions are more to your liking.

That's called "help".

[David L. Donald]

I thought we were talking guitar/steel amps,
not extended multi speaker p.a. systems,
a TOTALLY different animal. And MUCH more finicky.

Apples and Oranges.

Not really. Whether it's a 30,000-watt PA system, a musical instrument amp, a home stereo system, or a pocket radio - the load must be within acceptable parameters. Amps don't count speakers, they don't even care if there are speakers. They just look at the load presented them (or lack thereof), and react accordingly.

As noted a little higher Ohms will work within acceptable parameters
But too low will fry the amp.
Some amps have larger acceptable tolerances.
Others want ONLY design exacts.
Most fit in somewhere in between
and lean toward idiot proof tolerances.

You can put 5 totally different speakers on a one speaker load,
by balancing the resistances, using serial and parallel,
to equal, in the end, that one it likes best.

Thought that takes no consideration of speaker efficencies, and thus balanced sound.

[Curtis Alford]

To the Gentlemen who do not agree with resistors as load corrective,DO USE IT.
Jim, I hope you put as much effort in the waterproofing and coating projects as you have to make me sound stupid.You of coarse are an Elect.Eng.
God Bless us all and God Bless America
Curtis

[David L. Donald]

Curtis, I am not trying to make you sound stupid.

I have been saying I don't think using these
big resisters serves much purpose in Bill's case
as it is SO FAR understood.

Bill Moran,
exactly what amp and speakers
are your talking about using this on?

Why were you wishing to use it?

Is the amp running with no problems now?

Is there anything 'sounding off' with your amp,
that is making you want to use a matching resister?

[Ken Fox]

I would never suggest a load resistor either. The only place I use one is in my shop. I have 2 at 4 ohms rated at 200 watts each that I load test amps at 4 and 8 ohms via a selector switch.


My background was also 30 years as a low voltage electrician and engineer (local 48, IBEW). I was also licensed as an inside wire-man/electrician,

I designed fire alarm systems (NICEET Certified, level 4) designed PA systems, access control, nurse call systems, distributive sound systems (25 and 70 volt) school intercoms, MATV (RF systems), prison security, video systems, and much more. Not just old tube amps! I spent my last 8 years inside as an engineeer, project manager and estimator.

Public address systems are often 70 volt distribution systems with transistor power amps. Typically each 4" or 8" speaker has a matching transformer on it to allow .5/1watt/2 watt taps. Some have 1/8 ohm taps as well. A 1/2 ohm tap at 70 volts is a 9600 ohm reflected impedance on the primary. That limits the speaker to 1/2 watt at 70 volts RMS. In school intercom systems we used 25 volt systems. A speaker with a 1/2 watt tap would be at 1250 ohms reflected impedance at the primary of the matching transformer. The idea for, example with a 100 watt amp at 70 volts, is to allow 200 speakers tapped at 1/2 watt for the maximum load. Hence the audio is distributed amongst many speakers. That is a max on that amp, less speakers could of course be used. Even one speaker for that matter!

In the old days the Rauland, Stromberg, Dukane and Bogen tube amps used for distributive sound used a load resistor so the amp could operate at a no load condition. A transistor amp does not have to have a load. A tube amp's output transformer can be destroyed without a load on it.

Why would anyone want or need a resistor on an LTD 400 amp? You are going to waste 1/2 of the power across the 8 ohm dummy load. The amp will run cooler with an 8 ohm speaker and still get as much or more power without a dummy load. The amp is rated at 200 watts. 100 watts would be wasted at the dummy load! If ran at 8 ohms, the amp would produce 130 watts, from what I recall from the old spec sheet. And it would run the output transistors a lot cooler.

I will tell you that at full power I can get those 200 watt shop load resistors so hot you can't even touch them! I surely would not want to try 25 watt resistors in the shop for load testing!!

Just some of my thoughts and rambling on!!!

[Jim Sliff]

Thanks Ken - you brought up the really important point; why use a dummy load AT ALL?

Thanks also for reinforcing the heat issue. That can't be stressed enough in this thread - posting bad technical advice is one thing, but when dangerous technical advice is posted it needs to be pointed out by as many qualified individuals as possible before someone gets hurt...or kills their equipment.

[Ken Fox]

We used to try all sorts of impedance matching and padding in the old days! I burned up several resistors experimenting with that on speakers.

