Quencho092
New member
im thinking of grabbing a vintage jbl d130f 15 inch speaker on ebay. It's 8 ohms...is it possible to have a dummy load suck up another 8 ohms to make the total load 4 ohms, so i dont damage my bandmaster....
Can I use one 8 ohm speaker and a dummy load to have 4 ohms?
- Thread starter Quencho092
- Start date
Artie
Peaveyologist
Re: Can I use one 8 ohm speaker and a dummy load to have 4 ohms?
Sure . . . just keep in mind, it'll have to be a fairly large resistor. It should be rated at least half the amps power. Even then, its going to get hot, not to mention, hard to find, and expensive.
What kind of power are we talking about?
Edit: I just checked Mouser.com. A 10-ohm, 50-watt resistor is only about $10. So, not as expensive as I thought.
Sure . . . just keep in mind, it'll have to be a fairly large resistor. It should be rated at least half the amps power. Even then, its going to get hot, not to mention, hard to find, and expensive.
What kind of power are we talking about?
Edit: I just checked Mouser.com. A 10-ohm, 50-watt resistor is only about $10. So, not as expensive as I thought.
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Quencho092
New member
Re: Can I use one 8 ohm speaker and a dummy load to have 4 ohms?
meh 40 tube watts. Im coupling this with a big jbl d 130f speaker, i dont know the handling on this speaker but i know it can handle a fender showman and thats 85 tube watts.
How much handling should i get the resistor to be? I figure a whole lot of wattage would be necessary so i dont reduce the total speaker wattage handling, cause isnt the amount of wattage a set of speakers could hold the smallest rated speaker times 2?
Like if the jbl d130f could handle 130 watts, if i get a 50 watt resistor, then speaker could only handle 50 watts.... help me out here...
meh 40 tube watts. Im coupling this with a big jbl d 130f speaker, i dont know the handling on this speaker but i know it can handle a fender showman and thats 85 tube watts.
How much handling should i get the resistor to be? I figure a whole lot of wattage would be necessary so i dont reduce the total speaker wattage handling, cause isnt the amount of wattage a set of speakers could hold the smallest rated speaker times 2?
Like if the jbl d130f could handle 130 watts, if i get a 50 watt resistor, then speaker could only handle 50 watts.... help me out here...
Quencho092
New member
Re: Can I use one 8 ohm speaker and a dummy load to have 4 ohms?
and thanks a ton artie, you saved me from a heart attack since i had 190 bucks bidded on that jbl....oh and by the way, its a set of 2 15's, one needs reconing, ill sell it to you if youre interested....
and thanks a ton artie, you saved me from a heart attack since i had 190 bucks bidded on that jbl....oh and by the way, its a set of 2 15's, one needs reconing, ill sell it to you if youre interested....
Quencho092
New member
Re: Can I use one 8 ohm speaker and a dummy load to have 4 ohms?
hey can someone help me find a 60+ watt 8 ohm resistor on mouser....
hey can someone help me find a 60+ watt 8 ohm resistor on mouser....
mongrollo
New member
Re: Can I use one 8 ohm speaker and a dummy load to have 4 ohms?
Hey... YOU CAN'T USE A RESISTOR!!!
Speakers aren't resistors, they behave like inductances. If you use a resistance assuming it will work like an speaker you will end in an imballance load to your amp that will wear out you tubes very quikly.
You need something that behaves like a speaker but doesn't produce any sound. The only thing it cames to my mind it is an weber attenuator. Those are speakes without cone.
If you have more than one speaker on your amp the power will be equally share between all the speakers (no matter if the configuration it is in series or in parallel). So all the speaker must have at least a maximun power handling equal to the amp wattage divided by the number of speakers.
As an example: Imagine you have a 75 watts amp with 3 speakers, let 75/3=25wats. Each speaker will recieve 25 watts from the amp, then they shoud be rated 25watts or higher to work properly.
Hey... YOU CAN'T USE A RESISTOR!!!
Speakers aren't resistors, they behave like inductances. If you use a resistance assuming it will work like an speaker you will end in an imballance load to your amp that will wear out you tubes very quikly.
You need something that behaves like a speaker but doesn't produce any sound. The only thing it cames to my mind it is an weber attenuator. Those are speakes without cone.
