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Post by teleking36 on Mar 7, 2006 17:28:05 GMT -7
is it true that i could, theoretically speaking, plug an 8 ohm cab into a 16 ohm output and not have any problems?
just curious, as I have a peavey classic 20 with a 16 ohm out and i was wondering what it would sound like with my ZBest. i think i heard somewhere that you can have a cab a step down in ohms, but not higher than the output.
can anyone verify?
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Post by zdogma on Mar 7, 2006 18:06:14 GMT -7
Well, its a good question. Depends on who you ask. Most quality tube amps can handle a 1 step mismatch, up or down (ie 16 to 8 or vice versa), but a few can't.
The general consensus is unclear, but its my impression that it is worse to step up (8 ohm output to 16 ohm cab) and I can tell you from experience the sound is often not great.
2 steps is probably a bad idea in any case.
Stepping down puts a bit of strain on the output transformer, and depending on the bulid quality, it may or may not handle it.
With my ghia I have many times run it 8ohm out to 4 ohm cab with no ill effects, but the o. transformer is pretty impressive on that amp.
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Post by Curt on Mar 7, 2006 19:26:32 GMT -7
It is my understanding that if there is a 1x mismatch the load shoud be of the greater number, i.e 8 ohm jack to a 16 ohm load.
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Post by zdogma on Mar 8, 2006 5:09:49 GMT -7
I guess we're not helping very much. Why don't you ask miles?
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Post by bluzsteel on Mar 8, 2006 6:12:00 GMT -7
Its my understanding when you go down in Ohms you can damage your Transformer
this came off of the Weber Site 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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Post by zdogma on Mar 8, 2006 6:55:24 GMT -7
Agreed bluz, you can damage the transfomer going down, it was pretty common for the old Marshalls to croak when they were hooked up to a mismatched cab. I read a long thread on this same problem over on the gear page and there were 20 different answers. Apparently stepping down (16>8) puts more strain on the transfomer. Going the other way you can get voltage that somehow zaps (flyback voltage, as weber says) back through the output transformer and can damage the amp. When I got my Kingsley combo the manual said a 1 step mismatch in either direction was ok for this particular amp, and he implied that most handbuilt amps are similar. I also read an interview with the doc a few years ago, when ghias had only one output (8 ohm) and he said there was no problem with an 4 ohm cab from that output. I have used my ghia that way with no problems (8 to 4 ohm cab). It sounds good and the breakup is a bit earlier. I have also put it into a 16 ohm but it sounded pretty wooden and thin. The problem is, I really don't know if what I've done with the ghia and kingsley applies to production amps. Here's an answer from John Phillips from the TGP, he said it was OK to reproduce it, just to contrast what weber says. This comes up so often but it seems that it needs repeating...
A mismatch in either direction by a factor of two is not dangerous to the amp, in almost any case. That means that running into half the correct load (eg 16-ohm amp into 8-ohm cab) is NOT dangerous.
For a TUBE amp. It's certainly dangerous for a solid-state amp, which is probably where the confusion comes in.
But in fact for a tube amp, running into too low an impedance is safer than running into too high, all things being equal, and provided you don't go outside about a factor of two.
Running into too low a load is harder on the tubes and will wear them faster, but is unlikely to do any sudden or serious damage. It's still not a good idea to run into under half the right load for any length of time though, since you do run the risk of tubes failing, which could in theory cause other problems (though not in a well-designed amp really).
Running into too high a load is potentially dangerous since higher than normal flyback voltages are developed and stepped up through the output transformer which can exceed the insulation rating of components. This can cause sudden, serious damage to the transformer, the tubes or the tube sockets depending on where the insulation breaks down first. There's no real time factor, the damage either occurs or it doesn't, and it may not necessarily happen immediately. But it's definitely unwise to run into more than double the correct load with any tube amp.
There is a good reason why many amps (eg BF and SF Fenders) have a shorting switch in the speaker jack - it's to deliberately short the output in the case of no speaker being plugged in. This is because although it's not a great idea for the tubes, it's far safer for the amp than being left with an open circuit. Really.
There are a few specific cases where OT ratios may make a too-low load more dangerous than normal (eg old Marshalls, but they have three-way impedance selectors so you shouldn't have to), but by and large if you have the choice of too low or too high, go with too low.
Tonally you will hear a difference too - both too low and too high mismatches reduce power (typically about 1/3 for a factor-of-two mismatch), but they sound different from each other, and from a match. Too low gives a more mushy, thicker sound; too high gives a flatter, more midrangy sound. A correct match always gives the most power and headroom and widest frequency response. __________________ John P So maybe the best idea is to stick with the stated rating unless you have experience with specific amp and its ability to cope.
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Post by zdogma on Mar 8, 2006 7:03:15 GMT -7
Hey bluz, do you think teleking will actually read all this longwinded stuff we've put up??
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Post by bluzsteel on Mar 8, 2006 8:01:17 GMT -7
should, it will keep your amps healthy
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Post by zane on Mar 8, 2006 10:33:28 GMT -7
I run a 5 . something ohm cab thru my 4ohm jack on my Ghia...that's how the DR runs the Maz "Super"...guess it's alright ;^)
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