I did a real in depth study years ago on impedance matching, gain and gain overlap on rack systems. It was interesting to take different pieces of gear with different impedance and gain levels and match them up with "T", "H" and impedance matching pads. Then after that inject a signal into them and monitor each piece to make sure they arrived at clipping at the same time with a scope. Now days they have input LED's to warn of such and input variable pads. A great idea and useful if one understands the importance of matching equipment properly.

All that stuff is still somewhere in the cobwebs of my mind! I kept a lot of notes over the years so i could refer to it if needed. I just can't seem to keep all the tech stuff in my old head anymore!!!

[Donny Hinson]

Just one last note...


Back when I used to build and service amps, there were some cheaper amps that used audio-frequency chokes across the speakers to match speaker impedances. One of them was a twin-twelve Harmony tube amp. I suspect they got a good deal on some speakers that didn't match the output transformers they used, and they chose this as a "safe way" to balance the impedances. Audio chokes are still available, but the selection and power-handling capabilities are quite limited nowadays.

[David L. Donald]

Just finished wiring a choke into a Dumble clone earlier.

[Jim Sliff]

To the Gentlemen who do not agree with resistors as load corrective,DO USE IT

I assume that was a typo and you meant to say "DON'T USE IT"?

It's not a matter of "not agreeing", Curtis. And it's not a matter of making you "look stupid"...no one is making you "look" anything but yourself.

Technical discussions on this and other forums rang from opinions (what kind of speakers do you prefer in z XYZ amp?) to fact ("how do you calculate speaker impedance using parallel vs serial wiring"?)

Then there are the uncommon posts that give technical advice that is 1) useless and or 2) dangerous. That's where people with technical backgrounds in the subject (David is extremely experienced as is Donnie - I have about 40 years as a part-time amp tech and taught electronics; Ken Fox can bury all of us and has a professional amplifier repair/restoration business plus decades of heavy-duty electronics experience.). When experienced folks see incorrect advice given they WILL correct it, ESPECIALLY is it's dangerous to people, equipment or both. That's exactly what happened in this thread. If you took offense, it may have simply been your own fault for posting technically dangerous information in the first place. And sorry, misdirected attempts to discredit those with experience or waving the flag doesn't cover up bad information.

[David L. Donald]

Engineering has tolerances,
the name of that game is BALANCING those tolerances
with practical limitations.

But one of them is NOT, "It doesn't matter what you do."

Now an interesting tangental question is:

If I have :
One 15" 8Ω Speaker; 95db @ 1 meter for a given reference voltage
In a Fender Tone Ring style cabinet

and wired it in parallel with

two 10" 4Ω speakers; 97db @ 1 meter at same reference voltage
wired together in series
in an semi open back Dumble style cabinet

Giving 4Ω total at amp.

What kind of balance of tones might I expect?

Should I increase or decrease the 15" speakers efficiency?

Should I pick a 15" with more high end roll off and MORE efficiancy?
Less?

Ok this is a practical exercise, but subject to MANY opinions.
The 8||4+4 series is correct for a 4Ω amp setting,
but could also have been 16Ω a 8+4+4 too.

Next part is power handling
what about on a 50w tube amp
One 50w 15" and 2 25w 10"s?
which might run cleaner
and not add too much speaker non-linearities.

or

One 25W 15" and two 15w 10"s
Which might add more speaker growl to the equasian.
But might also run closer to the edge when the amp is pushed hard.

Safer with one 30w 15 and two 20w 10s?

What brands/models or mixes of brands might one say?

Now I can't fault ANYONE an opinion on this set of questions,
because it is potentially so subjective.
Have at it campers!

[Jim Sliff]

David - great questions!

I've tried many different combinations and found that there is NO consistency - the math can be identical but the csound totally different depending on the speakers used.

The first wild-card is speaker efficiency (not the "pure" definition, but the "practical" one - comparative volume at identical amp settings)not tied to power ratings (A power rating example: the Acme low-B bass speaker will handle something like 400 watts, but when used in a 100-watt amp it may be almost inaudible...and most certainly thin sounding). Fender speakers of the 60's and 70's are a unique mix of similar power handling capabilities but dissimilar tone and efficiency. A 50-watt Fender-badged Jensen or CTS will almost invariably be louder and seem more responsive to attack than most of the Utahs or Oxfords.

The second variable is frequency response. It will also skew the results, as a certain mix of speakers/impedance may sound better than another with steel while the reverse may be true with six-string (also depending on style).