If you have more than one speaker on your amp the power will be equally share between all the speakers (no matter if the configuration it is in series or in parallel). So all the speaker must have at least a maximun power handling equal to the amp wattage divided by the number of speakers.
As an example: Imagine you have a 75 watts amp with 3 speakers, let 75/3=25wats. Each speaker will recieve 25 watts from the amp, then they shoud be rated 25watts or higher to work properly.
Artie
Peaveyologist
Re: Can I use one 8 ohm speaker and a dummy load to have 4 ohms?
If your amp is 40 watts, then placing a resistor in parallel with the speaker, will absorb approximately half the power. Lets say you're actually running at full power. The resistor will absorb 20 watts. The thing is, you don't want to run the resistor at its full load. Getting one of around 60 watts should cover you, but thats the minimum I'd go.
You probably aren't going to find one that's exactly 8 ohms, but you shouldn't need to. Remember, if you use a 10 ohm, it will still be 8.88 ohms total, in parallel with your speaker. That should be fine.
I'm not sure what mongrollo is refering to. You can certainly use a resistor in the manner you're asking about. The amp doesn't care where its load comes from. I used dummy resistor for audio loads all the time, when I did pro-audio repair. Although, he is correct in one area. If you're running your amp at say, 20 watts, half that power is being wasted as heat across the resistor. So, you are, in affect, "using up" your tubes faster. But there's no damage, per se, other than that.
If you do this, be sure to mount the resistor somewhere where it won't be close to anything. You might even want a small fan blowing on it, like those ones inside a computer case. Just depends on how "hot" you run your amp.
If your amp is 40 watts, then placing a resistor in parallel with the speaker, will absorb approximately half the power. Lets say you're actually running at full power. The resistor will absorb 20 watts. The thing is, you don't want to run the resistor at its full load. Getting one of around 60 watts should cover you, but thats the minimum I'd go.
You probably aren't going to find one that's exactly 8 ohms, but you shouldn't need to. Remember, if you use a 10 ohm, it will still be 8.88 ohms total, in parallel with your speaker. That should be fine.
I'm not sure what mongrollo is refering to. You can certainly use a resistor in the manner you're asking about. The amp doesn't care where its load comes from. I used dummy resistor for audio loads all the time, when I did pro-audio repair. Although, he is correct in one area. If you're running your amp at say, 20 watts, half that power is being wasted as heat across the resistor. So, you are, in affect, "using up" your tubes faster. But there's no damage, per se, other than that.
If you do this, be sure to mount the resistor somewhere where it won't be close to anything. You might even want a small fan blowing on it, like those ones inside a computer case. Just depends on how "hot" you run your amp.
Artie
Peaveyologist
Re: Can I use one 8 ohm speaker and a dummy load to have 4 ohms?
I wanted to add one more thing. I see you've already bid on that speaker, but this really isn't the best way to go about doing this. I don't know if those JBL's have something "magical" about their sound, but there's a wide range of 15" speakers out there. I would think you'ld be much better off just getting the proper speaker to begin with.
You will, after all, be wasting half your amps power in heat.
I wanted to add one more thing. I see you've already bid on that speaker, but this really isn't the best way to go about doing this. I don't know if those JBL's have something "magical" about their sound, but there's a wide range of 15" speakers out there. I would think you'ld be much better off just getting the proper speaker to begin with.
You will, after all, be wasting half your amps power in heat.
mongrollo
New member
Re: Can I use one 8 ohm speaker and a dummy load to have 4 ohms?
I will try to explain it but firstly i have to beg your pardon for my english.
An amp it is a device that takes the weak guitar output and amplifies it so it can drive and speaker. (We all agree in this
) But from the guitar to the speaker there is a long way. The signal gets into the amp, firstly goes thru the preamp, where it is modified by the eq's, effects and so... Then to the amplification section where the signal is amplified. When it comes out this section the signal has a great voltage and a small current. To drive an speaker you need exactly the opposite: a great current and an small voltage. To achive this and output transformer it is needed. This can be seen as something that maches the impedance the amp expects with the impedance that the speaker has. The output transformer it is selected in a way that, for a given speaker impedace, all the sections of the amp will be working in the right way. All the internal voltages and currents will optimal.
