#1
If I run my 300 watt all tube Ampeg head at 4 ohms into a 1x15 Ampeg cab that handles 200 watts at 8 ohms will it damage anything? From what I understand I just won't get as much volume but it won't damage the head or the cab. Is this correct?
#2
no you can't. You can realy damage the amp if you do.
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#4
so if you read something why would u come on here and disagree with someone who gave you the answer you didnt want?
#6
Quote by Jabels225
so if you read something why would u come on here and disagree with someone who gave you the answer you didnt want?


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#7
no not with tube amps, and you have it backwards.
With a SS amp (solid state) you can mis match ohms, but only like this EX: 8 ohm head will work with 8 ohm or 16 ohm cab. The 16 ohm cab will use about 1/2 the amps power not all of it like the 8 ohm cab. You can not go from an 8 ohm cab to a 4 ohm cab it will fry the OPtransformer.

Tube amps must match ohms or you will run the risk of ruining it.

Remember you can only mis match ohms with ss not tubes, and you can not go lower than the ohms of the amp.
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Last edited by Robbgnarly at Aug 5, 2011,
#8
Yes, TS it should be fine. I've mismatched my 8 ohm head into a 16 ohm cab with no harm. The manual of my amp even said this was fine.

You just can't mismatch the other way, bad things can happen.

If anything, mismatching with a higher impedance cab is the best thing for your amp as it doesn't push as hard as it needs to for matched impedance, although it usually doesn't impact your tone in a good way.

Quote by Robbgnarly

Remember you can only mis match ohms with ss not tubes, and you can not go lower than the ohms of the amp.

You can mismatch with tube amps. At least my Mark IV... the manual says it's okay.
Last edited by Ignite at Aug 5, 2011,
#9
A 2:1 mismatch is fine, provided the cab has the higher impedance. You'll get less output power and a lower bandwidth but unless you're using a cheap budget amp which uses an OT barely scraping by already, nothing bad should happen.

EDIT: ^Mesa actually encourages mismatches to get different tones in some of their manuals. Sometimes I run my Flexi at 4 ohms into an 8 ohm load to take a bit of the edge in the highs off.
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Last edited by mmolteratx at Aug 5, 2011,
#10
Quote by Ignite
Yes, TS it should be fine. I've mismatched my 8 ohm head into a 16 ohm cab with no harm. The manual of my amp even said this was fine.

You just can't mismatch the other way, bad things can happen.

If anything, mismatching with a higher impedance cab is the best thing for your amp as it doesn't push as hard as it needs to for matched impedance, although it usually doesn't impact your tone in a good way.


You can mismatch with tube amps. At least my Mark IV... the manual says it's okay.

He wants to go from an 8 ohm head to a 4 ohm cab it wont work, and just cause your mesa says its ok, many amp companys will void your warantys if this happens, I wonder why? Because not all tube amps are built the same. Your Mark is of a substantualy higher build quality than any Ampeg tube amp, so the parts inside are to much higher specs than other amps.
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#12
Quote by Robbgnarly
He wants to go from an 8 ohm head to a 4 ohm cab it wont work, and just cause your mesa says its ok, many amp companys will void your warantys if this happens, I wonder why? Because not all tube amps are built the same. Your Mark is of a substantualy higher build quality than any Ampeg tube amp, so the parts inside are to much higher specs than other amps.


He's using a 4 ohm head into an 8 ohm cab. And Ampeg's (at least before they moved production to China, and even then they still aren't bad at all) are built to very high standards. They're tanks.
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#13
Quote by mmolteratx
He's using a 4 ohm head into an 8 ohm cab. And Ampeg's (at least before they moved production to China, and even then they still aren't bad at all) are built to very high standards. They're tanks.

