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Discussion Starter #1
So I have the opportunity to pick up a PositiveGrid Bias head with the BIAS II software, which is the most current. It's something I'm seriously considering because I like how it focuses on amps and seems to do it quite well. I love my analog pedals and I find most emulation style companies across the board (fractal, line 6, etc), don't do things like drives very well, especially Fuzz - which I love to no end.

Anyway, my main question is about the power section, and I want clarity on the pro's and con's of such. It sports a 600watt (@ 8ohm) class D power section and lots of people use this amp through your standard 2x12/4x12 guitar cabs live without any issue. With the knowledge of the concept that 1watt is 1watt, tube style or SS, how is this able to be run through seriously underrated cabs, at full live volumes, without causing speakers to essentially blow up? I ask this because it's how I would use it myself if I were to get it.

Only a couple examples here, but Andersons Music reviews it on YouTube going through a Marshall 4x12 with greenbacks, and Ola Englund uses his 2x12 with V30's I believe, neither of which shows any concern of using a 600watt powered head through a 100watt cab..

If anyone could explain how this is possible, whether it be because of PositiveGrid's unique design, or there is some general rule of thumb that allows this, that would be great. Thanks to all!!
 

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If memory serves it isn't 600W at 8 ohms, it's 600W at 4 ohms. That means it would be about 300W at 8 ohms, or 150W at 16 ohms.

Assuming the master volume pot is a standard audio taper, 300W at 8ohms class D with the master volume at 5 would only be about 30W-60W of output. That's way more than enough wattage for big stage tone and well under the rating of most 2x12 and 4x12 cabs.

That's how it's possible. Hope that helped!
 

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You (most often) do NOT blow speakers with more (clean) power. Speakers are blown by underpowering (pushing the amp too hard so that it's clipping - tube amps are a minor exception to this due to the soft clipping nature of tubes as well as their compression characteristic).

With a modelling amp like this, where you are not trying to get your tone from an overdriven power sefction, you just turn the master down and you're fine. The rest of the power held in reserve becomes headroom, which with class D (vs tube) you really need because it sounds horrible when driven; you want the power section as clean as possible; get yer tone from the emulations.
 

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Discussion Starter #4
If memory serves it isn't 600W at 8 ohms, it's 600W at 4 ohms. That means it would be about 300W at 8 ohms, or 150W at 16 ohms.
Here's a screenshot if the power specs from their website..
Screenshot_20180721-154630_Chrome.jpg

Though I think I get what you're saying. Basically the idea is the amp would get more than loud enough before the power getting to most speakers reaches critical mass?
 

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Discussion Starter #5
You (most often) do NOT blow speakers with more (clean) power. Speakers are blown by underpowering (pushing the amp too hard so that it's clipping - tube amps are a minor exception to this due to the soft clipping nature of tubes as well as their compression characteristic).
That's certainly interesting!! Glad I asked because this has already been so helpful. My big concern and reason for asking st all is because of the fact that I like to play obnoxiously loud live, when possible. When I'm playing a show where the only thing mic'd may be vocals and maybe the kick, and there's no real sound guy to worry about (I play quite a few smaller metal/hardcore/noisy doom shows) I basically crank my amp so that the drummer is almost drowned out, and go down a bit from there. Or I'll crank it and tell my friend, the drummer, to just hit harder!!! Muahahahaha!!!
HNG^%$
 

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Though I think I get what you're saying. Basically the idea is the amp would get more than loud enough before the power getting to most speakers reaches critical mass?
Yes.

And as Granny pointed out, power is only one piece of the puzzle in a blown speaker.
 

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I basically crank my amp so that the drummer is almost drowned out, and go down a bit from there. Or I'll crank it and tell my friend, the drummer, to just hit harder!!! Muahahahaha!!!
HNG^%$
Yikes! I hope you wear ear plugs. Your ears are valuable man. Don't waste them.
 

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Discussion Starter #8
Yes.

And as Granny pointed out, power is only one piece of the puzzle in a blown speaker.
Yeah, I know dB also plays a huge role too, as every speakers has a rating for max dB, then age if the speaker, etc etc.. but just that big RMS watt rating seemed more concerning to me.

