#1
Hi, I have a 100watt, combo amp, and it has one output for adding a cabinet, but my amp is 4omh, and alot of cabinets are 8ohm. for example :

i buy an cabinet with these features :

Features:

Power: 120 watts
Impedance: 8 ohms
Speakers: 4 x 12 in. Marshall/Celestion
custom-designed speakers (30W each)
Dimensions (W x H x D): 26.4 x 26.4 x 14.0 in.

Will it work, and will it be louder than my 1x12 70w speaker i have atm?

thanks alot, i know theres alot of info about this but im very dumb and stupid, a cow has higher IQ than me

EDIT 1: my amp is :

Behringer 100FX virtube (solid state)

and another thing, at the example cabinet above, it says 30W each, does that mean my 1x12 70w speaker is louder? because it also says power:120W.

lol my grammar fails like hell.

EDIT 2: my amp is 100w, what do those 100w do? do they give 100/120w to the example cabinet and and than the cabinet is only 100w and speakers only 25w each? or does the cabinet gets power from somewhere els?
Last edited by teli1337 at Dec 3, 2011,
#2
Amp ohm rating is the minimum rating a cab can use. So 4 ohm -> 8 ohm is fine, but not the other way around.

Obviously matched is best
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#3
Don't be so hard on yourself, TS. You may think you're dumb, but asking questions is how we learn. Ask enough questions and suddenly you're the go-to guy, because you know more than everyone else.

As already mentioned, never use a cab with a lower impedance rating than that of the amp. You can always go higher than the amp's impedance on the cab, but never lower. To do so will overheat the amp and damage it. On a tube amp, it will destroy the output transformer.

See? You've learned something. Now you're smarter than a cow and working on being smarter than us.
#4
^ pretty solid advice but saying it will destroy the OT isn't really accurate. some amps can take that mismatch (Splawn and most Mesas for example). Some guitarist actually would do that on purpose to get more 'sag' or whatever in their tone.


This is the formula I live by and it has been validated by countless amp builders/techs.

For Tube amps:
Amp Head Ohms > Cab Ohms = Possible blown Power Valves and/or Output Transformer.
Amp Head Ohms < Cab Ohms = Strain on the Output Transformer, Power loss.

*Some amps can take a 1 step mismatch but not all.

For Solid State Amps:
Amp Head Ohms > Cab Ohms = Broken amp
Amp Head Ohms < Cab Ohms = Power change, no strain



So I usually ask - what amp?

Wait, is this an MG and a MG cab you are considering purchasing? The cab will give you a tiny volume boost and will 'feel' louder yes. Similar to if you hiked your amp up on a table so it was ear level.


v ... You always have the best, most thorough answers. You should copy/paste your responses sometimes and keep them handy so you don't have to retype shit. We were going to do a 'Catch All' thread once. Damn, I forgot all about that.
Last edited by 311ZOSOVHJH at Dec 3, 2011,
#5
i am not agreeing with what is being said here, these answers leave out some inportant stuff. edit: except 311's, he hit it on the head in a 1/8 of my words.

Quote by teli1337
Hi, I have a 100watt, combo amp, and it has one output for adding a cabinet, but my amp is 4omh, and alot of cabinets are 8ohm.


what amp do you have? depending if you are asking about a tube amp or a solid state amp heavily effects the answer you need. please let me know the make and model of you amp. i don't know how these guys can give you a real answer without knowing that.

if you are running a tube amp, then the impedance mismatch will stress you output transformer (the most expensive part of your amp). the output transformer is there to optimize the transfer of energy from your amplifier head to your cab and the idea is to use a cabinet that has the same impedance as your amplifier's output (irl, it'll never be perfect, the idea is to get it nominally close, so some 'mismatch' is always present). If you don't match impedances with a tube amp you do stress the output transformer which could lead to critical failure, but i am more concerned about how it will sound. when your guitar signal is not properly matched to the cabinet it sounds different, usually pretty damn bad (too bright, overly muddy, unfavorable mids, that kinda stuff).

