OK...please help an idiot. I know that going one step up in an ohm load (4 to 8 or 8 to 16) is normally OK. Would there be any problems other than volume loss when putting a 4 ohm load into a 16 ohm speaker?
4ohm load into a 16 ohm speaker?
- Thread starter edgar_allan_poe
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Actually, that's totally wrong. It's safer to go lower in impedance, ie 8 ohm out into a 4 ohm load, than to go up.
Simplified, the power tubes are current sink devices, and they drive current into the output transformer (the OT). The current is stepped up by the turns ratio of the primary and secondary, and driven into the load. The higher the load resistance, the higher the voltage will have to rise to achieve the current that's being driven. It's Ohm's Law, and that never changes. Then, that voltage will be reflected back to the primary, and it will get stepped up accordingly by whatever the turns ratio of the OT is.....you can easily end up with 1000VDC and more on the plates with mismatching. SOme amps can take it, some can't. You ever wonder why so many old Marshalls had blown output transformers, while it was rare with Fenders?
The load ratings are not set in stone either, rather they're more of an average. Everything changes depending on frequency, etc. Sometimes you might find that an amp actually sounds better with a mismatch, but keep the above in mind.....it's just safer to stay within the set parameters.
I don't think so.... I had amp techs/companies tell me to never do that... It's the same concept of chain to many cab in to your P.A. ... look at the back of many amps.. Wattage goes up as the speaker load drops...which results in more heat....
I could be wrong on this but I've never had a amp failure with a higher speaker load... I've had a power burst into flames with a lower power load.
Old Fenders are 4 ohms, and Marshalls higher. Wouldn't tha make Marshalls the more fragile of the two?
I actually had one of the Two Rock builders tell me to run my Custom 8 ohm tap into a 16 ohm cab; he liked the way it sounded.
But all in all, it is best to stay with the plan.
Tube amps have inherently high output impedance. It's the output transformer that lowers output impedance to make a match with various nominal speaker impedances. The lower the speaker load (its rated impedance), the lower the amplifier's output impedance has to be, and the main way to do that with a tube amp is to add windings to the transformer. In other words, the 16-ohm tap uses the fewest windings, the 8-ohm tap uses more, and the 4-ohm tap uses all the windings the transformer has to offer. Tube amps that can safely drive a 4-ohm load wil be the heaviest and have the biggest output transformers to perform this trick.
I have a Trace Elliott Velocette, which is a killer little combo amp that also has a speaker output. The back panel and the operating manual both issue dire warnings not to use this amp to drive any impedance less than 16 ohms. Why? The Velocette weighs only about 22 lbs., so it's not packing much of a transformer; it only has enough windings to lower the output impedance enough to make it safe to drive a 16-ohm load.
As for Poe's question, it should be fine to use the 4-ohm tap to drive an 8-ohm or 16-ohm speaker. The question I have is, if the amp has a 4-ohm capable transformer, why doesn't it also have an 8-ohm tap as well? The lower the output impedance, the higher the damping factor, which is a good thing. So a 4-ohm output should be able to drive a 16-ohm speaker load with better-than-average cone motion control. The overheating usually happens the other way--using the 8- or 16-ohm tap to drive a 4-ohm cabinet.
I have a Trace Elliott Velocette, which is a killer little combo amp that also has a speaker output. The back panel and the operating manual both issue dire warnings not to use this amp to drive any impedance less than 16 ohms. Why? The Velocette weighs only about 22 lbs., so it's not packing much of a transformer; it only has enough windings to lower the output impedance enough to make it safe to drive a 16-ohm load.
As for Poe's question, it should be fine to use the 4-ohm tap to drive an 8-ohm or 16-ohm speaker. The question I have is, if the amp has a 4-ohm capable transformer, why doesn't it also have an 8-ohm tap as well? The lower the output impedance, the higher the damping factor, which is a good thing. So a 4-ohm output should be able to drive a 16-ohm speaker load with better-than-average cone motion control. The overheating usually happens the other way--using the 8- or 16-ohm tap to drive a 4-ohm cabinet.
The theory I described is Ohm's Law, and you can't change it. Think about it. A lot of old Fenders use a direct-to-ground shorting jack on the output, so that if you play a dumbass for a moment and forget to plug in a speaker cab, you won't blow the OT as long as you realize your mistake in time....this is because you have a HUGE load (direct short). If that jack does NOT short to ground (it remains open - no load), you'll take out that OT in a matter of seconds. That's just the way that it is. Again I refer you to old Marshalls that were notorious for eating outputs, and they were set up at 16 ohms which results in higher voltages being reflected back. With higher resistance, it takes higher voltage to achieve the amount of current present. This is simple Ohm's Law.
Something just occurred to me....the techs/Factory you were talking to were most likely talking about TRANSISTOR amps. These are totally different animals than tube amps, in that a direct short like that will most definitely kill the output transistors in the blink of an eye, while an open condition will not bother them in the least. I automatically think in terms of tubes, since that's what I build. They definitely should not be confused.
