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Watts equalling thermionic dick waving; or possibly the forsight that come the stadium tour, one wont have to buy a new amp.
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Yeah, the power craze with guitar amps is really silly. I also find it amusing/sad that most guitarists pay deerly and drag around big amps while often only using 1/10th or so of the power available.
My homebuilt amp has Powerscaling. At max clean power it puts out just over 10W (2xEL84 cathode bias). I haven't seen a venue yet where it wasn't enough for stage volume (and anything else is better micked). For "bedroom" level, I need to go down to about 30mW (yes milli Watts) to get appropriate volume when cranking it...Comment
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You need the extra watts for cleans. That basic fact could be more widely known. I get plenty of young guys with 100W Marshalls saying, it needs more gain, or, can you make the gain less fizzy, when what they really need is less wattage. For them, 100W somehow means more distortion; big misunderstanding.Comment
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I don't want to get bogged down on the one detail, but...
cbarrow, when you say we need this standard way to compare amps, I keep coming back to WHY? I hear kids discussing amps and this one or that one is "better" because it is 60 watts instead of 50. NUts. SO if i am looking for a 50 watt amp - more or less - and I have the real ratings we are discussing and they all were to have them. I might find the Fender was really a 47 watt amp, but the Marshall was really a 52 watt amp. Now what?
WOuld that enter into the purchase decision? or would we think they were both basically 50 watt amps and move on to what they sound like?
I think the preamp is what has to push the power stage to whatever glorious heights of power there may be, but preamps are not all equal either. So it seems to me unfair to the amp to think it (the preamp) paramount to the tone but bypass and ignore it for amp comparisons.Education is what you're left with after you have forgotten what you have learned.Comment
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Enzo,
When it comes down to it, a guitar amp is an amp like any other amp. This is how amps are rated. The ratings, when done correctly & honestly, are to protect the folks who are not as technically savvy as all of us. When it comes to tube guitar amplifers, the issue becomes highly simplified due to the limits we have already discussed. When it comes to solid state guitar amplifiers, and there are many out there and many more to come, the issue is not so simple. A manufacturer could just plain old lie to people and tell them the amp puts out 2 x 3 times as much power as it actually does because there is no 'simple' way to verify the output. A standardized testing methodology solves all of this.
The preamp does not drive the power amp to its output. The driver section does that. The preamp amplifies an instrument level signal to a level suitable to inject into a power amplifier. As I pointed out before, in most of our guitar amps we also add a distortion "effect" a lot of the time but not always.
The standardized test methodology injects singal directly into the power amplifier (which always included a voltage multiplier stage followed by a current amplifier stage). If we tested guitar power amps by going through the preamp then we would get different power ratings based on the preamp settings (which could be drastically different in something like a Line 6 amp).
ChrisComment
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Not trying to be sarcastic or anything, but how about this: Injecting a signal into the power amp in an amp that has an effects loop is easy. I don't own an amp built after about 1971. None of them have effects loops. What do I do, pull the chassis and inject a signal into the phase inverter just to get an exact reading of the power the amp is putting out?
I tend to agree with Enzo, treating the pre and power amps as one system gives you a reflection of the system as it's used.
What if the power amp you are measuring doesn't get driven to it's full potential by the preamp driving it? A lot of vintage amps are probably low gain like that.Comment
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That is exactly what I was trying to say in my clumsy way.What if the power amp you are measuring doesn't get driven to it's full potential by the preamp driving it? A lot of vintage amps are probably low gain like that.
The preamp doesn't drive the output stage in a schematic sense, but the preamp is what sends the signal there, and it may or may not drive or cause to be driven the output to its full potential.
But I will agree, if we had standard testing to lab specs, then we would know that kid who bought the 60 watt amp because it was "10 louder" was really getting his extra 10 louds.Education is what you're left with after you have forgotten what you have learned.Comment
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Regis, what is "correct" and what is "convenient" are two different things. If we mix terminology we'll never reach a consensus. You are correct, on a lot of guitar amplifiers measuring this way is inconvenient but not at all impossible. The requisite connection can usually be made without unsoldering anything and connecting a clip lead to the input of the phase inverter to input the reference signal (1-3 volts).
A power amp typically needs between 1-3V at its input to drive it to full, rated, output. The larger the amp, the larger the voltage typically needed at its input. For a guitar signal, that means that you need a voltage gain in the preamp somewhere between 10-60 (allowing for low output guitar pickups, 10-30 for a guitar outputting 90-100mV). I can't think of a single preamp that doesn't meet those requirements (relative to the size of power amp they are driving).
I think maybe the best food for thought I can bring into this discussion are the stand alone tube power amplifiers that Marshall used to (still?) make. I believe Mesa has done these as well. Remeber this piece?:
http://guitargeek.com/gearview/768/
The circuitry for this amplifer is similar to the circuitry for any other typical marshall of the day. With this guitar amplifier, you have no choice but to directly inject the measurement signal into the power amplifier. If we were to measure this power amplifer in a typical Marshall head by inputting the reference signal into the preamp, as some here are suggesting, we would get two radically different output measurements. This would be true using either the scope method or the THD meter method. These amplifiers that Marshall made did have a voltage gain stage in front of the phase inverter. That was for a couple of reasons; DC isolation without a capacitor, good input impedance and a 0dB input level (about 0.707VAC) and that is a typical signal flow for a typical power amplifer (high gain voltage stage followed by a low gain, linear driver and current amplifer). With that stated input reference level of 0dB, that stage is part of the power amp in this case.
http://www.drtube.com/marshall.htm
To measure this stand alone power amplifier vs. the power amplifier in a regular marshall head you need to do it apples to apples. You have to inject the reference signal directly into the power amplifier.
ChrisComment
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Well I don't think it matters much what you do so long as you drive the power amp up to maximum clean. What I do is, signal genny into the input, scope the output, open everything up (genny too if necessary) till the wave clips, back it off till it's clean, measure the vAC on the output, do the sums. Worked just fine on the Mesa 20/20 power amp I had in the other day, just needed more output from the genny till it clipped. Even if it does go into clipping the vAC reading doesn't rise much. This may not be totally accurate but it isn't misleading either, the only thing it doesn't account for is pre-clipping compression, which is a little thing, and you can see some of it on the scope, just back off the volume till the wave looks good and clean.Originally posted by cbarrow7625 View Post
Preamps that are dirty at all levels do present a problem, but again I look at the wave and make a judgement as to when preamp dirt turns into power amp clipping.
I can't recall directly but I do think there are amps whose preamp output is designed not to clip the power amp, and so might be at or below 1vAC. Old tube PAs?Comment
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Sounds like a perfectly valid way to go about it. As long as you keep the preamp clean and provide enough drive to the power amp that should get you in the ballpark. The THD meter method would still show the additive effects of preamp & power amp distortion (there will still be some preamp distortion, even when "clean") but this would work fine for the scope method.
ChrisComment
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