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#1
I was thinking today, that amplifiers don't really amplify the signal coming into the amp very much. For example, if your signal coming out of your active pickups is 9V peak to peak, the RMS voltage is (9/2)/sqrt(2) = 3.18. If the amplifier amplifies this signal by 5, you have roughly 15Vrms. 15Vrms^2 / 8 ohms = about 30 Watts of power delivered to the speaker.

In addition to not amplifying the signal very much, active pickups use solid state components. An amplifier is more of an impedance matching device, which brings me to believe the "warm" sound from a tube amplifier comes from the output transformer, not from the tube.

Am I crazy or does this sound right?
Last edited by farmosh203 at Nov 21, 2010,
#4
wat

Since when is the signal 9V P-P coming into an amp? The highest ouput pups put out maybe 1V max. Most pups put out around 100mV P-P on average. An amp doesn't amplify 5 times either. The first stage in many amps multiplies gain by factors of 30-60. The rest of the stages are similar. The warm sound from tubes comes from the way they distort. And contrary to many people's beliefs, those even order harmonics we all love are primarily generated in the preamp. Ideally, a push-pull power amp will add only odd order harmonics though in practice small amounts of even order harmonics will be added as well.

That said, the OT does play a role in how the amp sounds.

Have you read any technical books or articles on amps? Particularly tube amps? If not, a good place to start is Randall Aiken's site. Merlin Blancowe's site is also great. Some books by Richard Keuhnel, Morgan Jones and Merlin Blancowe go much farther in depth with lots of math if that's your thing. I highly recommend checking out the Pentode Press site and ordering Richard Kuehnel's books. They're great.
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#5
Here are some links:

Pentode Press (now AmpBooks apparently):
http://ampbooks.com/

Aiken:
http://www.aikenamps.com/

Merlin Blencowe:
http://www.freewebs.com/valvewizard/

Merlin even just released a new book. Gotta order that. I don't know a whole lot about power supplies.

Also try music-electronics-forum.com and the AX84 forums. Both have lots of knowledgeable dudes happy to answer questions in depth.
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#6
Quote by mmolteratx
wat

Since when is the signal 9V P-P coming into an amp? The highest ouput pups put out maybe 1V max. Most pups put out around 100mV P-P on average. An amp doesn't amplify 5 times either. The first stage in many amps multiplies gain by factors of 30-60. The rest of the stages are similar. The warm sound from tubes comes from the way they distort. And contrary to many people's beliefs, those even order harmonics we all love are primarily generated in the preamp. Ideally, a push-pull power amp will add only odd order harmonics though in practice small amounts of even order harmonics will be added as well.


I just measured the output of my pedal (the stage going into my amplifier) and it's definitely above 4V peak to peak. Sort of having trouble plucking the note, then quickly grabbing the probes on my oscilloscope to measure the signal though.
#7
If that's all it took nobody would bother using all this trickery and even resort to computer modelling to attempt to capture the valve sound with SS would they? They'd just hang a transformer on the end and the job would be done. You don't think anybody has tried that before?
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#8
For starters, you'll be hard pressed to find an active pickup that puts out a signal over 1-2 volts p-p. And that is INSANELY hot compared to normal passive pickup signals which range in the 100-300 millivolt area. Most active pickups will be a bit under a volt p-p.

Secondly, tube gain stages amplify that signal quite a great deal. So much so in fact, that interstage voltage dividers are almost always necessary to dump a lot of the gain back to ground. Let's take an example:

12AX7 triode (one half of a 12AX7 tube)
300 volt plate supply
100k plate resistor
1M grid leak
1k cathode resistor bypassed by 1uf cap
1M following impedance

The above configuration would yield you an amplification factor of 59.3 (roughly). So it would turn a 100mV input signal into a 5.93v output. That's a 35dB gain.

Not only is the input signal amplified to the hilt in a guitar amp, it's also distorted considerably.
#9
If that's all it took nobody would bother using all this trickery and even resort to computer modelling to attempt to capture the valve sound with SS would they? They'd just hang a transformer on the end and the job would be done. You don't think anybody has tried that before?


