Ohms ?
- Thread starter floyd1
- Start date
Re: Ohms ?
Wouldn't it be safer to match the heads output with the speaker...that is ...get a speaker rated at 4 ohms that can handle 200 watts (man, that is at lot of power, especially if you dime it...glad you don't live next door to me. You could castrate a cat at 500 yards with that rig)
There might be better solutions. Wait for others to respond.
I just don't want you damage your amp.... or blow a speaker.
Good Luck
Dave
Wouldn't it be safer to match the heads output with the speaker...that is ...get a speaker rated at 4 ohms that can handle 200 watts (man, that is at lot of power, especially if you dime it...glad you don't live next door to me. You could castrate a cat at 500 yards with that rig)
There might be better solutions. Wait for others to respond.
I just don't want you damage your amp.... or blow a speaker.
Good Luck
Dave
That90'sGuy
DyzaBoyzologist
Re: Ohms ?
Always match the speaker out on your amp head to the resistance on the cabinet (in this case, if it's a 4 ohm output, stick with a 4 ohm cabinet... so I'd go with either (4) 16 ohm speakers in parallel or (2) 8 ohm speakers in parallel).
200w is insane and you'll have to keep that master pretty low to not blow out those speakers of yours. I recommend getting either a 100w head to match the cabinet or getting higher power handling speakers for the amp head to ensure you don't blow them.
Always match the speaker out on your amp head to the resistance on the cabinet (in this case, if it's a 4 ohm output, stick with a 4 ohm cabinet... so I'd go with either (4) 16 ohm speakers in parallel or (2) 8 ohm speakers in parallel).
200w is insane and you'll have to keep that master pretty low to not blow out those speakers of yours. I recommend getting either a 100w head to match the cabinet or getting higher power handling speakers for the amp head to ensure you don't blow them.
midnite_man
New member
Re: Ohms ?
Dave's right. If you don't have a switch somewhere on your amp that enables the output to have different resistance ratings such as 4 and 8 ohms, I would think the speaker wouldn't last long. Not sure about the effect on the amp itself.
Let's get more info from other members on this.
Dave's right. If you don't have a switch somewhere on your amp that enables the output to have different resistance ratings such as 4 and 8 ohms, I would think the speaker wouldn't last long. Not sure about the effect on the amp itself.
Let's get more info from other members on this.
devastone
Well-known member
Re: Ohms ?
Actually the first post it right on. It is a SS head that is rated at 200W at a minimum load of 4 ohms. Since it is a SS head, you can use a cabinet that is a bigger load than 4 ohms, so you are correct, plugged into a 8 ohm cabinet, the amp will put out 100W.
SS and Tube amps are different beasts, SS amps don't have output transformers with taps for different loads like tube amps, most of them don't have output transformers transformers at all, the output of the power transistors is tied directly to the load. Also, most SS doesn't get angry and blow up if you don't have a load hooked up to it at all, meaning no cabinet plugged in. But don't run it with a load less than the minimum rated load, in this case 4 ohms, and never plug in a cable that shorts the tip to the sleeve (into any amp tube or SS).
Actually the first post it right on. It is a SS head that is rated at 200W at a minimum load of 4 ohms. Since it is a SS head, you can use a cabinet that is a bigger load than 4 ohms, so you are correct, plugged into a 8 ohm cabinet, the amp will put out 100W.
SS and Tube amps are different beasts, SS amps don't have output transformers with taps for different loads like tube amps, most of them don't have output transformers transformers at all, the output of the power transistors is tied directly to the load. Also, most SS doesn't get angry and blow up if you don't have a load hooked up to it at all, meaning no cabinet plugged in. But don't run it with a load less than the minimum rated load, in this case 4 ohms, and never plug in a cable that shorts the tip to the sleeve (into any amp tube or SS).
Re: Ohms ?
Sort of.
An 8 ohm load is not a bigger load than 4 ohms, it is less of a load. As impedance goes down the load increases and vice versa. Most solid state amps will produce roughly 65% of their power into double the impedance.