if you are running a solid state amp... who cares. a solid state amp doesn't have an output transformer, just make sure you supply the minimum impedance or more. one thing to know though, when you hook a 'higher impedance' speaker cab to a solid state tube head you'll get less output. you see stats like: 100W @4 ohms, 65 watts @ 8 ohms, etc. also, different impedance speakers have different frequency responses. the higher the impedance, the more rolloff you get in the extreme low and high ends (this is true to tube amps or solid state amps)

Quote by teli1337
Will it work, and will it be louder than my 1x12 70w speaker i have atm?


depends on the speakers you use. holding the amp constant, the speakers should tell you what we be louder. whether the 4x12 is louder or the 1x12 is louder depends on pretty much 2 things:

1) speaker sensitivity - some speakers are louder than others (generally the lighter, looser ones are louder)
2) speaker projection path - if your speakers are pointed at your ears they will sound louder, cabs with more speakers have bigger coverage and sound louder to more people listening but aren't necessarily putting out more sound.

Quote by teli1337
thanks alot, i know theres alot of info about this but im very dumb and stupid, a cow has higher IQ than me


bag that BS, we all start somewhere. the questions you asked a CONSTANTLY undestood wrong on the beginner level. if you were asking how turn on your amp that would be one thing: read the manual; you're asking legit questions that are hard to find real answers on.
punk isn't dead, it's always smelled that way.

"A perfection of means, and confusion of aims, seems to be our main problem."
-ae
Last edited by gumbilicious at Dec 3, 2011,
#6
Quote by gumbilicious
if you are running a tube amp, then the impedance mismatch will stress you output transformer (the most expensive part of your amp). the output transformer is there to optimize the transfer of energy from your amplifier head to your cab and the idea is to use a cabinet that has the same impedance as your amplifier's output (irl, it'll never be perfect, the idea is to get it nominally close, so some 'mismatch' is always present). If you don't match impedances with a tube amp you do stress the output transformer which could lead to critical failure, but i am more concerned about how it will sound. when your guitar signal is not properly matched to the cabinet it sounds different, usually pretty damn bad (too bright, overly muddy, unfavorable mids, that kinda stuff).



Not so true. Every tube amp I've ever seen has had the capability to accept what's called a "safe mismatch." I currently have 4 tube amps in this room and every one of them will accept a cab with a higher rated impedance. To tell him that failure to match the impedance will lead to critical failure is wrong and spreads disinformation. Using a safe mismatch will slightly effect the tone. You may actually like the tone it provides. The thing is, ask questions and then experiment. Running a safe mismatch on a tube amp will NEVER cause component damage.

I will not speak towards solid state amps, because I don't use them. But I do know tube amps. So, let's make sure we state facts, rather than fiction. The bottom line is, if it's a tube amp, check with the manual before trying something new. Most manuals are fairly good about covering what can be done and what will damage your amp.
#7
SS amps usually just list a minimum load impedance and no maximum. Running higher impedance loads just cuts down their output power.
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#8
Quote by KG6_Steven
Not so true. Every tube amp I've ever seen has had the capability to accept what's called a "safe mismatch." I currently have 4 tube amps in this room and every one of them will accept a cab with a higher rated impedance. To tell him that failure to match the impedance will lead to critical failure is wrong and spreads disinformation. Using a safe mismatch will slightly effect the tone. You may actually like the tone it provides. The thing is, ask questions and then experiment. Running a safe mismatch on a tube amp will NEVER cause component damage.


i am not trying to argue 'safe mismatches' and 'unsafe mismatches' and what causes damage. my claims were very conservative, and i only mentioned a worse case scenario in a very generalized way. a impedance mismatch CAN lead to a OT failure, i don't even felt i doomsayed about anything. mismatches DO stress the OT because transformers are designed for a particular flow of electricity through their different windings. when the transformer gets away from it's optimal running specs there is more waste heat and even arcing can occur (in more extreme cases); this is stress.

furthermore, tone is effected because the above mentioned characteristics of a transformer also effect harmonic content of your tone. you hear when your transformer isn't running right in the harmonic content. safe or unsafe, mismatching sucks tone.

the truth is, there is nothing wrong with my post. a tube amp uses impedance matching via a transformer to load the circuit, that is the mathematical principle: to try and match the output of the amplifier with the cabinet rating. the transformer WANTS a particular load to work the best, if they didn't want to run your circuit to a particular load they wouldn't have put a transformer there with windings and taps to specfically do so.