It's real simple--the + and - leads of a speaker cable connect to the two ends of the same wire--the speaker coil. Whatever load there is is essentially the cumulative impedance of the length of wire. So the lower the impedance, the closer you are to a short circuit, which is 0 ohms. That's why the lower the impedance, the greater the heat, and if the amp's output stage is not set up to handle a lower impedance load, you're literally playing with fire.
Actually both tube and transistor, it shouldn't really make any difference anyway; the basics are pretty much the same. Asked Peavey a question about my P.A. And Hughes and Kettner about my Switchblade.. To the H&K question..all I had was 16 ohm extensions at the time... the internal is 8ohms.. Running these in parallel is 5.something ohms (I don't feel like calcualting the number right now close enough. H&K said do use the the 8ohm tap but to use the 4 ohm amp tap for the 5 ohm speaker load. Peavey said do not load up monitors to equal less than the 4 ohm amp output...
Bottom line is that I match my stuff anyway.. I converted all my cabs to 8ohms because I can run a balanced load with all my amps which like to see 4 or 8 ohms. Dave not trying to really argue and obviously do what you think is best.. the only major failure I ever had was with a P.A running a low speaker load...It was pretty spectaclar.. flame out the back and a lot of smoke /emoticons/default_biggrin.png
Well, it DOES make a difference....the only basics that are the same at this point is that they both amplify a signal. Beyond that, tubes and transistors are totally different in the way they operate. I'm not trying to argue here either, Craig, just stating facts which are based on electronics theory (and which are are proven through Ohm's Law). I stand behind what I said.
Here's a quote from Dr Z on the subject. I don't think this applies to Dr Z amps only.
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« Reply #3 on Mar 9, 2008, 3:45pm »
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Pete ALWAYS remember you can run a tube amp into a LARGER load and no damage will be done.
You will generate less heat.
You will generate less wattage.
And your tone will change depending on the speaker might be for the better or worse.
This is all with respect to Ohms Law.
I have a dear friend who use to run a Super reverb ( 4X10 , 2 ohm load) with a 4X10 2 ohm extension cab.
The total load was under one ohm, needless to say he went threw 6L6's like candy, overheated his transformer and blew a couple, but boy did the James Gang sound good.
That is the point lowering the load for increased power is BAD long term.
Running into a larger load lowers power and sometimes tone suffers.
And oh yes my dear friend was/is Joe Walsh.
Z
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« Reply #3 on Mar 9, 2008, 3:45pm »
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Pete ALWAYS remember you can run a tube amp into a LARGER load and no damage will be done.
You will generate less heat.
You will generate less wattage.
And your tone will change depending on the speaker might be for the better or worse.
This is all with respect to Ohms Law.
I have a dear friend who use to run a Super reverb ( 4X10 , 2 ohm load) with a 4X10 2 ohm extension cab.
The total load was under one ohm, needless to say he went threw 6L6's like candy, overheated his transformer and blew a couple, but boy did the James Gang sound good.
That is the point lowering the load for increased power is BAD long term.
Running into a larger load lowers power and sometimes tone suffers.
And oh yes my dear friend was/is Joe Walsh.
Z
Well good Lord, with a load of under 1 ohm it was almost a dead short! Of course he was eating tubes! Look at it like this:
A 1 ohm load is pulling a lot more current through the output, and the amp will be louder than with a 16 ohm load. With more cuurent there is more power being dissipated, and more heat. So, which is the bigger load, 1 or 16? One ohm is. The fact is, mismatching too much in either direction is a bad thing, but given a choice, I would mismatch in the lower ohm direction (bigger load) before the other direction. You will get less voltage on your tube plates, and less chance of arcing in the tubes or output transformer.
Dave,
I don't know if you're contradicting yourself or I didn't make myself clear in my original post. What I said was it is safer to go to a higher impedance ie speaker load than a lower one. Example 8 ohms from the amp to 16 ohm speaker. this is exactly what DR Z said. The question is whether it is safe to do this and apparently it is, of course matching is preferable and going into a smaller than optimal speaker load may damage your amp.
ArnieZ
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According to THD the Hot Plate should match the output of the amp. They make different units for each impedance. The Bad Cat unit has an adjustable impedance and the DR Z Airbrake is a nominal 8 ohm unit. Some amplifier companies say the warranty is voided if you use an attenuator. I can't speak to atennuator amp and cab mismatches, I am not knowledgeable enough. As far as amp speaker mismatches it is safer to connect the amp to a higher resistance load than a lower resistance load ie 8 ohm output into a 16 ohm cab.
ArnieZ
wouldn't shorted be NO load, not a huge load? And open is basically infinite resistance, or HUGE load. If I understand it right, Higher loads than designated are more likely to blow transformers, while lower loads are more likely to blow power tubes.
Mesa at least says it's ok to try mismatches, but I think they usually mean 1 step, 4 -> 8 or 8->16 or the otherway. Mesa are probably overbuilt enough to take a 2 step change, but I don't know for sure. If it's a multispeaker cab, might be easier to rewire it to lower the resistence if possible.