Solid state amps do not like direct shorts and can quickly burn up but will work all day without a speaker load.
Tube amps will be quickly destroyed with no speaker load on the secondary side of the output transformer. Tube amps can deal much easier with a shorted secondary.
Check out how the speaker jacks are wired on any Fender amp since they started making "piggy-backs" - the jack is a shorting jack to protect the output transformer just in case someone forgot to plug their head into the speaker cabinet.
Sort of.
An 8 ohm load is not a bigger load than 4 ohms, it is less of a load. As impedance goes down the load increases and vice versa. Most solid state amps will produce roughly 65% of their power into double the impedance.
Solid state amps do not like direct shorts and can quickly burn up but will work all day without a speaker load.
Tube amps will be quickly destroyed with no speaker load on the secondary side of the output transformer. Tube amps can deal much easier with a shorted secondary.
Check out how the speaker jacks are wired on any Fender amp since they started making "piggy-backs" - the jack is a shorting jack to protect the output transformer just in case someone forgot to plug their head into the speaker cabinet.
devastone
Well-known member
Re: Ohms ?
Posts like this cause people to burn up stuff everyday. Where do you get this? I actually have a BS in electronics, this is first semester basic stuff. 8 ohms is more impedance than 4 ohms, if it wasn't ohms would not be a very good unit of measure.
You are correct on this, but it is the same thing I said.
Not a bet I would take, with a direct short the lack of load will transfer to the output circuit, and based on a bad experience where a cat rewired a cabinet for me, you will see a flash, that would be output tubes, it also blew some traces off the board, but, the output transformer was fine. It didn't take long at all after the wires touched.
I don't have a Fender amp to check out, but, see my previous answer, I highly doubt they are shorting the the 2 speaker lines, I'm sure the designers know what would happen. In fact, all of the amp protection circuits I am aware of use a large 150 or so ohm resistor that is in line with the speaker jack when nothing is plugged into it to protect the amp in case nothing is plugged into it. Shorting is bad for SS or tube.
Posts like this cause people to burn up stuff everyday. Where do you get this? I actually have a BS in electronics, this is first semester basic stuff. 8 ohms is more impedance than 4 ohms, if it wasn't ohms would not be a very good unit of measure.
You are correct on this, but it is the same thing I said.
Not a bet I would take, with a direct short the lack of load will transfer to the output circuit, and based on a bad experience where a cat rewired a cabinet for me, you will see a flash, that would be output tubes, it also blew some traces off the board, but, the output transformer was fine. It didn't take long at all after the wires touched.
I don't have a Fender amp to check out, but, see my previous answer, I highly doubt they are shorting the the 2 speaker lines, I'm sure the designers know what would happen. In fact, all of the amp protection circuits I am aware of use a large 150 or so ohm resistor that is in line with the speaker jack when nothing is plugged into it to protect the amp in case nothing is plugged into it. Shorting is bad for SS or tube.
Re: Ohms ?
If you were not sleeping during the ohms law discussion, you may remember a derived formula: voltage squared divided by resistance equals watts.
If we assume the amp is adjusted to put out a 10 volt signal into an 8 ohm load we can do the math: (10*10) / 8 = 12.5 watts.
If we assume the same 10 volt signal into a 4 ohm load and do the math: (10*10) / 4 = 25 watts.
So, if 4 ohms is less of a load than 8 ohms, why would the amp produce less power driving it?
Oh, and I never said that 4 ohms was less of an impedance than 8 ohms - just less of a load.
You do not need to tear apart a Fender amp to look at how the speaker output jacks are wired. Just look at any schematic of one - and there are plenty available on the Fender Field Guide. If you look at the "tweeds" they had open circuit speaker jacks - very little chance of someone pulling the speaker on a combo amp. After the introduction of the piggy backs, all tube amps were eventually changed to a closed circuit configuration for the speaker jacks.
If you were not sleeping during the ohms law discussion, you may remember a derived formula: voltage squared divided by resistance equals watts.