SS amps lack the transformer and there is nothing 'stressed' when running a bigger load. this is impedance bridging, where the circuit actually doesn't care what impedance it gets as long as you provide a minimum impedance to load the circuit.

Quote by KG6_Steven
So, let's make sure we state facts, rather than fiction.


i am unsure of what you mean by this
punk isn't dead, it's always smelled that way.

"A perfection of means, and confusion of aims, seems to be our main problem."
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Last edited by gumbilicious at Dec 3, 2011,
#9
Yet to come across a tube amp yet where running a cab at 2x the impedance of the selected output on the amp wasn't a "safe mismatch."
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#10
that is because matching an amp to a speaker cab via a transformer generates an environment too complex to strictly classify in as simple a dichotemy as 'safe' or 'unsafe'. fact is 'sudden catastrophic OT failures' are not the norm, the whole story is how much heat can your transformer shed for how long before you windings start eventually arcing (which itself is not this concrete concept and is dependent on all types of factors). i have even read that old transformers with a coating of rust on them cut down eddy current waves that transformers generate and help protect it in more extreme circumstances.

the answer is completely dependent on design and components, and many tube amp manufacturers include engineering to help protect the transformer (like stringing diodes to protect from flyback voltage).

safe, unsafe, w/e. i know it's hard to kill a guitar amp, you have to try pretty hard to kill it just by plugging into off-rated guitar cab. all i am claiming is that this puts more stress on the transformer than running it at it's expected/rated load, and that this effects it's sound.
punk isn't dead, it's always smelled that way.

"A perfection of means, and confusion of aims, seems to be our main problem."
-ae
Last edited by gumbilicious at Dec 3, 2011,
#11
Well, I guess Mesa, Rivera, Peavey, Fender and others are wrong. Why would they state it's safe to do this, if it weren't safe?
#12
Quote by KG6_Steven
Well, I guess Mesa, Rivera, Peavey, Fender and others are wrong. Why would they state it's safe to do this, if it weren't safe?


wow dude, you seem to consistently miss the point. i am done trying to explain the fallacy of your paradigm, but if dichotomies are that appealing to you, buy all means tell me how wrong i got it. it would be interesting for you to tell me instead the purpose of a transformer between a tube amp's power section and speaker.
punk isn't dead, it's always smelled that way.

"A perfection of means, and confusion of aims, seems to be our main problem."
-ae
Last edited by gumbilicious at Dec 3, 2011,
#13
OP - I havent read through this whole thread as it started to get into an "argument" so sorry if this has been stated already. Looks like in your post you have some basic questions regarding specs of equipment. I'll do my best to explain.

It seams you are wondering if your 70watt speaker will be louder than a cabinet loaded with 4*30 watt speakers. While the answer given is very true here is some food for thought. That 100watt sticker on that back of your amp means its going to push out or 'deliver' a maximum of 100 watts of power. Remember watts are a product of voltage and current. Voltage X Amperage = Wattage

Think about this. Your amp wants to deliver power to a speaker. They designed that one to target a 4ohm speaker. They even put a sticker on the back that says 100watts @ 4ohms. So you might wander, what happens if i put an 8ohm load on it? Well, without getting into specifics cause OMG every amp handles this differently (mine has a switch for 4,8, or 16) but pretend thats not the case here. You load your amp with an 8ohm cabinet and it wants to deliver that 100 watts into 4ohms. but now it has TWICE the resitance. SO, you get half the power. (In theory, not in reality) just trying to help you visualize this is all.