If you use a speaker with a different load that it is advised the amplifier will be working out of the range that has being designed to. In a solid state amp, since the componets are very robust, the consecuence will be that you won't get a good tone out of it. In a tube amp this will cause the tubes to wear out much quicker since they will be forced to work in a way they haven't being designed to.
So, WHY CAN'T YOU USE A RESISTOR????
The impedance isn't the resistance, altought they are messured in the same units (Omh). The impedance it is the sum of the effects of resistance, the capacitance and the inductance. (Not the straight sum but the combination of the effects). If you place a resistor (8 Ohm) in parallel with a speaker (8ohm) you will be mixing an impedance of 8 ohm with a resistance of 8 Ohm. The result won't be 4 Ohm impedance nor a 4 Omh resistance!!! The calculus are much complicated...
Sorry, I can't explain it better.
I will try to explain it but firstly i have to beg your pardon for my english.
An amp it is a device that takes the weak guitar output and amplifies it so it can drive and speaker. (We all agree in this
) But from the guitar to the speaker there is a long way. The signal gets into the amp, firstly goes thru the preamp, where it is modified by the eq's, effects and so... Then to the amplification section where the signal is amplified. When it comes out this section the signal has a great voltage and a small current. To drive an speaker you need exactly the opposite: a great current and an small voltage. To achive this and output transformer it is needed. This can be seen as something that maches the impedance the amp expects with the impedance that the speaker has. The output transformer it is selected in a way that, for a given speaker impedace, all the sections of the amp will be working in the right way. All the internal voltages and currents will optimal.If you use a speaker with a different load that it is advised the amplifier will be working out of the range that has being designed to. In a solid state amp, since the componets are very robust, the consecuence will be that you won't get a good tone out of it. In a tube amp this will cause the tubes to wear out much quicker since they will be forced to work in a way they haven't being designed to.
So, WHY CAN'T YOU USE A RESISTOR????
The impedance isn't the resistance, altought they are messured in the same units (Omh). The impedance it is the sum of the effects of resistance, the capacitance and the inductance. (Not the straight sum but the combination of the effects). If you place a resistor (8 Ohm) in parallel with a speaker (8ohm) you will be mixing an impedance of 8 ohm with a resistance of 8 Ohm. The result won't be 4 Ohm impedance nor a 4 Omh resistance!!! The calculus are much complicated...
Sorry, I can't explain it better.
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Artie
Peaveyologist
Re: Can I use one 8 ohm speaker and a dummy load to have 4 ohms?
You are absolutely correct . . . up to a point. (btw - your English is excellent.)
Impedance is, in fact, a more complex function than simple resistance, however, when we're talking about amplifier "load", the only aspect of impedance that we're concerned with, is the resistance portion. If I completely replace the 8 ohm speaker of an amp with an 8 ohm resistor, that amp will run happily all day long and never know that its not producing a sound.
Solid state amps are actully happier with resistors over inductors, (speakers). Most SS amps have a damping resistor as the last component in the output chain specifically to suppress, or "dampen", the back EMF that's generated by the mechanical return of the speaker and the collapse of the field around the speaker coil.
Even in tube amps, the back EMF, and the current that goes along with that, adds to the overall current that the output transformer must bare. This is called "reflected" power. In a guitar amp, the amount is negligable, but its still an undesirable affect.
There really is no harm to come from using a resistor in the amps output. However, its just not a great idea for other reasons. As a temporary "fix", or for troubleshooting, its ok, but I wouldn't want to strive for that end.
You are absolutely correct . . . up to a point. (btw - your English is excellent.)
Impedance is, in fact, a more complex function than simple resistance, however, when we're talking about amplifier "load", the only aspect of impedance that we're concerned with, is the resistance portion. If I completely replace the 8 ohm speaker of an amp with an 8 ohm resistor, that amp will run happily all day long and never know that its not producing a sound.
Solid state amps are actully happier with resistors over inductors, (speakers). Most SS amps have a damping resistor as the last component in the output chain specifically to suppress, or "dampen", the back EMF that's generated by the mechanical return of the speaker and the collapse of the field around the speaker coil.
Even in tube amps, the back EMF, and the current that goes along with that, adds to the overall current that the output transformer must bare. This is called "reflected" power. In a guitar amp, the amount is negligable, but its still an undesirable affect.
