Power amp wattage.. - will I blow my speakers?

dwswift · #1 01-20-2001, 03:02 AM

I'm planning on buying a VHT2150 Power Amp. *For those who don't know about this amp: it's a stereo power amp 150 watts a side. *I'd like to run the power amp into two Legacy 4X12's. *

I know the Greenbacks in the Legacy's can only handle 25 watts each (or 100watts a cabinet).

Am I going to have problems with this power amp or will it be okay as long as I don't turn the amp up too loud?

Sorry for my ignorance on this subject... ohms and watts have always confused me. *Thanks

J Todd Beachler · #2 01-20-2001, 05:13 AM

more importantly, what preamp are you using?

dwswift · #3 01-20-2001, 02:06 PM

POD Pro... but does that really make a difference?
Thanks

rgr · #4 01-20-2001, 04:28 PM

Wow, 300W for guitar, that's alot of power, it's also alot of tubes when it's time to replace them. *You should be okay as long as you don't run everything wide open. *Speakers are more likely to get damaged from power amp clipping, especially ss power amps that clip with really sharp transients. *The 2150 should give you plenty of headroom and I can't imagine ever needing to run it on 10. *Since it runs on KT-88s, you can probably have it rebiased to run EL34s which will lower the output, consult VHT or the manual.

Roger

Doink · #5 01-21-2001, 05:42 AM

1st off, are your impedances going to be matched? If your amp pushes, say, 4 ohms, and you hook up an 8 ohm cab, you're only getting about 2/3 the power. Same with an amp that pushes 8 ohms and hooking up a 16 ohm cab. NEVER hook up a speaker load that has a LOWER impedance than what the amp can push.

2nd, 300 watts is a lot of power for a guitar amp. Are you going to be playing 4000 seat arenas? If not, then you won't need half that much power. But if you don't want to get a lower powered amp, you can always pull some tubes. Pull the 2 outside ones from each amp (a stereo power is 2 amps in one box). Now, here's where amps with 6 power tubes get confusing. My Super Twin has 6 6L6's, and for a 6 tube amp that pushes 4 ohms, to pull the 2 outer ones, the speaker load would be matched at 6 ohms. It's very hard to wire up a speaker configuration to ohms (the closest you can get is running a 24 [3 8 ohm speakers in series] ohm load and an 8 ohm load in paralell), but pulling 4 of the tubes and leaving 2 of them is match for 8 ohms. Confused?

3rd, you can usually hear the speakers crapping out when they're overpowered. When you hear them dying, just turn your amp down.

You might be able to run 4 cabs total, and not worry about blowing the speakers, it would depend on the impedances, again.

rgr · #6 01-22-2001, 01:09 PM

Doink, ? *

I agree with you on the "never hook up a lower impedance than the amp can push." *But the rest was kinda confusing and I know impedances pretty well. *Realize also that with the 24 ohm and 8 ohm loads in &#0124;&#0124; (&#0124;&#0124; means parallel) you will be dumping about 3X more power across the 8 ohm load (approx. 75W assuming a 100W amp) vs the 24 ohm load (approx. 25W assuming a 100W amp). *You have to be careful mixing and matching. *

I think the rule you are trying to communicate is that if you pull some power tubes it changes the impedance that the output transformer sees. *Ex. if you pull 2 tubes out of a 4 tube amp, you have to set the amp to 16 ohms to run an 8 ohm cabinet. *

The VHT is 2 power amps with 4 KT-88s each if I'm not mistaken. *KT-88s can put out more power than 6L6s and EL-34s, they also cost alot more. *It is also a new amp I would bet that it has an impedance selector for 4, 8 or 16 ohms, it wouldn't surprise me if it would work with a 2 ohm load too. *

dwswift,
You should still be fine with the 2 Legacy cabs but I agree that 300W for guitar is overkill for 99.9% of playing situations. *

Roger

dwswift · #7 01-22-2001, 05:33 PM

Thanks all for the input...