Now, going back to your 70watt vs 30 watt guestions. Speakers have a couple of specs right? Ohms, and wattage. Well, ohms is simply how much resitance and the wattage is HOW MUCH wattage they can handle before they start to fail. The cab your reference has 4 speakers. Here is a line to remember - Electricity will take the path of least resistance. So, you run a cable from your amp to your cab and now the power coming into the back of the cab has 4 choices of where to go. Know what it does? - It goes to all 4 evenly (Given that its wired that way 2s2p). So lets say you have that thing cranked up and your outputting roughly 50 watts of power @ the head. That hits the cab and that 50 watts gets distributed to each of the 4 speakers evenly. That means 50/4 = 12.5 watts per speaker. How many watts can this cabinet take? Well, each speaker can handle 30 so 30*4 = 120watts @ 8ohms.

Soooo, in a nutshell, the wattage listed on the speakers are how much they will handle
The wattage output is how much power your sending to your cabinet
The lower your volume of output the lower the wattage your sending
The higher your colume the more wattage your sending

Its best to match impedance amp/cab so the amp doesnt overheat (Unless its made to autoswitch) - Some amps dont care WHAT impedance you load them up with as they will autoswitch and output the same amount of power regardless of load.

That was pretty good advice above tho about the cab being a bit louder. One thing you can do right now to make your combo louder is raise it up on a stand. Just having the speaker at ear level makes ALL the difference in the world. Learned that one when i "dialed in the best sound ever" when i had my amp on the ground. Then stuck a mic to it and hated the recording. Got my ear down there and realized it wasnt the mic's fault. Thats exactly how my amp sounded!
#14
Quote by surjer
Now, going back to your 70watt vs 30 watt guestions. Speakers have a couple of specs right? Ohms, and wattage. Well, ohms is simply how much resitance and the wattage is HOW MUCH wattage they can handle before they start to fail. The cab your reference has 4 speakers. Here is a line to remember - Electricity will take the path of least resistance. So, you run a cable from your amp to your cab and now the power coming into the back of the cab has 4 choices of where to go. Know what it does? - It goes to all 4 evenly (Given that its wired that way 2s2p). So lets say you have that thing cranked up and your outputting roughly 50 watts of power @ the head. That hits the cab and that 50 watts gets distributed to each of the 4 speakers evenly. That means 50/4 = 12.5 watts per speaker. How many watts can this cabinet take? Well, each speaker can handle 30 so 30*4 = 120watts @ 8ohms.


when can do the whole thing if you want. another speaker statistic is called 'sensitivity', that is how loud a speaker is @1 meter away from the cone with an 1 watt signal being fed to it (sensitivity = @1m @1W).

i am unsure of the speaker, but i will guess the speakers and use the stats. guitar speakers are usually rated around 100 dB sensitivity, so we'll use that number. you want to use 50 watts to output (he has a 100 watt head, but we'll do calcs for 50 watts)

dB louder = 10*(LOG10(P2/P1))
P1 = power output 1
P2 = power output 2
dB louder is hoe much louder P2 is compared to P1
P1 = 1 watt
P2 = 50 watt

dB louder - 10*(LOG10(50/1)) = 17 dB

so the amp will be 17 dB plus it's sensitivity rating, speaker output would be 117 dB

now for 4 speakers at 100 dB sensitivity we use your 12.5 watt output you calculated.
P1 = 1 watt
P2 = 12.5 watts
dB louder - 10*(LOG10(12.5/1)) = 11 dB

each speaker would put out 111 dB. since you have 4 speakers, you add them as such

10*LOG10(10^(S1/10)+10^(S2/10)+10^(S3/10)+10^(S4/10))
S1 = speaker 1 output
S2 = etc

10*LOG10(10^(111/10)+10^(111/10)+10^(111/10)+10^(111/10)) = 117 dB

so the math says both cabs would be putting out the same SPL at 50 watts if they have the same sensitivity. if you ran the cabs to capacity and assumed the sensitivities were the same then:

-the 1x12 with a 70 watt speaker could handle 70 watts, putting out 118.5 dB, while the 4x12 could handle the full 100 watts and put out 120 dB. that makes the 100 watt cab potentially 1.5 dB louder. well, how loud is that. it's about 1.13 times as loud, not much.

but still, without knowing the head he has, or the speakers he is using we can't actually know. but that should give you an idea of how much one is louder than another. i wasn't going to bore anyone with the tedious math until i saw your post.
punk isn't dead, it's always smelled that way.