Another IS style - some speakers sound better only when played "clean", while others have superior tone when hit with a distorted signal.

Which tosses another ting into the mix - speaker breakup. If you look at the WeberVST site, he has very detailed descriptions of his speakers - and there are many (for example) 50-watt speakers with different breakup levels; they are all designed to handle the same *power*, but cone type, spider design, VC construction etc. is engineered to have the speaker predictably break up at a certain volume level.

This is why "speaker chasing" (a common term in the 6-string world) is both fun and REALLY expensive!

[David L. Donald]

I have consulted with Ted. And we have some ideas,
but always good for more input too.
Especially since I have reduced access
to a variety of speakers these days.

I have made 2 15" tone rings, and the wood shop is
JUST getting finishing construction to pop out boxes
by next week. It is 12m x 3.5m 2.5m tall.

So I will do 2 variations of the 15" front output tone ring cabs.
My buddy really likes them. But I also want some
more top tightness and open back sound at the same time.

I am 1/2 through wiring a Dumble clone amp, so 50w pushed hard is possible.
A lot of Tele/Strat twang bottom and crisp top too. At least that's the aim.
With a bigger amp the combination should be pretty nasty for C6 neck too.

I want the speakers just breaking up at around 45-50w since with tubes they could punch out 65-70w on stun,
and I want the speaker sound in the tone too.

Well it's ALSO at WHAT POINT do you measure your 50w at what distortion rating?
0.001 he fe jazz guitar fanatical

1% lo fi, it works but sounds a bit ratty

5% dirty clean guitar, basic every highschoolers home amp

10% cause I want to look bigger balled?
mini metal heads could care less if it ain't pure clean.

65-70w at 20% should have a pretty nice
hard rock sound for screaming leads.
Don't get too close now.


I also have two 12" tone rings and want to do a 112 TR cab for an 18 Marshal type amp.

Speaker chasing for me is QUITE problematic due to location.
Karioke speakers and BAD guitar boxes are the norm here,
or fly to Bangkok and hope to get luck or mail order.

[Larry Bressington]

4 ohms is a basic figure, 4 ohms is not static , an 8 ohm speaker will be warmer but headroom will be slightly diminished;

[David L. Donald]

Turns out speaker chasing just got easier
I just found that one of my repair part suppliers
P Audio, makes guitar speakers for the Mackie VT-12,
and Ampeg Jet J-20 which seems to be quite well thought of.

If I get the speaker I can voice the amp
and especially the cab to fit the speaker.

Sure will beat the individual shipping cost of Webers.
I hope I like them like the Webers...
They also said I can get short run custom ones,
if I reach a certain number. that's good to know too.

I thought they only did full range PA and Karioke speakers,
and was suprised when the tech said:
Yes we do OEM for X, X, X & X
who rebrand them... OK good on you mate.

[James Harrison]

An 8 ohm speaker on a 4 ohm output is less load not more and will not make much difference in RMS wattage output. A 2 ohm speaker on a 4 ohm output will overload the amp and damage it. Remember you are talking about RMS watts output and loads, not straight resistive loads. On a resistive load the small wire in a 200 watt speaker coil would burn out at a very low resistive load.

[David L. Donald]

An 8 ohm speaker on a 4 ohm output is less load not more and will not make much difference in RMS wattage output.
Sorry, but it is double the load.

A 2 ohm speaker on a 4 ohm output will overload the amp and damage it.

It will UNDER LOAD the tubes and output transformer.
Particularly bad on a tube amp,
because the tube will not see enough resistance
and can red-plate or draw too much current.

Remember you are talking about RMS watts output and loads, not straight resistive loads.

RMS or PEAK will not differ to a loads jjob it is just two ways of measuring a potential.
Root Means Square is just an averaging mechanism,
nothing to do with frequency interacting in an inductor.

On a resistive load the small wire in a 200 watt speaker coil would burn out at a very low resistive load.

It is an INDUCTIVE load and thus is frequency sensative
relative to higher frequencies,
resitance goes up significantly at higher frequencies.
as well as to a lower resonance frequency load point.

2, 4 , 8 16 are nominal reference points for lowest loading.


Too little load is the amp killer for tubes.

[Larry Bressington]

Valve amps are far more forgiving than solid state, i think most have of us have even proven that over the years, but its true, so if you ran a 4 ohm speaker on an 8 ohm amp that was valve, you would get away with it, transistors will get too hot and burn out, i prooved that on a test in my shop just to see,Mosfets are the best for withstanding punishment, better than a valve, use a big heat sink!
Technically, i am incorrect, but in practice it has happened, your ohms are not static at 4, thats a ballpark figure, it changes with the signal.