That's what makes the tube amp expensive though...

For sake of argument, let's just say the input signal coming into the amplifier is 5V peak to peak after going through a pedal (which I measured on my oscilloscope). The amplifier doesn't need to amplify this very much to produce a high amount of power.
#10
Quote by farmosh203
That's what makes the tube amp expensive though...

For sake of argument, let's just say the input signal coming into the amplifier is 5V peak to peak after going through a pedal (which I measured on my oscilloscope). The amplifier doesn't need to amplify this very much to produce a high amount of power.

They aren't that expensive. You can buy an OT for a 100W JCM800 for $60 - and that's at retail prices.
Read what Craig just said.
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#11
And contrary to many people's beliefs, those even order harmonics we all love are primarily generated in the preamp. Ideally, a push-pull power amp will add only odd order harmonics though in practice small amounts of even order harmonics will be added as well.


If you're implying that you need a tube pre-amp for this, doesn't having a solid state active pickup (pre amp) ruin all the signal content immediately after this stage?
#13
Quote by farmosh203
If you're implying that you need a tube pre-amp for this, doesn't having a solid state active pickup (pre amp) ruin all the signal content immediately after this stage?


what.
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#14
dude your measurements are wrong.


Shall I take a scope plot and prove you wrong? The signal coming out of a pedal is definitely above 4V.
#15
Why would it? The tube preamp adds the harmonics on top of whatever AC signal is put in. It doesn't matter if it's passed through a SS signal path. And yes, you need a tube preamp for that specific type of clipping. Transistors simply won't distort like tubes do. MOSFETs can get kind of close but the cut off point is still sharper. I think the chapter of Blencowe's book he has available on his site covers this.

EDIT: Min, a 4V P-P signal is reasonable depending on the pedal. I know of a design that puts out 18V P-P that's meant to slam the **** out of the amp's input.
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Last edited by mmolteratx at Nov 21, 2010,
#16
Quote by farmosh203
That's what makes the tube amp expensive though...

For sake of argument, let's just say the input signal coming into the amplifier is 5V peak to peak after going through a pedal (which I measured on my oscilloscope). The amplifier doesn't need to amplify this very much to produce a high amount of power.


The mystique of tube amplifiers is not a matter of nominal amplification. It's the harmonic distortion characteristics. That's where the "warmth" comes from. You can have a single 12AX7 preamp into a phase inverter and on into an output section and have yourself a guitar amplifier--even without a 5v input signal. Might not be that "warm" or "rich" to your ears though.

There is a lot more going on in an amp than simple voltage (or current) amplification. The coveted sound comes from manipulating the signal being fed which usually has little to do with how much it is amplified. And it is absolutely independent of output power. Output power is an indicator of potential clean volume above all else.
#17
Why would it? The tube preamp adds the harmonics on top of whatever AC signal is put in. It doesn't matter if it's passed through a SS signal path. And yes, you need a tube preamp for that specific type of clipping. Transistors simply won't distort like tubes do. MOSFETs can get kind of close but the cut off point is still sharper. I think the chapter of Blencowe's book he has available on his site covers this.


Hmmm, I guess I didn't consider MOSFET vs. Tube clipping, since there is no clipping during the active pickup / pedal stage.
#18
Quote by mmolteratx
Why would it? The tube preamp adds the harmonics on top of whatever AC signal is put in. It doesn't matter if it's passed through a SS signal path. And yes, you need a tube preamp for that specific type of clipping. Transistors simply won't distort like tubes do. MOSFETs can get kind of close but the cut off point is still sharper. I think the chapter of Blencowe's book he has available on his site covers this.

EDIT: Min, a 4V P-P signal is reasonable depending on the pedal. I know of a design that puts out 18V P-P that's meant to slam the **** out of the amp's input.


right that's out of a pedal...which goes into the v1 input buffer of the amp...which immediately gets dumped out the other side into a volume pot.

all that initial 4v is doing is distorting that first stage...which ironicaly is what the tube amp is there for...adding harmonic distortion content.