I appreciate all your opinions. *I realize that 150watts a side is overkill, but you don't buy an amp that loud (they ain't cheap) if you don't crave overkill

*

at 4 Spaces and 60Lbs this purple faced beauty is a tone monster. Probably the best sounding power amp ever...
:biggrin:

rgr · #8 01-23-2001, 12:29 AM

Where ya gonna play it? *I love power tubes because of the tone they get when you push them hard. *I have trouble pushing my 60W per side amp hard though.

wyzenhymer · #9 01-23-2001, 05:56 AM

I realize there is a huge difference between tube and solid state power amps. I run a solid state Rocktron F-300 through 2 4x12s and sometimes 3. When on a small stage I run both channels stereo into the 2 *sides of a Marshall stereo cab. I often have to crank this setup at well over 50% power. With the Chameleon preamp I get the same sound all the time, low volume or high. I'm not saying it sounds good:biggrin: just loud!
My 2nd guitarist in my band blew a Laney 4x12 with a Peavey head, and it just quit. Didn't sound fuzzy or clipped beforehand, just...stopped. I haven't popped it open to look for fuses or breakers, (it's not mine, why should I care?!) but that was the 1st 4x12 I've ever seen give up.
Any comments?
G.M. *

rgr · #10 01-23-2001, 01:20 PM

Sometimes speakers just give up, like anything else. *It could have got hit with a power spike too, especially if it is a ss amp. *There are lots of reasons, sometimes these things just happen. *If you ohm out the speakers I'll bet one of them is open, replace that one with one of the same impedance and it should be fine. *

Roger

Doink · #11 01-25-2001, 01:28 AM

rgr
I don't think the majority of the power would go to the 8 ohm load. I think it'd be divided evenly. I don't know for sure. Maybe if I get bored this weekend, I'll do some experimenting.

When a speaker blows with no warning, it's usually the voice coil that blows. That is most commonly due to an SS amp clipping (tube amps usually clip "softer".
When a speaker has too much power going into, you can usually hear the cone "blarfing" and crackling.

wyzenhymer
I doubt he blew all the speakers at the same time. Could be something as simple as a connection that shook itself loose due to the rumble.

rgr · #12 01-25-2001, 04:30 PM

Doink,

You're right on about the speakers blowing, but,

Quote:

I don't think the majority of the power would go to the 8 ohm load. I think it'd be divided evenly.

Unfortunately, yes, it would (not be divided equally, that is). *That's basic first year electronics or physics Ohm's Law calculations, BTW, I have a 4 yr degree in it. *The 8 ohm side would have less resistance to the current being dumped into it than the 24 ohm side. *

Damn, gotta get out my calculator again, assuming a 100W amp:

Total impedance of 8 ohm &#0124;&#0124; 24 ohm = 1/((1/

+ (1/24)) = 6 ohms.

There would be about 24.5V across each load, V is the same since they are &#0124;&#0124;, V=sqrt(100W*6ohms). * *

Now, the current through the 8 ohm leg = 24.5V/8ohms = 3.06A and the current through the 24 ohm leg is 24.5V/24ohms = 1.02A.

We can check this by finding the total current, 100W/24.5V = 4.08A which is equal to 3.06A + 1.02A

Now, the power dissipated in the 8 ohm leg = 24.5V * 3.06A = 75W (74.97 but I'm rounding).

The power through the 24 ohm leg = 24.5V * 1.02A = 25W.

Basic Ohm's law equations:

Rseries = *R1 + R2 + R3 + ... + Rn

Rparallel = 1/((1/R1) + (1/R2) + (1/R3) + ... *+ (1/Rn))

V = IR *(V = Voltage, I = current, R = Resistance)
therefore, I = V/R and R = V/I

P = IV (P = power)
therefore, P = Isqrd * R = Vsqrd/R *(sqrd means squared)

These are the tools you need to figure loads (impedance) and how power, voltage and current is divided in circuits. *For series/parallel resistance circuits figure the series Rs in each leg first then do the parallel calculation. *Capacitors and inductors add a new dimension, their impedance (called reactance) is frequency dependent but you don't have to worry about them in speaker load calculations. *PA spkrs with built in crossovers have caps in them but you can usually just figure the load based on the cab rating and ignore the fact that the caps are there.

Hope this is helpful, e-mail me if anybody has any questions,
Roger

(Edited by rgr at 1:32 pm on Jan. 25, 2001)

(Edited by rgr at 10:27 pm on Jan. 25, 2001)

Doink · #13 01-26-2001, 07:33 PM

Wow, 4 years? I didn't even have the attention span to last 6 months in an electronics class in high school. I wish I would have stuck it out the full 2 years and I wish I woulda paid more attention. I kick myself everyday because of it!

But I am familiar with ohms law(s).

I conceed to your knowledge.

rgr · #14 01-27-2001, 01:14 AM

Well I spent 4 years in school for engineering, I wish now I would've followed my gut and went to music/recording school but I caved to family pressure. *15 years later and I'm still living the backup plan and barely find time to play. *After school loans, etc... I never got back to plan A, DOH!
*