There really is no harm to come from using a resistor in the amps output. However, its just not a great idea for other reasons. As a temporary "fix", or for troubleshooting, its ok, but I wouldn't want to strive for that end.
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moog1000
New member
Re: Can I use one 8 ohm speaker and a dummy load to have 4 ohms?
Once more Artie you astound me with your breadth of knowledge and pyscho-babble!!
ArtieToo said:You are absolutely correct . . . up to a point. (btw - your English is excellent.)
Impedance is, in fact, a more complex function than simple resistance, however, when we're talking about amplifier "load", the only aspect of impedance that we're concerned with, is the resistance portion. If I completely replace the 8 ohm speaker of an amp with an 8 ohm resistor, that amp will run happily all day long and never know that its not producing a sound.
Solid state amps are actully happier with resistors over inductors, (speakers). Most SS amps have a damping resistor as the last component in the output chain specifically to suppress, or "dampen", the back EMF that's generated by the mechanical return of the speaker and the collapse of the field around the speaker coil.
Even in tube amps, the back EMF, and the current that goes along with that, adds to the overall current that the output transformer must bare. This is called "reflected" power. In a guitar amp, the amount is negligable, but its still an undesirable affect.
There really is no harm to come from using a resistor in the amps output. However, its just not a great idea for other reasons. As a temporary "fix", or for troubleshooting, its ok, but I wouldn't want to strive for that end.
Once more Artie you astound me with your breadth of knowledge and pyscho-babble!!
mongrollo
New member
Re: Can I use one 8 ohm speaker and a dummy load to have 4 ohms?
I don't think I agree with this statement.
In a very simple way the impedance it is defined as the "resistance to the AC current". The real value of the impedance in a device like an speaker depends on the frecuency of the current that goes thru it. The given inductance of a speaker (4,8,16... Ohms) it is the one measured with a fixed frecuency current. But it can be any, just depends on the signal that goes trhu it. In a guitar amp (or any HI-FI amp) the amplified signal it is, in the end, a AC current with a variable frecuency. The output of the amp it is a AC current that goes thru the speaker, and the impedance of the speaker varies with the frecuency and this means that the amp load should change with frecuency too. So, IMO, if you place a resistor as a load for an amp it won't react in the way an amp expects. It's impedance value will be allways the same.
May be for an amp that amplifies a DC or an fixed voltage the load could be a resistor since the output of the amp will be a DC current. It won't change with frecuency since doesn't have any.
Anyway, thanks for your comment about my English!!
ArtieToo said:You are absolutely correct . . . up to a point. (btw - your English is excellent.)
Impedance is, in fact, a more complex function than simple resistance, however, when we're talking about amplifier "load", the only aspect of impedance that we're concerned with, is the resistance portion. If I completely replace the 8 ohm speaker of an amp with an 8 ohm resistor, that amp will run happily all day long and never know that its not producing a sound.
I don't think I agree with this statement.
In a very simple way the impedance it is defined as the "resistance to the AC current". The real value of the impedance in a device like an speaker depends on the frecuency of the current that goes thru it. The given inductance of a speaker (4,8,16... Ohms) it is the one measured with a fixed frecuency current. But it can be any, just depends on the signal that goes trhu it. In a guitar amp (or any HI-FI amp) the amplified signal it is, in the end, a AC current with a variable frecuency. The output of the amp it is a AC current that goes thru the speaker, and the impedance of the speaker varies with the frecuency and this means that the amp load should change with frecuency too. So, IMO, if you place a resistor as a load for an amp it won't react in the way an amp expects. It's impedance value will be allways the same.
May be for an amp that amplifies a DC or an fixed voltage the load could be a resistor since the output of the amp will be a DC current. It won't change with frecuency since doesn't have any.
Anyway, thanks for your comment about my English!!
Artie
Peaveyologist
Re: Can I use one 8 ohm speaker and a dummy load to have 4 ohms?
You are explaining this exactly correct. Where I think you may be getting messed up is, that change in impedance with frequency, is the bad aspect of inductive loads. Its an undesirable, and unavoidable, side affect of speaker loads. Amplifier engineers work to prevent that change from harming there amps, with different type of filtering and damping added to the amps circuit so that it doesn't oscillate.