Also, I definitely to wear ear plugs to any and all shows I go to, but unfortunately usually end up playing my set without them. If I am in my project I do vocals in it's just yelling style vocals which sucks to have ear plugs in, because all I can hear is my own dumb voice!! Haha
 

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That's certainly interesting!! Glad I asked because this has already been so helpful. My big concern and reason for asking st all is because of the fact that I like to play obnoxiously loud live, when possible. When I'm playing a show where the only thing mic'd may be vocals and maybe the kick, and there's no real sound guy to worry about (I play quite a few smaller metal/hardcore/noisy doom shows) I basically crank my amp so that the drummer is almost drowned out, and go down a bit from there. Or I'll crank it and tell my friend, the drummer, to just hit harder!!! Muahahahaha!!!
HNG^%$
Basically if your current head (I know you have a few) is 100 to 200 watts, you just use that much of this amp via the master (though it's not quite that simple because bass notes especially will be louder due to the headroom or current reserve this amp will have if you're running it at 33-40% vs your other amp cranked) If the speaker is the same as before then it will be just as loud, without cranking the new amp. If you did dime it you probably would blow the speaker, but I doubt you would because you'd hurt yourself before you got there. Frankly, with solid state amps (including class D) it's usually best to not turn the master volume to more than 1-3 o'clock at most (5-5:30 being max).

Also just to clarify based on what you said there: wattage is power that the amp puts out - electricity. Db is a measure of how loud something is. And a speaker has an efficiency rating (aka sensitivity aka SPL) - how good it is at converting the amp power into sound waves (vs just heat). This is measured in Db per watt at 1 meter (or Db per X Volts at Y feet - you get the idea). This does not mean you get e.g. 90 db for every watt the amp is putting out, but rather that you get 90 Db with the first watt when you listen from 1 meter away from the dustcap. After that it's +3db for every doubling of wattage.
 

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Discussion Starter #12
bass notes especially will be louder due to the headroom or current reserve this amp will have if you're running it at 33-40% vs your other amp cranked
That's actually something I was considering against it for me. I have heard that because of what you stated it's actually more difficult to get these modeler style amps to have the same low end and sustain than a cranked valve amp. Part reason I want to give this a go was to see about getting a Sunn Model T style tone to use as a pedal platform for loud, full stack doom tones. Try and reach the volume and low end of the ultra linear powered, 150watt Model T through a 600watt monster class D power section. Apparently it isn't that easy an equation though.
Maybe my Morris XS3 is better suited. Modded with 6550's for about 113watt clean power, before any distortion on the oscillioscope. Still doesn't have the volume of the mighty Model T, but the Morris easily surpasses it in its inherent tone. XS3 is a plug in and play amp; leave the dirt boxes at home!!
 

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Discussion Starter #14
Most of the bands we play with dont either (except maybe the combo bit).
I know, I was just being cheeky! The real truth is because neither of my current bands have been around for very long. My 2-piece band just played our first show on the Thursday just passed. I'm sure it'll happen eventually though; our music scene isn't that big.
 

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Even when using lower wattage amps (e.g., 22W dual 6V6 or 45W dual 6L6), most musicians won't run them full out. Bear in mind that many efficient speakers can produce considerable acoustic power with even minimal power. Looking at the specs for the 12" Fane Ascension F70, it shows an efficiency rating of 100db at 1 metre with 1 watt of power applied. The Eminence Cannabis Rex 12" clocks in at 101.8db. Plenty of speakers are considerably less efficient than that, coming in at 94 or 96db. Still, the efficiency of many speakers is such that , in a decent cab, one won't be applying more than maybe 10W on any sort of continuous basis, and only exceed 50W on VERY brief peaks. Higher power is generally only required by bass, and even there it's for handling slaps cleanly.
 