"A perfection of means, and confusion of aims, seems to be our main problem."
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Last edited by gumbilicious at Dec 3, 2011,
#15
@gumbilicious - Awesome post, I didnt know about the sensitivity! Thanks for the explanation!
#16
Quote by KG6_Steven
Well, I guess Mesa, Rivera, Peavey, Fender and others are wrong. Why would they state it's safe to do this, if it weren't safe?

He didn't say it wasn't. What he said was true. Running a mismatch does cause extra stress, in most cases not enough to affect the life of the OT because nobody is stupid enough to design them right on the limit. The impedance of speakers changes depending on manufacturing tolerances and even what frequencies you are running. I should imagine that chugging away in drop A compared to strumming away on the top three stings up at the 12th fret causes a bigger difference in OT current than running a 2X mismatch.
Nothing in domestic electronics runs at that tight of a tolerance that it can't tolerate being slightly off-spec. When you add up all the tolerances of the components in an amp you get such a wide room for error that it's amazing that anything ever works at all.
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#17
^^sensitivity was a bit key. there is a guy on this site i got a bunch of that info from. he is the actual speaker guru, i just read some of his stuff. if you wanna know about speakers then check him out

http://profile.ultimate-guitar.com/Phil+Starr/contributions/columns/

^thx cath, i was starting to wonder if i was being really dumb about trying to get my point across. i think i seems really obvious i have been overly long-winded.
punk isn't dead, it's always smelled that way.

"A perfection of means, and confusion of aims, seems to be our main problem."
-ae
#18
Quote by gumbilicious
wow dude, you seem to consistently miss the point. i am done trying to explain the fallacy of your paradigm, but if dichotomies are that appealing to you, buy all means tell me how wrong i got it. it would be interesting for you to tell me instead the purpose of a transformer between a tube amp's power section and speaker.



Wow. If you had worked one more big word into that paragraph, you would've qualified for the U-G toaster oven.

I'm not here to argue with you. Nor have I "consistently missed the point of the fallacy of my dichotomous paradigm."

Quote by gumbilicious
If you don't match impedances with a tube amp you do stress the output transformer which could lead to critical failure, but i am more concerned about how it will sound. when your guitar signal is not properly matched to the cabinet it sounds different, usually pretty damn bad


This is one of your statements I take issue against. These are your words. My point is the stress induced on a transformer, by using a higher impedance cab, is not enough to cause a problem. You say it could lead to component failure. I disagree. My point is, the transformer still sees an acceptable load and is not allowed to create an unloaded magnetic field in the core. I could take any one of my amps and run them at 2:1 impedance and never experience a "catastrophic failure." Again, while we, ideally, want to see a perfect match between amp and speaker, a 2:1 mismatch is perfectly safe. Does a 2:1 mismatch cause stress? A marginal amount. Will it cause failure? No. Does a "no load" or less than 1:1 mismatch cause stress? You bet it does.
#19
Steven, i do appreciate what you are saying. the truth is, as i had mentioned earlier, you are going to have a VERY hard time killing a guitar amp with JUST an impedance mismatch. frankly most guitar manufacturers anticipate us hooking up a bunch of wrong impedances to your tube amps and over engineer an amplifier to handle these abuses.

this designing has nothing to do with how you are supposed to run a transformer optimally though, if it is not run optimally it is stressed and will at least sound different. i will once again state: A transformer expects a particular impedance, if it does NOT get the rated/expected impedance you will be stressing it more than if it was getting the rated/expected impedance

for all the words i have spent on this subject already, i am going to sink a bunch more into it and still barely touch the true complexity of the subject matter. certainly this explanation is a more elaborate and vague conclusion than: "a higher impedance cabinet doesn't hurt a tube amp" but it does reflect the reality of the situation much better.

alright, transformers are fairly crazy complicated, they work off of flowing current through some windings (primaries i think?) and inducing current in closely located but separate windings (secondaries?). the complications come when A/C is applied and the windings ressist and react to the current, furthermore when the currents get more intense then flux leakage starts to occur. i have a hard time think about flux leakeage, but i kinda think of it as leaking pipe that is contaminating the process the transformer is trying to perform.

now trying to get an idea what is going on in that environment bends my mind, and most engineers end up really dumbing it down so that guitar players can understand it. this tends to lead to misunderstandings. in a quest to get an idea of what is going on in a transformer during impedance mismatch i ran across this FAQ response at Ted Weber's site (great guy, very knowledgeable, he will be missed).

