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Post by sydsbluesky on Jan 10, 2010 4:29:55 GMT -5
Okay, this is probably a simple issue for you guys, but this still isn't my area of expertise.
I have one 4x12 cab that offers inputs of either 4 or 16 ohms in mono (series wiring vs. parallel wiring, correct? This is selected via a small slider in back) or 8 ohms stereo. I would like to use this with two different heads. One head is my little 5 watt blackstar, which runs in 8 and 16 ohms, so that has one match. The other head is a Marshall AVT which only run at 8 ohms... It's actually the top of a combo amp, so they probably didn't have this exact use in mind when they made it.
I'm thinking that it would be nice to get that cab to run some configuration at 8 ohms.
Now, my understanding is somewhat limited, but I'm going to guess that right now the cab can be switched to run either four 4ohms speakers in series for 4 ohms, or four 4ohms speakers in parallel for 16 ohms. Both in mono.
But stereo wiring would be using two 4ohms speakers per input, wired in parallel to make 8 ohms, right?
So... I'm thinking (uh-oh.)
Since I don't feel the need to run a cab in stereo, could I run two of the 4ohms speakers in parallel, and then run them in series with the other two, being wired in parallel, and thusly achieve 8 ohms?
I'm almost thinking there would still be a way to run the amp in stereo, right?
Let me know what you guys think... And yes, before anyone asks, I tried running the 8ohms Marshall through the 16ohms setting and I didn't like it... maybe just my imagination... maybe just a violent urge to re-wire something...
Jesse
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Post by sydsbluesky on Jan 10, 2010 4:33:47 GMT -5
...Or could I just run a Y cable out of the back of the Marshall and use the stereo jacks?
That just seems way too easy... and that would change the resistance seen at the end of each cable, right?
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Post by JohnH on Jan 10, 2010 5:04:32 GMT -5
Sdysbluesky - your cab has 4 x 16 Ohm speakers, and to use them all in mono mode, the switch converts them from all parallel (4 Ohms) to series/parallel (16 Ohms). You cant get all 4 to work together off one amp at 8 ohms - not mathematically possible!
You should set your cab for 16 Ohms and use it with the Blackstar, matched to 16 Ohms. Tube amps like to have matched amp and cab Ohms settings.
Your AVT head has a solid state output section however. As such, it is happy with any cab ohms greater than the minimum. So if 8 ohms is its minimum allowable value, it is fine at 16 Ohms. The only downside is less power available, but it will only be 3db less which is not much of a drop.
John
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Post by sydsbluesky on Jan 10, 2010 13:25:27 GMT -5
*big salty tears*
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razbo
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Post by razbo on Apr 15, 2010 11:40:50 GMT -5
Your AVT head has a solid state output section however. As such, it is happy with any cab ohms greater than the minimum. So if 8 ohms is its minimum allowable value, it is fine at 16 Ohms. The only downside is less power available, but it will only be 3db less which is not much of a drop. I picked up a used Peavey 4x12 cab the guy said was wired to 4ohm. I wanted to test it using my Fender Vibro Champ's external speaker line which is 4 ohm. In the event that the cab is wired for more than 4 ohm, I take it from this post that I'm not in danger of blowing the amp or anything, ...right? For that matter, with a mis match, will either side blow or do anything bad? 4 into 16 or 16 into 4? Would it be just a matter of degraded sound quality or output? The VC is a "hybrid" I guess they call it. Has tubes + DSP if that makes a dif. [Edit:] Oh, and can I test "ohmage" with a meter? Sorry for the newb questions.  |
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Post by JohnH on Apr 15, 2010 15:53:42 GMT -5
If it has a solid state output section, then you are OK at any cab ohms equal or greater than the amps minimum. If you go below minimum, it could blow, above minimum, all is fine but you lose max power output.
If you have a tube output section, you should match Ohms between cab and amp, or possibly risk the output transformer if driven hard.