OH sorry the thread title has it as 4 ohm head and 8 ohm cab.
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#14
Quote by Robbgnarly
He wants to go from an 8 ohm head to a 4 ohm cab it wont work, and just cause your mesa says its ok, many amp companys will void your warantys if this happens, I wonder why? Because not all tube amps are built the same. Your Mark is of a substantualy higher build quality than any Ampeg tube amp, so the parts inside are to much higher specs than other amps.


Actually I want to run my 4 ohm head into my 8 ohm cab. Not the other way around like you said.
#15
Quote by Ignite

You can mismatch with tube amps. At least my Mark IV... the manual says it's okay.


I think it depends on the OT on whether you can do it or not. And when you can do it with a tube amp, isn't it usually the opposite of SS, that with the ampeg you could run a 2ohm load from the 4 ohm tap, but not an 8 ohm load (SVTs have a 2ohm out, but you get the idea)

I could be way off here, but sometimes folks at TB talk about running 2 ohm loads out of amps, like traynors, which only have 8 and 4 ohm connections and I thought this was the rule put forth.
#16
Quote by Robbgnarly
OH sorry the thread title has it as 4 ohm head and 8 ohm cab.


Also, both my head and cab are made in USA. Not China.
#17
Quote by ethan_hanus
It'll put too much stress on the OT, thus frying it if you run that way long enough. Now, if it was the other way around, then the amp would be ok, but the speaker would not, after a while.


What is the OT?

I think I see what you're saying. So since the cab is 8 ohms and the head is only 4 ohms, the head has to work harder which can hurt the head over time?
Last edited by Andrewtmayes at Aug 5, 2011,
#19
Quote by Andrewtmayes
What is the OT?

I think I see what you're saying. So since the cab is 8 ohms and the head is only 4 ohms, the head has to work harder which can hurt the head over time?

No, the head has to work LESS than normal, so it's okay.
#20
Quote by Ignite
No, the head has to work LESS than normal, so it's okay.


Oh okay. Then what is he saying about the OT?
#22
Quote by Andrewtmayes
What is the OT?


output transformer

here is how it works, the output section of your tube amp has a very high impedance output signal (too high to effectively 'bridge' the output of the amp to the speaker). this means tube amps need a output transformer to convert the high impedance signal to a low impedance signal (a signal more appropriate for driving the speaker). these concepts use engineering principles that are beyond me, but the important part to know is the tube amps use a transformer to change the signal so it can drive a speaker.

now, this transformer's goal is to convert the high impedance from the amp into a low enough impedance signal to drive the speaker of a specific nominal impedance rating. meanwhile the speaker itself has it's own impedance, in other words: the speaker provides electrical resistance/reactance (aka 'the impedance') to the signal getting passed through it. the important part to know is that the impedance the speaker 'shows' in the circuit is dependent upon the input signal.

what does that mean? it means when you play your open high E string it actually provides less impedance than when you play your low E string. actually, the impedance of the speaker looks like this:



that is a 4 ohm speaker (but it's characteristic impedance curve is very similar to most speakers). notice how this speaker can actually provide as much as 6.4 ohms or as little as 3.5 ohms?

pretty revealing huh? all impedance ratings are 'nominal', which means they can derive a 'hard' impedance value from a speaker that actually gives different impedances depending on the input. so 'impedance values' of a speaker aren't as concrete as people claim.

so this begs the question 'so how much can you mismatch an amp and be fine'. there is no hard rule, and it is almost completely in the realm of 'how much did the designer take into consideration people mismatching their amps?' here is Ted Weber's explanation of mismatching and what it actually does to the amp

Quote by Ted Weber
From: Chuck

I have heard various views on impedance mismatches between the amp and the speakers. One is that you should always match the impedance (4 ohm amp = 4 ohm speaker or two 8 ohm speakers in parallel), or you can blow your transformer. The other is that it is fine to mismatch, but you may lose power. Should the impedence match? If yes, then how quickly could you damage your amp when you have a mismatched impedence?

