More stupid tube amp questions about ohms n' volume n such

[deadair]

follow up questions, only somewhat related......

so, i'm playing a show tonight (guitar), and want to play out of two heads/cabs to spread my sound on both sides of the drums/bass/etc...... and am trying to figure out a set-up. having some gear of my own, and having friends that practice in our space, there's a good bit of gear at my disposal, this is what i'm playing around with:

heads:
jcm800 100watt (switchable (4/8/16) ohms)
bassman 100watt (silverface 70s style) (4 ohms)
traynor yba-1a with marshall mod and marshall o.t. making it about 50watts (8 ohms)

cabs:
marshall 4x12 (1960a?) 4 or 16 ohms
ampeg v4 4x12 8 ohms

basically the jcm800 is my normal amp and i'm trying to find a good compliment for live.

so...... first i have the jcm800 into the marshall cab (set to 16 ohms each), and the traynor into the v4 cab (both 8 ohms). the marshall is way louder which means the traynor doesnt help fill much of anything out.

so, then i try the bassman through the marshall cab (so both are set at 4 ohms) and the jcm800 through the v4 cab (both at 8 ohms). the marshall seems a little louder than the bassman, but theyre pretty equal and nice and full. but in toggling between playing them both seperately i realize the jcm800 is a lot quieter and a little darker then it was through the marshall cab.

so, i decide to see if i leave the jcm800 through the v4 and then put the traynor through the marshall cab (set at 16 ohms, so it has 8 going into it) how the volume will work out.... now the amps are about the same volume! the traynor seems louder than before, and the marshall is still quiter through the v4.

do the ohms control the volume in some way? or is it just the cabs? it seems really bizarre to me, or am i missing the stupid answer of: 'hey asshole, the marshall cab is just louder than the ampeg', and hoping theres more of a science to it......

thanks again for any insights!

[thunderboy]

check my post in your other thread...

jt

[deadair]

thunderboy, i had read your post before and it actually doesnt make sense with what i've described/noticed.....

the 8 ohm traynor head seemed louder when plugged into a 16ohm cab than an 8 ohm cab. in your post you suggest it should be the other way around (which i believe you about, which is part of why i'm confused)

also, the jcm800 went from being 16 ohm into 16 ohm, to being 8ohm into 8ohm. it was much quiter in the 8 ohm case. this scenario doesnt really apply to your description of ohms and volumes either.....

if you have any more thoughts/info please share them......

[thunderboy]

If the output impedance of your amp (let's say 8 ohms) is greater than the impedance of your cab (4 ohms), then the amp will push MORE current.
If the output impedance of your amp (8 ohms) is less than the impedance of your cab (16 ohms), then the amp will push LESS current.

More current =
Less current =

Again, this is a generalization. Different speaker configurations will have an effect as well.

jt

[deadair]

communication breakdown.

anybody else?

[Allcalm]

try this
the physics of this are
ohms law
volts = current x resistance
power = volts x current or current squared x resistance
(watts)

In plumbing terms resistance is how big the pipe is


copied:
amps don't have impeadences, cabinets do. When an amp's specs give its power (say 200 watts) at X ohms (say 8 ohms) it means the amp puts out 200 watts when presented with an 8 ohm load. The idea is similar to the word "impeadance:" higher ohms (impeadance) "impeade" (reduce) your amp's power. So, for an amp which is rated at 200 watts at 8 ohms and 300 watts at 4 ohms, an 8 ohm cabinet lets your amp put out 200 watts while a 4 ohm cabinet lets your amp put out 300 watts. Get it? (If you don't its not your fault because amp companies like to confuse things by using the 4 ohm or 2 ohm rating to advertise the cabinet.