If we assume the amp is adjusted to put out a 10 volt signal into an 8 ohm load we can do the math: (10*10) / 8 = 12.5 watts.
If we assume the same 10 volt signal into a 4 ohm load and do the math: (10*10) / 4 = 25 watts.
So, if 4 ohms is less of a load than 8 ohms, why would the amp produce less power driving it?
Oh, and I never said that 4 ohms was less of an impedance than 8 ohms - just less of a load.
You do not need to tear apart a Fender amp to look at how the speaker output jacks are wired. Just look at any schematic of one - and there are plenty available on the Fender Field Guide. If you look at the "tweeds" they had open circuit speaker jacks - very little chance of someone pulling the speaker on a combo amp. After the introduction of the piggy backs, all tube amps were eventually changed to a closed circuit configuration for the speaker jacks.
devastone
Well-known member
Re: Ohms ?
Nope, didn't sleep through that class at all. In this discussion load and impedance are essentially the same things, all things equal, a circuit can put more power into a smaller load (RL for theoretical purposes in my texts). A lower R (or X for impedance) will allow the circuit to generate more current, hence more power. This is why too small of a load will cause the amp to try to generate more current than it can until it burns itself up which will happen quite quickly with a dead short.
I checked some of the schematics, that short also seems to short the feedback loop to the PI tube to ground which would also sink the signal going into the PI tube to ground essentially killing the signal to the power tubes. No signal in means the power tubes aren't trying to put out anything. I could be wrong, I didn't go through all the schematics.
If you don't believe me, and apparently you don't, contact someone whose opinion you will trust, try Bruce at www.missionamps.com, Mike or Bill at www.soldano.com, or there are lots of other resources, as always, be careful of forum posts (including here), anyone can post on a forum.
Nope, didn't sleep through that class at all. In this discussion load and impedance are essentially the same things, all things equal, a circuit can put more power into a smaller load (RL for theoretical purposes in my texts). A lower R (or X for impedance) will allow the circuit to generate more current, hence more power. This is why too small of a load will cause the amp to try to generate more current than it can until it burns itself up which will happen quite quickly with a dead short.
I checked some of the schematics, that short also seems to short the feedback loop to the PI tube to ground which would also sink the signal going into the PI tube to ground essentially killing the signal to the power tubes. No signal in means the power tubes aren't trying to put out anything. I could be wrong, I didn't go through all the schematics.
If you don't believe me, and apparently you don't, contact someone whose opinion you will trust, try Bruce at www.missionamps.com, Mike or Bill at www.soldano.com, or there are lots of other resources, as always, be careful of forum posts (including here), anyone can post on a forum.
Re: Ohms ?
I do not trust anyone's opinion. I trust facts.
The basic definition of a larger load is one that requires more power. In that regard, a lower impedance load is the greater load to an amp. Sorry that is not as easy to understand as it is an inverse relationship. Resistance does not the same relationship to power as for example, physical weight has to the power required to move it a certain distance in a certain amount of time.
Even in the pickup forum people agree that a lower volume pot (e.g. 250K) represents a greater load than a numerically higher pot (e.g. 1 meg) to a pickup. There is no need to grant a special exception to an amp with a speaker load over a pickup and pot load, as electronics does not know the difference.
We do agree that a complete short on the output of a solid state amp will destroy it.
For tube amps, I suggest you go back to your BS roots. You are ignoring what impedance is reflected back to the PRIMARY side of the output transformer when the SECONDARY side is either open or closed.
When output transformers go - it is 99.9 percent of all cases on the primary side - due to an open secondary.
I do not trust anyone's opinion. I trust facts.
The basic definition of a larger load is one that requires more power. In that regard, a lower impedance load is the greater load to an amp. Sorry that is not as easy to understand as it is an inverse relationship. Resistance does not the same relationship to power as for example, physical weight has to the power required to move it a certain distance in a certain amount of time.
Even in the pickup forum people agree that a lower volume pot (e.g. 250K) represents a greater load than a numerically higher pot (e.g. 1 meg) to a pickup. There is no need to grant a special exception to an amp with a speaker load over a pickup and pot load, as electronics does not know the difference.