[James Harrison]

David the way you see it a short circuit would be no load and an open circuit would be a dead short. I still say a 2 ohm load is greater than a 16 ohm load in a 4 ohm speaker output circuit.
Using an 8 Ohm Transformer Secondary
Connecting a 4 ohm speaker load will draw 2 times the marked wattage on the transformer
Connecting a 16 ohm speaker load will draw 1/2 the marked wattage on the transformer
The total impedance load should not be less than the minimum output impedance of the amplifier.
If I am wrong, I was taught wrong in the late 1950s and I appoligize.
I say what we don't know, won't bother us, its what we do know that is not right.

[Michael Strauss]

The amp is rated at 200 watts. 100 watts would be wasted at the dummy load! If ran at 8 ohms, the amp would produce 130 watts, from what I recall from the old spec sheet.

FWIW I have a Fender Jazz King (140W@8ohms). I was told by the folks @ Fender it's the same as the Steel King (200W@4ohms) with the speaker change. I'm no expert, but that's what I was told.

[Jim Sliff]

A couple of other points (and I agree with David's analysis regarding loads for the most part):

1. A 2 ohm load on an OT that's looking for 4 ohms will not "damage the amp" - it will only damage the amp if the amp is designed with strict tolerances, which most guitar amps are not. Fender tube amps, with a few exceptions, will tolerate a 100% mismatch either direction without any problem. They were designed that way. The only one that is a bit shaky is the Super Reverb, where you simply have to be careful not to go down to 1 ohm load...they'll handle a theoretical 1.5 (knowing the actual impedance changes during use) but most techs will recommend NOT using the Ext jack on a Super unless you REALLY know what you have as far as speakers.

The one technical omission/error I've seen in the last several posts is that while too low a load (i,e, 2 ohms on a 4 ohm output) won't damage the amp, just wear out tubes faster and run hotter (things that bias changes can sometimes compensate for as long as you don't screw up the tone going TOO cold) it's too HIGH a load that is most dangerous with tube amps.

The Fender Bassman head, designed for 4 ohms, was/is often used with an 8-ohm load without any problem. But if you plug a 4x10 Marshall 16-ohm cab into it, a sries-wired 2x12 Fender cab with 8 ohm speakers, or even worse, a 4x12 cab of 8-ohm speaker wired in series (32 ohms) you stand a good chance of killing the output stage due to "flyback". 200% high is pretty much guaranteed to cause flyback, and is much more damaging than 200% on the LOW side, which you'll probably recognize by blown output tubes (unless you see the virtual lava-lamp reflected off the wall in back of the amp.. ). Flyback takes out transformers, which excessively low impedance is more apt to cook tubes first.

This has all concerned Fender amps - a few others are not as forgiving. Marshalls, for the most part, don't handle even a 100% mismatch very well, specifically the JCM800's and 900's (900's are especially finicky, hence the LED warning lights to let you know a tube has failed...as if you couldn't already tell!). That's why most Marshalls have different taps and a switchable impedance, and why it's critical that you have the switch in the right place!

100% too high or too low in a Marshall is trouble; 100% too high or too low in a Fender means a bit of power loss and a colder/hotter running amp.

[David L. Donald]

I'll take "for the most part"

Think of a short circuit as NO RESITANCE.
so no load holding back the signal flow.

Think of a open circuit as TOTAL RESITANCE
Nothing can get through because the load is too big.

At least that's how I see it on a basic level.

This page might be useful.
http://www.maxim-ic.com/appnotes.cfm/an_pk/4164
I steal a bit below

Ceramic speaker impedance, as seen by a driving amplifier, can be modeled as an RLC circuit with a large capacitance as its main element (Figure 1). Across most audio frequencies, the ceramic speaker is mostly capacitive. The speaker's capacitive nature dictates that impedance decreases as the frequency increases. Figure 2 shows the similarity of ceramic speaker's impedance versus frequency to that of a 1µF capacitor. This impedance also has a point of resonance above which the speaker is most efficient at producing sound. The dip in impedance around 1kHz indicates the speaker's resonant frequency.

fig 1

fig 2


Of course tha actual needs of the speaker for flat respoinace are inverse.



Plenty more on the page, this was just a teaser.