I don't think the op realizes that he doesn't actually need an amp...he can plug that pedal right into the PA.
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#19
I don't think the op realizes that he doesn't actually need an amp...he can plug that pedal right into the PA.


You're saying a pedal can drive a speaker?
#20
Quote by farmosh203
Your saying a pedal can drive a speaker?


yes that's what the pa is. a single speaker.

wait actually yes you can drive headphones with active speakers.

problem solved.
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Last edited by AcousticMirror at Nov 21, 2010,
#21
I'm pretty sure pedals are not meant to be used to drive speakers, they are just pre-amps.
#22
Quote by CECamps
The mystique of tube amplifiers is not a matter of nominal amplification. It's the harmonic distortion characteristics. That's where the "warmth" comes from. You can have a single 12AX7 preamp into a phase inverter and on into an output section and have yourself a guitar amplifier--even without a 5v input signal. Might not be that "warm" or "rich" to your ears though.

There is a lot more going on in an amp than simple voltage (or current) amplification. The coveted sound comes from manipulating the signal being fed which usually has little to do with how much it is amplified. And it is absolutely independent of output power. Output power is an indicator of potential clean volume above all else.

The coveted 18W Marshall uses half a 12AX7 into the PI. That's one rich sounding blues amp right there.
But as I said, if all the sound characteristics came from the OT valve guitar amps would have disappeared decades ago along with all the other valve gadgets.

Re using a pedal to drive a speaker - he was taking the piss out of you.
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Last edited by Cathbard at Nov 22, 2010,
#23
Quote by farmosh203
I'm pretty sure pedals are not meant to be used to drive speakers, they are just pre-amps.


pretty sure what's possible > ignorance.
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#24
pretty sure what's possible > ignorance.


Well the pedal runs off 9V, so [(9V/2)/(sqrt(2))]^2/8 ohm = 1.2Watts

Exactly why pedals don't make a great amp (and who knows what the current limit of the output amplifier is in the pedal...).
#25
Well, I might be completely wrong, but I think of it the way I think of voltage in a car, even though it's DC. A simple component, like a relay, takes 12 volts to activate, the battery puts out a constant 12 volts, and 400 amps, you would think that 400 amps going through a relay would fry it, but it doesn't, because the relay only pulls what amps it needs. So your voltage may not change, but your amps can change wherever you want them to, depending on how much you need.

So an amp may be getting a whatever milivolt input at whatever miliamp, I believe the purpose of all the circuitry is to change the amount of amps so it puts out a higher wattage, thus giving more volume, while not changing the actual input. I'm prolly wrong, but idc, I tried.

#28
Quote by farmosh203
I'm pretty sure pedals are not meant to be used to drive speakers, they are just pre-amps.


No they are not, but they can. In fact, if you have a small enough speaker that needs some ridiculously small power to work, you could drive it with just your guitar. Basically, you just need to up the signal a lot so that it can drive a larger speaker if all you want to do is make sound. The tubes and such come in now days more for the quality of sound that they make, as with a couple op amp circuits, you could easily boost the sound enough to drive a cab. It still does require a lot of amplification though. The signal is not very big coming out of a guitar and it needs to be big if you want a loud amp.

The reason that your pedal is over 4 v is that the pedal has a power source in it (the 9 v battery) which is being used to boost the sound. Basically it's being a very small amp that also flavors the sound in some way.
#29
So an amp may be getting a whatever milivolt input at whatever miliamp, I believe the purpose of all the circuitry is to change the amount of amps so it puts out a higher wattage, thus giving more volume, while not changing the actual input. I'm prolly wrong, but idc, I tried.