A purely resistive load solves that problem. Having an impedance that doesn't change with frequency is ideal. As a side note, many high-power resistors are wire-wound, and do present a degree of inductance into the equation.
mongrollo said:I don't think I agree with this statement.
In a very simple way the impedance it is defined as the "resistance to the AC current". The real value of the impedance in a device like an speaker depends on the frecuency of the current that goes thru it. The given inductance of a speaker (4,8,16... Ohms) it is the one measured with a fixed frecuency current. But it can be any, just depends on the signal that goes trhu it. In a guitar amp (or any HI-FI amp) the amplified signal it is, in the end, a AC current with a variable frecuency. The output of the amp it is a AC current that goes thru the speaker, and the impedance of the speaker varies with the frecuency and this means that the amp load should change with frecuency too. So, IMO, if you place a resistor as a load for an amp it won't react in the way an amp expects. It's impedance value will be allways the same.
May be for an amp that amplifies a DC or an fixed voltage the load could be a resistor since the output of the amp will be a DC current. It won't change with frecuency since doesn't have any.
Anyway, thanks for your comment about my English!!
You are explaining this exactly correct. Where I think you may be getting messed up is, that change in impedance with frequency, is the bad aspect of inductive loads. Its an undesirable, and unavoidable, side affect of speaker loads. Amplifier engineers work to prevent that change from harming there amps, with different type of filtering and damping added to the amps circuit so that it doesn't oscillate.
A purely resistive load solves that problem. Having an impedance that doesn't change with frequency is ideal. As a side note, many high-power resistors are wire-wound, and do present a degree of inductance into the equation.
Quencho092
New member
Re: Can I use one 8 ohm speaker and a dummy load to have 4 ohms?
ok the speakers are coming, and i got a plan.
I got word from lew that you could safely run an amp intended for 4 ohms at 8 ohms if necessary at the loss of a negligible amount of wattage. I'm going to put one speaker in my peavey, recone the second one and put it in one my dad's old electrovoice closed back tuned bass cabinet. (The baffle is already cut for 15 inches and the cab sounds great)
So on normal jam dates ill just use my peavey cab with one JBL at 8 ohms and on bigger practices/gigs ill use both the peavey cab and the electro voice at 4 ohms.
Is it possible for me to get some sort of set impedence attenuator at 8 ohms so i won't lose power? It's a fender bandmaster head, all tube so i figure that would be better....
ok the speakers are coming, and i got a plan.
I got word from lew that you could safely run an amp intended for 4 ohms at 8 ohms if necessary at the loss of a negligible amount of wattage. I'm going to put one speaker in my peavey, recone the second one and put it in one my dad's old electrovoice closed back tuned bass cabinet. (The baffle is already cut for 15 inches and the cab sounds great)
So on normal jam dates ill just use my peavey cab with one JBL at 8 ohms and on bigger practices/gigs ill use both the peavey cab and the electro voice at 4 ohms.
Is it possible for me to get some sort of set impedence attenuator at 8 ohms so i won't lose power? It's a fender bandmaster head, all tube so i figure that would be better....
Quencho092
New member
Re: Can I use one 8 ohm speaker and a dummy load to have 4 ohms?
help, what's the best thing to do in this case to make these speakers work properly? I know that if i run both it will all be fine at the proper impedance.
Is it better just to run 8 ohms (will this hurt the amp??) or get some sort of inductance load?
help, what's the best thing to do in this case to make these speakers work properly? I know that if i run both it will all be fine at the proper impedance.
Is it better just to run 8 ohms (will this hurt the amp??) or get some sort of inductance load?
Quencho092
New member
Re: Can I use one 8 ohm speaker and a dummy load to have 4 ohms?
the 61 bandmaster ran one 12 inch speaker at 8 ohms fine, the 63 was intened for 2 12's at 8 ohms.
Will it make any diff if i plug into the extension speaker outlet?
the 61 bandmaster ran one 12 inch speaker at 8 ohms fine, the 63 was intened for 2 12's at 8 ohms.
Will it make any diff if i plug into the extension speaker outlet?
skyydogg01
Active member
Re: Can I use one 8 ohm speaker and a dummy load to have 4 ohms?
is your extention speaker 4 or 8 ohms?
is your extention speaker 4 or 8 ohms?