We do agree that a complete short on the output of a solid state amp will destroy it.
For tube amps, I suggest you go back to your BS roots. You are ignoring what impedance is reflected back to the PRIMARY side of the output transformer when the SECONDARY side is either open or closed.
When output transformers go - it is 99.9 percent of all cases on the primary side - due to an open secondary.
devastone
Well-known member
Re: Ohms ?
The problem here is that you are talking about Load as a Current Load or the amount of current that a specific impedance will try to draw.
I'm speaking of Load as in R subscript L or Resistive Load, which will draw less current as it increases.
Yes, I agree that a 250K volume pot will "load" down the circuit more than a 500K pot.
And trust me, you don't want to short you across the secondary of your amp and try to play it, it will create nasty smells and some ugly flashes out the back of the amp. The reason that failed transformers usually fail due to an open, is because someone forgetting to plug in a cabinet is the leading cause of failure, not plugging in a shorted jack.
Unfortunately what I'm afraid we have accomplished is to further confuse the original poster. I think we do agree that if his solid state amp is rated at 200W at a minimum load of 4 ohms, it is perfectly safe to use an 8 ohm cabinet and the only consequence will be that the amp will put out less power, roughly 50% of its maximum output.
Right?
The problem here is that you are talking about Load as a Current Load or the amount of current that a specific impedance will try to draw.
I'm speaking of Load as in R subscript L or Resistive Load, which will draw less current as it increases.
Yes, I agree that a 250K volume pot will "load" down the circuit more than a 500K pot.
And trust me, you don't want to short you across the secondary of your amp and try to play it, it will create nasty smells and some ugly flashes out the back of the amp. The reason that failed transformers usually fail due to an open, is because someone forgetting to plug in a cabinet is the leading cause of failure, not plugging in a shorted jack.
Unfortunately what I'm afraid we have accomplished is to further confuse the original poster. I think we do agree that if his solid state amp is rated at 200W at a minimum load of 4 ohms, it is perfectly safe to use an 8 ohm cabinet and the only consequence will be that the amp will put out less power, roughly 50% of its maximum output.
Right?
Last edited:
Re: Ohms ?
Quote Devastone: Unfortunately what I'm afraid we have accomplished is to further confuse the original poster. I think we do agree that if his solid state amp is rated at 200W at a minimum load of 4 ohms, it is perfectly safe to use an 8 ohm cabinet and the only consequence will be that the amp will put out less power, roughly 50% of its maximum output.
Right?
Glad somebody finally mentioned this.
I'm wondering if Floyd1 is still reading...you there Floyd1?
LOL
Quote Devastone: Unfortunately what I'm afraid we have accomplished is to further confuse the original poster. I think we do agree that if his solid state amp is rated at 200W at a minimum load of 4 ohms, it is perfectly safe to use an 8 ohm cabinet and the only consequence will be that the amp will put out less power, roughly 50% of its maximum output.
Right?
Glad somebody finally mentioned this.
I'm wondering if Floyd1 is still reading...you there Floyd1?
LOL
Last edited:
Re: Ohms ?
Roughly 65% of it's maximum output.
And there is a good reason for it. With the lighter 8 ohm load the output transistors draw less current. With less of a current requirement (approximately 50%), the power supply rails (B+ and or B- depending on the circuit) will remain at a higher voltage - in other words the power supply sags less. This enables the circuit to put out roughly 15% higher voltage into the load before the same level of distortion is obtained.
But I agree it is really picking nits because you probably will not hear the difference between 50% and 65%.
Roughly 65% of it's maximum output.
And there is a good reason for it. With the lighter 8 ohm load the output transistors draw less current. With less of a current requirement (approximately 50%), the power supply rails (B+ and or B- depending on the circuit) will remain at a higher voltage - in other words the power supply sags less. This enables the circuit to put out roughly 15% higher voltage into the load before the same level of distortion is obtained.
But I agree it is really picking nits because you probably will not hear the difference between 50% and 65%.