[thunderboy]

woo. I'm intrigued!
Could have everything to do with the cabs...are they both closed-back? What speakers do you have in them? If the Ampeg had Greenbacks or some other low-power drivers, and the Marshall had some bad-ass EV 200's (or even Vintage 30's)...that might explain it...hmmm.
Kick down with the specifics!

jt

[soundguy]

Deadair-

have you heard a few different V4 rigs? You've perfectly described what my V4 cabinets do, and pretty mcuh every V4 cab I have ever heard. If you have the original speakers in the cab, the ones with the square magnets and ribbed cones and tiny dust cover, I have no idea who made em or what they are, there's no label on them. If you have those speakers, thats totally what that cab sounds like and thats exactly its behaviour. I dont think the issue at hand is primarily impedance, but the sound of those speakers in that enclosure. Those cabs are WEIRD. If you put different speakers in there, you'll get a much more defined sound. Guaranteed.

dave

[deadair]

my friend actually just got the v4 cab (2 actually) (for bass), and while i do have another friend with one i've only heard it in a live setting... not under this more critical listening.

meanwhile the v4 speakers probably are original..... i've only taken the grill cloth off (not looked in the back) and the speakers are ribbed and look fairly old.

both cabs are closed-back 4x12s though.... as for the marshall, i haven't popped it's back open either, but judging from the info i can find on the cab the speakers in it should be Celestion G12T-75.

perhaps it is just the beast that is the v4 stock speakers that are responsible. unfortuantely it doesn't give list how many watts it can handle on the back, so it's all uncertainties about those speakers for now..... maybe i will pop the back open......

thanks for the knowledge......

[biasvoltage]

There's another piece of the puzzle that we've been touching on here, and that is called Sensitivity. No, not the Dan Fogelberg kind of sensitivity, the quantification of how many dB's a particular speaker will generate with a given input. It can also be referred to as efficiency. You see this specification way more often on home audio equipment than you do on guitar cabs, but it's still a factor.

Sensitivity is expressed in dB's at 1 watt mesaured from 1 meter from the speaker. My speakers at home are 88db SPL @ 1W 1M.

If the Ampeg cabinet can only generate, say, 72db at 1W 1M and the Marshall can generate 82db at 1w 1m, well, then the Marshall will be TWICE as loud for a given input

Other goofy facts, at least I hope I'm right about these: 10db more = twice as loud, but twice the amount of watts into a given speaker will only be a 3db increase which is barely audible, and closed back cabinets are usually less efficient than ported or open back cabinets. And traynor amps are cool.

chuck

[soundguy]

deadair-

the ampeg V4b heads are among the loudest amplifiers I have ever worked with. L O U D. I wouldnt worry about blowing the speakers. I would have loved to have heard one of those cabs new, back in the day, but as it is now, they are the most unefficient speaker I can think of, their transient response is total crapola, but that makes up the character of those cabs. Excellent choice of cab if you want to combine two different cabs on one amp. Nothing sounds like those cabs. Your should be front loaded, I think they all were.

dave

[AGCurry]

SoftSupply has it nailed. The Marshall cabs are more efficient (aka sensitive).

Ohms and watts have far less to do with loudness than does speaker efficiency.

The Ampeg drivers were made by CTS, by the way.

And: deadair, if you need more than your Marshall, you're playing WAY TOO LOUD.

[deadair]

ever since the first post i've been waiting for someone to tell me i was crazy for wanting more volume than the jcm800..... glad it happened! who are we kidding though, volume is an amazing thing, and sometimes a good band that is also an incredibly loud band is even better (ie: high on fire). maybe that sounds stupid, but i know it works for me and my friends. hell, we're about to play some 7 piece show with 4 100watt guitar amps (2 jcm800s, an orange, and a fender twin) with two 300+ watt bass amps in a goddamn basement!

anyways, volume is not my only reason for wanting two amps running together. being amongst a scene that plays basements, halls, and small rooms on college campuses with no soundperson (which i don't mind), if theres only one guitarist in a band and you're standing on the bass side, sometimes you just can't hear enough guitar. Thats what i'm trying to counter by having an amp on both sides. played a show like this last night, and it didnt make much volume difference where i was, but i'm sure it did on the other side.

anywhoo......... effeciency, makes sense... i didnt realize this would be such a complicated answer, but i guess i'm glad it is.

i remeber hearing at one point that 3db increase "seemed" twice as loud...? but that was in a college class, so who knows......

soundguy:
i actually like how the v4 cab sounds, sometimes the marshall sounds too bright... it's great because it cuts through, but the darkness of the v4 has a nice character to it as well, and seems a little more real/tangible if that makes any sense. my friend has a v4 head he uses for bass but i've never played around with it, someday i definately should.

softsupply:
thanks for the info, and yes traynors do rule, i actually have two, one un-modded, and one modded. both sound great and theyre such a damn deal (one was $119).