Chuck, technically, you should always provide a load that is recommended by the manufacturer of the amp. The designer of the amp chose a particular output device (tube) and specified all of the operating voltages for the output stage so the tube would work at its optimum efficiency while delivering maximum power to the load with minimum distortion.

Ok, so let's discuss the problems associated with mismatches. When you use a load that is lower than the intended load, the output has to drive the load (speaker) with more current because it is a lower impedance than is expected. Two inherent problems associated with transformers are flux leakage and regulation. Flux leakage is also referred to as leakage inductance. It is related to the current in the secondary, and these problems increase as the current increases. As the current draw in the secondary increases, the primary has a more difficult time transferring the signal to the secondary, so the secondary signal to the load gets squashed, or 'soft-clipped'. This soft clipping is called regulation.

While regulation is desireable in a power supply, it is undesireable in a transformer. In other words, in a power supply, if the input voltage or the output load current changes, we don't want the output voltage to change. In a transformer, we want the output voltage to follow the input voltage and not regulate at all. When you put a heavier load on the output than was intended, it will pull the output voltage down, hence regulation. The leakage inductance problem arises because the current from the heavier load causing the regulation to occur reduces the efficiency of the transformer by not allowing the output to follow the input.

Transformer designers simulate or view this problem as having extra inductance in series with the primary. The extension of this idea then, is that with the heavier load, you could affect the efficiency of the transformer, alter the frequency response (due to the extra leakage inductance in series with the primary), and cause other distortions to occur.

OK, on to mismatching the other way. A speaker is a current operated device in that it responds to the current through it to generate a magnetic field that works against the magnetic field of the speaker magnet to make the cone move in and out. Thinking in very short amounts of time, when the output charges up the voice coil with current, then the signal goes away or gets reduced, the cone system moves the voice coil back to its home or resting position. As it is moving back, it generates a voltage that is fed back up the line into the transformer and appears in the output circuit of the amp. This generated voltage is often referred to as flyback voltage, because we are charging up an inductor, then when we disconnect or stop charging the inductor, the magnetic field in the inductor collapses and induces this big voltage into itself. This big voltage then 'flies back' to the source of the charging current.

There is a mathematical formula to determine how big the voltage is and it is related to the inductance of the voice coil, the amount of time it was fed current, and how much current it was charged with. The bottom line is that the voltage fed back to the output circuit is oftentimes much higher than the voltage that was used to drive or charge up the voice coil initially. This voltage gets transformed up by the turns ratio of the output transformer, and in many cases can be over 1,000 volts. What happens then is that arcing can occur between the pins on the output tube socket. Once this has occured, a carbon path forms on the tube socket between the pins. The carbon path allows a steady current to flow between the pins and eventually burns up the socket due to the heat that is generated. For example, it wouldn't be too uncommon to see a transformer turns ratio of 30:1.

If we had a voltage fed back from the voice coil that was around 50 volts, 30 times 50 would be a 1,500 volt spike at the plate of the output tube. This is why you often see designers connect diodes in a string between the output tube plates and ground. They are trying to suppress these spikes and dissipate the energy in the diodes rather than allowing an arc to occur at the tube socket. So, when you use a higher impedance load on a lower impedance tap, the turns ratio is higher and resulting fed-back (flyback) voltage gets multiplied up higher than what it would have been with the correct impedance load.

It's just about impossible for me to answer how long an amp would last under these conditions. It all depends on how the designer took these potential problems into account in his or her design with regards to the quality of the tube sockets, the use of stringed diodes, the output circuit operating voltages, etc.


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Last edited by gumbilicious at Aug 5, 2011,
#23
Quote by Robbgnarly
He wants to go from an 8 ohm head to a 4 ohm cab

Reading comprehension fail.

Well done.
Everyone is entitled to an opinion.