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.


well, it seems that running too high of an impedance can actually create a flyback voltage that could eventually break past that string of diodes and arc across one of your output tubes pin set. and even if it doesn't cause catastrophic failure immediately, you're still stressing your amp with excessive flyback voltage.

i am still waiting for you to tell me why a tube amp has a transformer and what it's purpose is (if it is not the one i propose, than what is the purpose). i would also like to see some type of technical backup on why 'big impedances are OK for tube amps' too (i have seen no valid technical conclusions on such ideas, only people's opinions what they think 'safe' and 'unsafe' is. how can it be that Ted Weber, who designed music equipement his whole life, cannot tell you what a 'safe' and 'unsafe' mismatch is but you can? i fell you are putting too much faith in an oversimplified theory.
punk isn't dead, it's always smelled that way.

"A perfection of means, and confusion of aims, seems to be our main problem."
-ae
Last edited by gumbilicious at Dec 4, 2011,
#20
Quote by gumbilicious
well, it seems that running too high of an impedance can actually create a flyback voltage that could eventually break past that string of diodes and arc across one of your output tubes pin set. and even if it doesn't cause catastrophic failure immediately, you're still stressing your amp with excessive flyback voltage.

i am still waiting for you to tell me why a tube amp has a transformer and what it's purpose is (if it is not the one i propose, than what is the purpose). i would also like to see some type of technical backup on why 'big impedances are OK for tube amps' too (i have seen no valid technical conclusions on such ideas, only people's opinions what they think 'safe' and 'unsafe' is. how can it be that Ted Weber, who designed music equipement his whole life, cannot tell you what a 'safe' and 'unsafe' mismatch is but you can? i fell you are putting too much faith in an oversimplified theory.


Exactly. But what do we consider too high? Is 2:1, as I've suggested is fine, too high? What about a 4 ohm head and a 16 ohm cab? This is 4:1.

The purpose of an output transformer? Easy. Tubes are voltage devices, as opposed to transistors, which are current devices. As such, tubes are high impedance devices, while speakers are low impedance. The transformer performs the function of impedance matching - matching the output from the high impedance tubes to the low impedance speakers, for an efficient transfer of energy. Believe it or not, not all tube designs have used transformers. Transistors do not require a transformer due to the fact they are low impedance devices.

Another important distinction to make is that transformers do not have impedances, they have impedance ratios. In other words, transformers change impedance as a pure ratio. Tube design is what dictates their output impedance. The transformer is designed to accomodate this impedance at the primary and match it to a given load, typically 4, 8 or 16 ohms. And Cathbard is correct - impedance does change at a given frequency. Just to be clear, my assertion that it's safe to run your amp at 2:1 assumes good transformer design. If the transformer is poorly designed, running a mismatch may push it past its capabilities. Again, this is why I stated that TS should check his manual to determine if this was safe, or not. I have never seen a tube amp that wouldn't do 2:1. Would I recommend doing 4:1 or higher? Probably not. You may be pushing your luck, but I feel confident you could run at 2:1 all day long and not have a problem.

This is one of the areas where some folks feel adamant that it's not safe, while others feel it's okay.
#21
Quote by Cathbard
Yet to come across a tube amp yet where running a cab at 2x the impedance of the selected output on the amp wasn't a "safe mismatch."

don't really wanna hijack the thread, but does that mean it's safe to connect my fender champ reissue to an 8 ohm cab even though it's rated at 4 ohms?
Rig Winter 2017:

Fender Jazzmaster/Yamaha SG1000
Boss TU-3, DS-2, CS-3, EHX small stone, Danelectro delay
Laney VC30-112 with G12H30 speaker, or Session Rockette 30 for smaller gigs
Elixir Nanoweb 11-49 strings, Dunlop Jazz III XL picks
Shure SM57 mic in front of the amp
#22
Ted Weber -

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.