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That's actually something I was considering against it for me. I have heard that because of what you stated it's actually more difficult to get these modeler style amps to have the same low end and sustain than a cranked valve amp. Part reason I want to give this a go was to see about getting a Sunn Model T style tone to use as a pedal platform for loud, full stack doom tones. Try and reach the volume and low end of the ultra linear powered, 150watt Model T through a 600watt monster class D power section. Apparently it isn't that easy an equation though.
Maybe my Morris XS3 is better suited. Modded with 6550's for about 113watt clean power, before any distortion on the oscillioscope. Still doesn't have the volume of the mighty Model T, but the Morris easily surpasses it in its inherent tone. XS3 is a plug in and play amp; leave the dirt boxes at home!!
So what I said is theory tested by my recent experience going from a 50-60 watt 6CA7 based Garnet to a Sunn 1200s (4x6550 like the T, but nowhere near as much preamp gain; 120 W advertised but that is known to be understated, as was the T). At jam it doesn't go above 2 on the preamp volume. The Garnet was on the edge of breakup; preamp not dimed but giving it more was just distortion - I was never able to be perfectly clean live. It is also rather full in the low end on top of that; voicing change in addition to more power). I'd never use the Sunn for guitar except as a clean pedal platfrom (I rec guitar thru it; also use it for reamping vox). The Solarus I had (2x6550 for about half the power) was much better for guitar, but still I never got close to diming it (except once just to see; not sure I would have needed that even at a PA-less show). So the Morris should be fine.

In any case a compressor (set to be more of a limiter; no low pass sidechain filter) would mostly take care of the additional bass, as could most amp EQ (on the Sunn the bass lives at 0-2 and it's still very full, but that's just the Sunn - I used to crank the bass more with the Garnet), but I figured for doomy stuff it'd be just the ticket. I mean, there is no way SunnO)))) dimes all of those stacks at the same time.
 

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Discussion Starter #17
Just to follow up, I decided to keep my AX8 for now, rather than trade it for the PositiveGride BIAS head. After all that was said in this post, and some independant research, I figured I am not ready to give up on a tube driven power section. I'm messing with the AX8 with a Peavey 60/60 and it sounds pretty dang glorious!! Keeps the low end perfectly in tact, even for mid-gain doom tones; just like I like it.
Honestly, it's even making me think twice about selling the AX8 at all. I'm even starting to think I should look into buying a better, higher wattage power amp to pair it with. Mesa 295 Simul-Class, VHT, etc... Someone should help me out by buying my '99 Godin LGX off of me..
;) hahaha

Thanks to all who commented in this post. It's very cool to be a part of an online community that strives to teach, rather than just immediately put someone down who doesn't know as much as their peers.
WOO!!!
 

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Something that should be mentioned:

"600 watts @ 8 ohms" is a nearly meaningless spec. It's like saying a car is rated at 40. 40 what? Years old. MPG? MPH max? Not enough info.

The big things I would like to see is 'peak or RMS' (RMS being the tougher, more real spec - but we see peak used more for marketing) and at what percentage of distortion (usually shown as THD - total harmonic distortion). Without at least those two pieces of info, the spec is vague at best.

And as Mark mentioned, we usually don't use all, or even most, of that power. Most playing might be at 1%, maybe 5% of total power, with peaks at 50% or more. Maybe not in your case, though!

Add to that, most speakers are rated somewhat conservatively. If a speaker is rated at 50 watts, it won't go poof at 51. It will probably take sustained playing of something over 50 watts (so not peak levels but RMS levels) to get it to heat up enough to break. And by then, you'd start to hear it. You know, if your ears are still working. :D


Another thing, there is much misunderstanding of 'dB'. 'Decibel' on it's own is not an absolute measurement. It is only a ratio. When people refer to 'speaker dB', there are meaning 'dB SPL'. Which is one of many absolute dB measurements. dB can also be used to measure absolute voltage (dBv) or absolute power (dBm).

For example, 0 dBm is defined as 1 milliwatt of power. So a 1 watt amp is +30 dBm. A 10 watt amp is +40 dBm. A 100 watt amp is +50 dBm. That 600 watt amp would be +57.781 dBm (I cheated on the last one and used an on-line calculator, but the others are daily use for me at work). And these numbers have no relationship to the 'loudness of your speakers' in dB.

That, as they say, is the rest of the story.
 
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