Feel free to express yours so I can make an informed judgement about how stupid you are.
#24
Quote by gumbilicious
output transformer

here is how it works, the output section of your tube amp has a very high impedance output signal (too high to effectively 'bridge' the output of the amp to the speaker). this means tube amps need a output transformer to convert the high impedance signal to a low impedance signal (a signal more appropriate for driving the speaker). these concepts use engineering principles that are beyond me, but the important part to know is the tube amps use a transformer to change the signal so it can drive a speaker.

now, this transformer's goal is to convert the high impedance from the amp into a low enough impedance signal to drive the speaker of a specific nominal impedance rating. meanwhile the speaker itself has it's own impedance, in other words: the speaker provides electrical resistance/reactance (aka 'the impedance') to the signal getting passed through it. the important part to know is that the impedance the speaker 'shows' in the circuit is dependent upon the input signal.

what does that mean? it means when you play your open high E string it actually provides less impedance than when you play your low E string. actually, the impedance of the speaker looks like this:



that is a 4 ohm speaker (but it's characteristic impedance curve is very similar to most speakers). notice how this speaker can actually provide as much as 6.4 ohms or as little as 3.5 ohms?

pretty revealing huh? all impedance ratings are 'nominal', which means they can derive a 'hard' impedance value from a speaker that actually gives different impedances depending on the input. so 'impedance values' of a speaker aren't as concrete as people claim.

so this begs the question 'so how much can you mismatch an amp and be fine'. there is no hard rule, and it is almost completely in the realm of 'how much did the designer take into consideration people mismatching their amps?' here is Ted Weber's explanation of mismatching and what it actually does to the amp




From what you said I would think it's fine. But from what Ted said I wouldn't want to risk it.
#25
If you are TL;DR on the Weber quote... The short answer is that having a higher speaker ohm translates into higher voltages in the output circuitry. Enhancing the possibility of arcing and destroying the circuit. If you want an answer you should contact Ampeg support and see what they say.
#26
An Ampeg will run into an 8 ohm load from the 4 ohm output all day long without any problems.
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#27
Quote by Andrewtmayes
What is the OT?

I think I see what you're saying. So since the cab is 8 ohms and the head is only 4 ohms, the head has to work harder which can hurt the head over time?


Output Transformer, in tube amps it's what helps output the power to the speaker, but don't quote me on that, it may just convert the power so the power tubes can output them, still not sure exactly how it works.

On SS amps, transistor chips output the power, so they don't have a OT, just a normal transformer. Transistor chips can take considerably more stress than a OT, because they just burn off the extra energy in the form of heat, but you can damage them if it's too much stress.
#28
Quote by ethan_hanus
Output Transformer, in tube amps it's what helps output the power to the speaker, but don't quote me on that, it may just convert the power so the power tubes can output them, still not sure exactly how it works.
An OT converts the internal high voltage/low current to external low voltage/high current. Since power is a linear relationship between voltage and current, you get the same power by dividing voltage by the same amount you multiple current.
#29
Quote by Robbgnarly
OH sorry the thread title has it as 4 ohm head and 8 ohm cab.


WTF? This is the second time of the thread your lack of comprehension skills has shown. You are saying the thread title caused your confusion, even though you quoted it as being right....

To the TS. You will be fine, Ampeg's use quality OT's and will not blow, I do this all the time with my amp heads to get different sounds.
#30
Quote by Andrewtmayes
From what you said I would think it's fine. But from what Ted said I wouldn't want to risk it.


yeah, but it really boils down to 'what did the amp designer account for'. Ted gives a couple of tricks to use to account for a mismatched impedance.

i personally don't like using it cuz it sounds different, especially the feedback. response for the amp get 'pinched' or 'flat' when using mismatched loads.
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#31
Quote by Wylde_Life
WTF? This is the second time of the thread your lack of comprehension skills has shown. You are saying the thread title caused your confusion, even though you quoted it as being right....

To the TS. You will be fine, Ampeg's use quality OT's and will not blow, I do this all the time with my amp heads to get different sounds.

Dont be a Douche, I thought it said 8ohm head 4 ohm cab. Honest mistake
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