power amp/ohm/new guitar cabinet

M

MegaGoo

New member
hi all. heres my situation. i play through a 2 channel power amp right now with my j-station. into a mesa 8ohm cabinet. mono

i am getting a new cabinet and have my choice of wiring. as of now im getting 4ohm stereo

but on my poweramp, there is no switch to go between ohms. i was told the amp will switch itself (depending on what its hooked up to i guess) as far as ohms go

has anyone ever heard of that? i wanna make sure i dont blow up my speakers or my amp.

also, according to the spec sheet, the amp is 270 watts per channel at 4 ohms.

im getting 2 celestion vintage 30s, and either 2 classic lead 80s or 2 v12-80s (classic silver series). the 30s are 60 watts each and the other 2 are 80 i believe. so thats 140 watts per pair. if im going stereo at 270 watts per channel, doesnt that mean im putting 270 watts into 1 pair of speakers. i was told this is ok as long as you dont crank the power amp up too high.

does that make sense? i dont have a problem with turning the levles on the power amp and leaving it there perminatly, and just fooling with the volume as i go, on the j-station.

thanks for your help

eddie
 
With a solid state amp you just need to make sure that the load satisfies the "minimum load" spec, usually 2 or 4 ohms. Anything higher than that is ok. The amp does not "adjust itself", the speaker load or impedance (measured in ohms) present at the output stage dictate how much current flows thru the circuit; the more impedance in the circuit (output stage of amp + speaker load), the less the amount of current that will flow. If you have no load connected, the circuit is not completed, so no current flows, no damage done. Now TUBE amps, because of the output transformer needed to convert the high voltage/low current output of the tubes to the low voltage/high current that speakers can handle, need the rated impedance; not significantly higher or lower.
The output rating of your amp, 270 watts, is the MAXIMUM amount it is able to deliver; cranked to the max. With 4 12" speakers, that would be frickin' loud indeed ! If you are using it in your bedroom, and are listening at a comfortable level the output may only be 5 watts or so, the louder you crank it, the higher that output wattage will go. You ears will tell you when to stop.
Speaker ratings are a "nominal" rating; to be used for approximation and comparison, it is not an absolute rating like a 3 amp fuse or a 1000 ohm resistor. there are a lot of factors involved in the frequency and type of signal being amplified. You don't have to worry about closely matching an amps output watt rating to a speaker's, as long as its within reason. As long as you have a legitimate guitar audio signal going thru those speakers, your ears would be bleeding before they blow. What usually blows a speaker is a "bad" signal such as DC, which you will only hear when it first hits (like when you put a 9 volt battery across a speaker's terminals), some really strong very low frequency, or when a solid state amp is driven into clipping. Most, if not all power amps today have a limiting circuit to prevent clipping, but otherwise you are actually more likely to blow a speaker by using too low of a power amp to achieve a desired sound level, since clipping may occur, therefore overheating the voice coil.
Probably more info than you wanted.
 
no thats not too much info. i appreciate it

ok well i have my friends 4x12 mesa cabinet and im going into it stereo 4ohms through this power amp. i can crank it to about 6 or so while keeping the volume at the normal level on the j-station, before i get a little worried about blowing the speakers. maybe its the volume, maybe im hearing the speakers distort slightly, maybe both. i cant tell.

heres a question. say i put the power amp speaker volume on something high and, put the j-station volume level sort of low. is it possible to blow the speakers by only increasing the j-station's volume?

also, is there a way to scientifically determine how loud i can go before i will be pushing too much wattage through, like playing while holding a volt meter on the output terminals and doing some math, or something similar?

or is this simply an ear thing. if you sense the speaker is being pushed to the max, back off the bass eq (i dont think the celestion v30s are made to have a lot of bass pumped through em), or the volume.

