RyanWasHere, sorry man but I have to disagree with you. It is totally ok to run a 4ohm amp into a 16 ohm cab. The rule is that you can mismatch ohms as long as you mismatch lower rather than higher. Example....I have a Traynor Bassmaster which is a 8 ohm amp. I run this amp into my Marshall speaker cabs which are 16 ohm cabs. I have mismatched the ohms, but the amp puts out less ohms than the speakers. This is ok. I have played for several hours with this setup and my amp and cabs work without any problems. If you have a 16 ohm amp into 8ohm speakers, you will blow the speakers and possibly the output transformer of the amp.
You can plug a 4 ohm amp into a 16 ohm cabinet.
Sorry Gtr_Pkr, kittenface, but RyanWasHere had it dead right!
Common misconceptions there, but dead wrong...
(Also you are also getting more than a tad confused: Your example above of an 8 ohm amp into a 16 ohm cab is a high
direction mismatch, not a low
direction one. Though you did say 16 ohm cabs
. If you meant running into two 16 ohm cabs then there would be no mismatch at all...)
Anyway... here we go again. (I've been here before in my other forum persona as annoying impedance man Andy H.)
Fact is that if ever impedance mismatching a VALVE amp it is FAR less dangerous for the amp itself to run into a too low rather than a too high impedance. Too high impedance loads are the quickest way to kill your valve amp!
There are very good reasons why that is so...
Think about it: what kills a valve amp quickest? Running with no speaker load attached is what. That means trying to run it into fresh air = a very high impedance load. Conversely if you short circuit the speaker connections of a valve amp (touch the +ve and -ve together --> very low impedance, near zero ohms!) it'll almost always survive. (Not recommending you try that, but it is true all the same!*) If anything goes with a low direction mismatch it'll be the power valves. But in fact they (unlike the output power transformers of a solid state amp) are pretty much self limiting as far as power output is concerned. Rather than trying to put out more and more power into a lower impedance, (exactly the the problem which destroys the output sections of ss amps), the impedance mismatch means an efficiency loss and LESS power output. (Also meaning you WON'T blow the speakers running a 16 ohm amp into a suitable 8 ohm cab BTW as the amps power output is reduced.) The output valves do
have to work harder though so the main problem with low direction mismatches is that they are hard on your output valves. They will take a lot more wear, but probably won't fail immedaiately unless already weak. If they do happen to fail of course THAT in turn can potentially cause further amp damage. Though with good HT fuse protection this is actually pretty unlikely.
Power output does also drop with high direction mismatches, that is correct. All
mismatches, low and
high, lead to less efficient running and less power output!
The real big danger for the amp itself though (specifically the output transformer - the most expensive bit of your amp!) is going into a too high impedance load.
What is too high? Well, most valve amps (with healthy valves and output transformers) will survive a mismatch within the (low to high) range of 1/2 to 2X the correct matching impedance. (So 8 ohm amp into 16 ohm cab is just about okay - you will indeed almost always
get away with doing that.) Go much above 2X though and especially at high power output levels you risk suddenly inducing very high 'flashback' voltages in the OT. That
is what can cause breakdown of the insulation of the windings = transformer burnout.
Remember you can safely turn on and turn up a solid state
amp with no speaker attached. It'll sit quite happily all day like that. Short circuit the speaker output though and you'll burn out the output transistors before you even have time to start to think about swearing. Exact opposite of valve amps.
Like I said this is a very
common misconception. I think there are a couple of main reasons for it.
Firstly, as RyanWasHere says the rules for valve and solid state amps are very different - basically opposite from each other. People sometimes don't realise that, or if they do they may mistakenly think they are reading about one type of amp when the discussion was really about the other. Reading in context there is vital.
Secondly, I think people often mistake 'high impedance load' for a 'high amp load' - again wrong - and may misread things not realising the difference.
Think about the situation with a typical solid state amp. Fact: the lower the impedance it runs into the more power it tries to deliver into it. This means an increased current demand, so a low impedance load is actually a harder
load for the amp to run. (To do so it needs much higher current rated power supplies and output components.)
And in the opposite case a high impedance load is in fact a much easier
load for the amp to run as the current demand is reduced. The highest impedance of all is the NO LOAD situation!!!
easy to get confused there, but it is important:
A high amp load = low impedance, an 'easy' amp load = high impedance, no load at all = near infinite impedance.
Previous rant (as Andy H.) on exactly this:
A couple of the Gear Page forum links I listed there don't work any more. So here are a couple that still do in their place:
Basic valve amp / speaker mismatching 'rules'.
Post #6 here:
http://www.thegearpage.net/board/showth ... adid=67069
*Don't believe me about valve amp surviving having their outputs shorted out? Fender valve amps usually have shorting jack sockets fitted to their speaker outputs! Posts #5 and #6 here:
http://www.thegearpage.net/board/showth ... p?t=126001
The really important point guys, and the reason I spend a lot of time on this stuff here, is not that these misconceptions are wrong. (There are much worse things than simply being wrong.) It is that they are REALLY DANGEROUS for valve amps!
Don't go risking your valve amps output transformer thinking running into high impedances is okay.
It isn't okay!!!