That just confused me... but I think I know what you're trying to say. Basically it's just impedance matching but the amplifier still needs to amplify the signal a little.
#30
So here's a question, if the output of active pickups is never above 1V, why do people do the 18V mod to their EMG's?
#31
Quote by ethan_hanus
Well, I might be completely wrong, but I think of it the way I think of voltage in a car, even though it's DC. A simple component, like a relay, takes 12 volts to activate, the battery puts out a constant 12 volts, and 400 amps, you would think that 400 amps going through a relay would fry it, but it doesn't, because the relay only pulls what amps it needs. So your voltage may not change, but your amps can change wherever you want them to, depending on how much you need.

So an amp may be getting a whatever milivolt input at whatever miliamp, I believe the purpose of all the circuitry is to change the amount of amps so it puts out a higher wattage, thus giving more volume, while not changing the actual input. I'm prolly wrong, but idc, I tried.



Yup. You're wrong.

A guitar amp is a voltage amplifier. It takes a small AC signal from a pickup (~100mV P-P or so) and multiplies the signal in each stage depending on that stages design. The current output of a guitar's pickup is negligible to the point of not mattering.
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#32
Quote by farmosh203
That just confused me... but I think I know what you're trying to say. Basically it's just impedance matching but the amplifier still needs to amplify the signal a little.



Yeah, but it doesn't have to by amplifying the volts, it can just up the amps to pull more current, which if done right wont change your volts at all. Everything in a car runs on 12 volts, headlights, blinkers, radio, AC controls, power windows, power seats, everything. The only thing that changes is the amount of amps being pulled for each certain function, the volts never change. But comparing DC and AC circuits can be a very bad move on my part.
#33
Quote by Warrior47
No they are not, but they can. In fact, if you have a small enough speaker that needs some ridiculously small power to work, you could drive it with just your guitar.


Not true. Speakers require current, not voltage to be driven. A transformer takes a high voltage, low current signal and transforms it into a low voltage, high current signal to drive speakers. The more voltage you can feed the transformer, the more current it can feed the speakers. This is the precise reason why output tubes are required in tube guitar amplifiers.

Because transistors are current amplifiers, transformers are not required. They can drive speakers directly.
#34
Quote by farmosh203
So here's a question, if the output of active pickups is never above 1V, why do people do the 18V mod to their EMG's?


The 18v mod increases the supply voltage of the active electronics from 9v to 18v in order to increase the output. But it doesn't mean the guitar is putting out an 18v signal. 18v is the DC supply voltage to the active electronics.
#35
Quote by ethan_hanus
Yeah, but it doesn't have to by amplifying the volts, it can just up the amps to pull more current, which if done right wont change your volts at all. Everything in a car runs on 12 volts, headlights, blinkers, radio, AC controls, power windows, power seats, everything. The only thing that changes is the amount of amps being pulled for each certain function, the volts never change. But comparing DC and AC circuits can be a very bad move on my part.


yes. bad move.
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#36
If anyone cares, I measured the output directly out of my guitar, you can get a 2V peak to peak signal coming out of an EMG pickup strumming hard.
#37
Quote by farmosh203
So here's a question, if the output of active pickups is never above 1V, why do people do the 18V mod to their EMG's?

Because the input voltage of the preamp in an active pickup isn't the same as its output. Running the preamp at 18v changes the way the preamp reacts to the input signal, but it doesn't mean its putting out 18v.
#38

The 18v mod increases the supply voltage of the active electronics from 9v to 18v in order to increase the output. But it doesn't mean the guitar is putting out an 18v signal. 18v is the DC supply voltage to the active electronics.


The supply voltage of the active electronics doesn't matter unless the signal is coming close to the positive rail of the power supply. The only reason to increase the voltage supply of an amplifier would be to prevent the signal from clipping if it was coming close. The gain is set by the feedback resistors, not the voltage supply.
#39
Because the input voltage of the preamp in an active pickup isn't the same as its output. Running the preamp at 18v changes the way the preamp reacts to the input signal, but it doesn't mean its putting out 18v.


I'd like you to explain how running the supply voltage at 18V changes the signal.
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