thanks
eddie
 
It doesn't matter if you turn the volume down on the J-station or the power amp; coventional wisdom in in the sound reinforcement community is to leave the amp turned all the way up, and adjust the volume upstream (in this case the J Station), thus leaving the maximum headroom available. In your application I don't think it matters much, but an accidental twist of one of the J Stay's gain, level, or master knobs could be painful. If you were to turn down the amps volume, it might help prevent this. Also a cardinal rule; ALWAYS TURN THE POWER AMP ON AFTER EVERYTHING ELSE IS POWERED UP, WITH THE VOLUME ALL THE WAY OFF, AND WHEN THE GIG IS OVER, POWER IT DOWN FIRST. After applying power, turn up the amp's volume to the desired level. This will prevent any speaker/ear damaging signals that may occur from a device in its powering up or powering down phase, or alert you if there is a problem in the signal chain like a bad cord or something.
Speakers do distort, that's part of "real amp" distortion. They can only handle so much signal until they start to reach a point of saturation, where a distortion of the signal occurs. You might also be hearing a little bit of distortion or an ugliness inherent in the J Stay's patch you are using. Celestions are desirable guitar speakers for many players because they DO break up in a particular way at a particular sound level. Some guys like JBL's and EV's because they don't break up as much and have more attack, punch and headroom available at higher volumes.
As far as measuring the wattage, I guess you could try to measure with a meter, but actually the needle would probably be flipping back and forth too much to get an acurate reading. As far as a decibel meter, it wouldn't account for the frequencies of the signal - you are correct about the low end - the lower the frequency, the more power is used to reproduce it . That's why a bass player with a 200 watt amp can be blown away by a 50 watt tube guitar amp. I think you might be worrying too much about it. Rad Shack has a decibel meter for 40 or 50 bucks, you might want to check the level to make sure you aren't in the ear damaging range, which your rig is certainly capable of producing.
the best advice I can give you, if you have the $$ would be to ditch the power amp, get a 50 or 100 watt tube guitar amp head. If you want you can plug the J Stay in to the effects return or power amp in of the amp using it only as a power amp. Using a tube power amp first of all should sound better, you won't blow your speakers unless you feed it a bad signal, and will be plenty loud for any real world application...
 
with measuring the wattage, the needle would be moving? hm well i've seen people say to put a sine wave through it. i have a sine wave but wouldnt the volume of it change the wattage also, leaving that as an unwanted variable? or are you just looking for the wave to give a constant steady signal to take a reading on the volt meter?

with the amp thing, i happen to have a 100 watt mesa nomad head sitting right on top the amp. i never thought of, or knew that you could use that just as a power amp. the head isnt mine, but i hooked it up, and it sounded honestly, as good as the power amp. if better, not by much. but it was late when i tested it and didnt wanna leave it cranked to the max for too long. but at least this way, i can kinda get comfortable with a good max volume by playing through the 100 watt head, and not surpassing that approximate point with the power amp.

the nomad (tube amp) actually sounded a little bit higher in the mids. i had to cut them out totally for it to sound cool. and none of the eqs worked on the nomad. infact, only thing that worked was the SOLO switch on the pedal, and the master and solo volumes.

all in all, i should be able to hear some type of speaker distortion when im reaching the break point huh? i'll have to just train my ears a little

also i've read, that the more likely way you'll blow a speaker is if you're amp is clipping, cause then you're sending bad DC signals to the speaker (or something like that)

<excess banter>
i would play through it long term but a) its not my amp, b) thats a 1400 dollar head so why not play through the $150 power amp if it sounds similar or the same, c) the person whom it belongs to is our keyboard player who plays guitar on some songs, but not many, but its nice as a second guitar rig (this guy also has a triaxis but doesnt touch it partially because the midi pedal is being screwy, and he doesnt have a mesa 2:90 which would bring out most of the options/tones of the triaxis. hes thinking about getting a pod and just playing through that, just to have some kind of effects)

thats the bad thing about having a really nice head, or a triaxis. if you want effects, you needa buy the individual pedals. of course you'll have superior tone, but my j-station isnt all that bad, and with the press of a button, i have my selection of 7 or 8 effects, etc
</excess banter>

thanks. keep the replies coming anyone who has something to add

eddie
 
I've got a 60 watt Silver series celestion and it's OK, but I would not buy another. At some point I'll replace it for another Mesa celestion that I use. The Silver series seems a little harsher sounding.
 
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