John
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razbo
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Post by razbo on Apr 15, 2010 16:00:58 GMT -5
If it has a solid state output section, then you are OK at any cab ohms equal or greater than the amps minimum. If you go below minimum, it could blow, above minimum, all is fine but you lose max power output. If you have a tube output section, you should match Ohms between cab and amp, or possibly risk the output transformer if driven hard. John I can't even answer that question. Maybe I can find out online somewhere. So far I'm glad I'm playing it safe. Thanks for the input! |
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Post by sydsbluesky on Apr 15, 2010 23:28:03 GMT -5
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razbo
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Post by razbo on Apr 16, 2010 7:37:50 GMT -5
That is the amp. Was there something in the specs there that let you know this? Or is it just something you know about these amps? 16 into 4 will be very bad. 4 into 16 will just sound very bad. Makes sense that an "overload" (?) would be much worse than an "underload". But what happens?? The amp sends what it sends, how is the return signal, after going thru some coils, going to damage the amp? Something more for me to educate myself on! Hope that helps a bit... I'm unsure what your last reply was even supposed to mean, so... I only meant that, contrary to my normal "try it and see" approach, I held off until I had some answers. I was writing while at work, so afterwards I was able to measure the impedance of the cab, which was about 4.8 (I'm sure there is some tolerable variance). So I went ahead with my check out. Had it all cranked for a couple of hours and everything was fine, so I guess it was ok. And on the really upside of things, I now have a 4x12 Peavey cab - unblown! - for 40 bucks. Sometimes even I get lucky!  |
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Post by ashcatlt on Apr 16, 2010 12:18:57 GMT -5
The amp sends what it sends, how is the return signal, after going thru some coils, going to damage the amp? Because your statement is false. The amp delivers (or tries to) what is "asked" of it. Let's take the example of an amp designed to deliver 5W into an 8Ω speaker. What does that 5W mean in terms of Voltage and Current requirements? Well, we know that Power = Voltage x Current: W = VI ...but there are two unknown variables there, with an infinite number of solutions. Luckily, thanks to Mr. Ohm and his Law, we also know that: V = IR I'm gonna shoot for voltage first if that's alright with you. We can manipulate Mr. Ohms thing to say: I = V/R and then drop that into the wattage deal: W = V 2/R which means V = sqrt(WR) = sqrt (5W*8Ω) = ~ 6V Now we can go back to Ohm's Law and say I = 6V/8Ω = ~ .75 A So, with an 8Ω speaker attached, the amp has to supply a maximum of 6V at .75A, and is likely designed so that these values fall within the safe zones of all the components involved in the power supply and output section. Now, what happens if we attach an 4Ω speaker? I'm making a bit of an assumption here, but I think that the amp will (try to) keep the Voltage constant. Mr. Ohm says that, in order to do this, the amp must now supply more current: I = V/R = 6V/4Ω = 1.5A That's twice the current requirement! Now, these numbers seem small, but remember that the parts just might not be spec'd to handle this big a difference. To make matters worse, impedance is not actually the same thing as a static resistance. It is frequency dependent. There are electronic reasons for this (the inductance of the coil, etc) as well as mechanical (dependent on the construction of the cabinet) and not exactly simple to work out. Suffice it to say that, for some (usually low) frequencies, the impedance of the speaker can actually be significantly less than it's nominal rating. It's quite likely that the amplifier was designed to be able to produce enough current for the very lowest impedance expected from an 8Ω speaker. With the 4Ω, these might be much lower, and outside the bounds of the design. Does that help any? BTW - a 4Ω speaker is technically a much larger load than a 16Ω because it demands much more Current for a given Voltage. PPS - The reading you got with your meter was the DC resistance of the coil. When measuring unknown speakers, we can usually assume that this will be somewhere close to the rated Impendance, but it is not the same as the Impedance, which is much more complex - like, with imaginary numbers and stuff. |
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Post by sydsbluesky on Apr 16, 2010 12:28:42 GMT -5
Well, they don't exactly make it easy for this, but when you see such a low wattage rating and a power tube of any kind listed, it's usually safe to say that it's "100% tube."
Now, I believe the fact that it has a power tube AT ALL qualifies it as such, but... I have never seen power tubes mixed with solid state in the power section... Maybe someone else has, so I don't want to say anything for sure.
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Post by sumgai on Apr 16, 2010 13:05:03 GMT -5
ash, Go to the head of the class! IOW, all that time you've been hitting the books has finally paid off!! ;D Well put, and 100% on-the-money. I guess what I'm tryin' to say here is that "I wish I'd said that (first)".  sumgai p.s. I ain't gonna quibble, but in your equations, you mixed the formal "I" for current (Intensity) and the informal V for volts, whereas voltage is formally known as E (Electromotive force). I say this only because viewers referencing other sites/books may not make the connection right away when they see the same formula with different letters. |
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razbo
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Post by razbo on Apr 16, 2010 13:09:06 GMT -5
It does if I just jump ahead to this part: V = sqrt(WR) = sqrt (5W*8Ω) = ~ 6V Now we can go back to Ohm's Law and say I = 6V/8Ω = ~ .75 A and follow thru from there. So (in the above scenario) the damage is caused by the amp trying to produce to transfer too much power to the speakers? ...Or I think it might be better described as the speakers being a "load" and I guess they are drawing too much power? Why wouldn't amps just have a fuse or a relay or something as standard equipment? This seems like something that must be happening all the time. Well, they don't exactly make it easy for this, but when you see such a low wattage rating and a power tube of any kind listed, it's usually safe to say that it's "100% tube." Now, I believe the fact that it has a power tube AT ALL qualifies it as such, but... I have never seen power tubes mixed with solid state in the power section... Maybe someone else has, so I don't want to say anything for sure. Cool, thanks. Sorry for the newb questions. I am just recently expanding my guitar-related awareness to amplifiers. Aside from just being a place to poke my lead.  |
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