I thought this was very interesting.


A lot of the people that use this forum have lower end tube amps so imo it is just easier to state what is safe vs trying to get technical about why it is OK to not play it safe.
#23
^+1. my sentiments exactly. tell the person asking the question what is going on and let the individual decide whether it is safe or unsafe for them. especially when you have make assumptions about transformer quality and amp design.

Quote by KG6_Steven
Exactly. But what do we consider too high? Is 2:1, as I've suggested is fine, too high? What about a 4 ohm head and a 16 ohm cab? This is 4:1.


i dunno, i personally won't tell other people what is 'safe' or 'unsafe'. you seem to feel confident in what you believe though. i would feel i am making too many assumptions commmenting on what is safe for someone elses equipment.

Quote by KG6_Steven
The purpose of an output transformer? Easy. Tubes are voltage devices, as opposed to transistors, which are current devices. As such, tubes are high impedance devices, while speakers are low impedance. The transformer performs the function of impedance matching - matching the output from the high impedance tubes to the low impedance speakers, for an efficient transfer of energy.


i don't know of any guitar tube amps that don't use transformers to match to speakers (only exotic hi-fi ones that use an ungodly amount of output tubes), but that doesn't mean they don't exist (i'll just limit my talk to tube amps that have transformers). since you obviously know what a transformer does, how does this conflict with anything i claimed?

seems to me you know better and just wanted to jump all over my ass cuz i happened to mention that an impedance mismatch can (not WILL, but there is the possibility that it can) ruin a transformer and your advocating the concept of strictly 'safe' and 'unsafe' mismatch. is this right? so, because i didn't advocate the concept of 'safe' and 'unsafe' mismatching i deserve this incredibly draw out interaction?

seems like we both believe the same thing, but you prefer to encapsulate your concept in a simple paradigm that is easily digested while i find the same concept heavily truncated and not very enlightening.

Quote by KG6_Steven
This is one of the areas where some folks feel adamant that it's not safe, while others feel it's okay.


in my original post, i actually tried to down play my feeling on what i feel is acceptable and i made an effort to just report what is basically happening. looks like i failed miserably.
punk isn't dead, it's always smelled that way.

"A perfection of means, and confusion of aims, seems to be our main problem."
-ae
Last edited by gumbilicious at Dec 5, 2011,
#24
Quote by Blompcube
don't really wanna hijack the thread, but does that mean it's safe to connect my fender champ reissue to an 8 ohm cab even though it's rated at 4 ohms?

Yes. People do that all the time. It will cut the output power almost in half but if you are running it into a better speaker it more than makes up for the loss of power. A Champ will run into an 8 ohm load all day without issue.

Pretty cheap to replace the output transformer on them though.
http://triodeelectronics.com/tfchxfwi48oh.html
$20 - hard to beat that. They are good quality ones made in Chicago too.
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#25
Quote by Cathbard
Yes. People do that all the time. It will cut the output power almost in half but if you are running it into a better speaker it more than makes up for the loss of power. A Champ will run into an 8 ohm load all day without issue.

Pretty cheap to replace the output transformer on them though.
http://triodeelectronics.com/tfchxfwi48oh.html
$20 - hard to beat that. They are good quality ones made in Chicago too.

thanks - i knew this happened with solid state amps but was always to never do this with tube amps. so even with the volume cranked that won't destroy the OT?
Rig Winter 2017:

Fender Jazzmaster/Yamaha SG1000
Boss TU-3, DS-2, CS-3, EHX small stone, Danelectro delay
Laney VC30-112 with G12H30 speaker, or Session Rockette 30 for smaller gigs
Elixir Nanoweb 11-49 strings, Dunlop Jazz III XL picks
Shure SM57 mic in front of the amp