farmosh203

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?

gregs1020

i cant wait to find out myself.

AcousticMirror

no. the end.

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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.

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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mmolteratx

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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farmosh203

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.

Cathbard

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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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.

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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.

Cathbard

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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Yamaha SBG500
Telecaster
Ibanez Iceman
Roland GP-8
Quadraverb
Abbey Harmonic II
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Marshall 1960A

Cathbard Amplification

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?

AcousticMirror

dude your measurements are wrong.

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AcousticMirror

what.

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farmosh203

Shall I take a scope plot and prove you wrong? The signal coming out of a pedal is definitely above 4V.

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.

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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.

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farmosh203

Hmmm, I guess I didn't consider MOSFET vs. Tube clipping, since there is no clipping during the active pickup / pedal stage.

AcousticMirror

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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farmosh203

You're saying a pedal can drive a speaker?