Pre-amp versus power amp distortion?

Yeah i'd like to know to, im looking at building a rig and with a mesa recroding pre, but i dont want to lose the tone with a SS power amp? would that happen?

To complicate matters, power amp distortion is very different between "single ended" Class A amps (which distort pretty nearly the same way as the "preamp" stages) and push-pull Class AB1 amps. And what you hear is heavily influenced by speakers.

Class AB is a lot more efficient so you can get more power out of a given set of tubes. They're always paired so one tube gets the positive half and one gets the negative half of the wave. Called "push-pull" operation. For larger powers, they may be paired in multiples so you have 2 power tubes, or 6... Class A can use a single tube, or multiples hooked up in parallel. This is called "single ended' operation also.

The real difference in sound between them is in how they distort when they're overdriven. Until they're pushed past maximum clean output, they sound pretty much the same.

When mildly overdriven, the "single ended" Class A amp will usually start clipping one end of the waveform a bit, either the positive or negative depending on which way the tube bias is off. It's rarely exactly in the center of the range where both peaks will reach their limit at the same time. When one peak is distorted, the waveform is asymmetrically distorted and contatins even harmonics. Mostly second harmonic. Sounds kind of like a twelve-string guitar. A little bit of that goes mostly unnoticed but adds "warmth" to the tone.

Push it harder, and two things happen: On the positive peak, the grid draws current from the electron stream. That effectively keeps the grid from going more positive. Softly rounds off that peak. On the negative end, the tube "shuts off," the negative grid stopping electrons from flowing from the cathode to the plate. That produces a sharper clipping on that end. Also, the current flowing from the grid to the coupling capacitor from the previous stage gives it a negative charge that keeps it from going more positive on that positive half of the wave, but when it turns negative again, it's more negative than it was. That charge can only slowly leak off to ground through the grid resistor, so it builds up and hangs around for a while. The tube's bias has gone up, more negative. Now it'll clip earlier and harder on the negative peak.Still has even order harmonics in it since it's asymmetrical, though as it clips harder on both ends it makes more and more odd harmonics. The limit of clipping is a square wave, which has a large content of harmonics, all of them odd.

Exactly the same thing happens with the double-ended or "push-pull" Class AB amp. Each tube is softly rounding the positive peak of the signal on its grid, and hard-clipping the negative half. But since the signal is reversed on the grids of the paired tubes, one tube taking the positive half and the other the negative half of the signal, that is symmetrical distortion. Odd order harmonics, mostly third harmonic. More importantly, as the grid current charges up the coupling capacitors and runs the tubes' bias up, each one cuts off just before the other one turns on and takes over. Now there's a "glitch" at the crossover point between the positive and negative halves of the signal. That's called "crossover distortion," and is the main thing that's heard in power stage distortion from a cranked Class AB amp. it's also all odd order harmonics, mostly third harmonic. Push-pull stages with matched tubes can't make any even harmonics. They're all cancelled out.

So you do get a different sound from the two when overdriven. "Preamp" distortion is of the single ended Class A type, so that's what you'll hear with a Class AB master volume amp if you turn up the gain but keep the volume down.

It's an interesting experiment to take a master volume amp and play a single note as you turn the gain up while keeping the sound level about the same with the master volume. As it just starts clipping on one end, the second harmonic starts becoming noticeable. It's an octave of the fundamental tone being played, so it blends right in and you have to listen very carefully to hear it. Like a twelve-string, kinda. Crank it on up more and you start hearing third harmonic from both ends being clipped. That's a perfect fifth musically. Makes a "power chord" sound. Keep cranking it up and you start to hear some fifth harmonic, which is a perfect major third. You're getting a major triad in a single note, from amp overdrive distortion! You've got to limit the frequency response of an overdriven amp, though, or you'll get too many higher harmonics coming through, beating together and making intermodulation frequencies that make the sound a muddy mess. You really don't want anything past the fifth or sixth harmonic to be audible in a distorted amp. For clean playing you often do want lots of harmonics coming through for a bright, acoustic-like sound. That's why folks rave about certain amps' wonderful clean tones, but don't like them for distorted playing. Different amps for different styles.

Excellent reply Ricochet!!!! :D

Enjoyed it. Thankx.
I'll never hear tube overdrive the same again :wink:

Ken

Thanks, Ken. I'm just trying to condense as much as possible what took me a long time of studying, pondering and experimenting to figure out.

Yes, Tube amp sound, operation, and understanding.... not to mention maintainence can seem pretty daunting sometimes.

I saw your other replies to the home/jam amp thread.

I agree with the whole not turning it up all the way point.
I love the blues, and all those guys would turn the amp up til it just started to sing - around 7.
That way, with pick dynamics youcould get soft clean sounds, or pick harder and get some overdrive.
As you were saying here, the more towards total output it gets, the more odd the harmonics get.
Which is very undesireable in my opinion.
The other thread you explained the extent of the volume you want to get to to keep even harmonics, and have triads
with single notes.
Very informative and well said.
It takes some time to find the sound your looking for - and to train your ears.
Great tone is hard to come by - sometimes by very easy means; Say a better understanding of tube works or
simply adjusting the pup hight and dialing that in to your specific guitar can make a huge tonal difference.

Thanks again,
and if I have any tube amp questions, I hope you'll be there. :D

Ken

Depends on what you want. Sometimes just totally blowing it out with all the odd harmonics and no dynamics exactly hits the spot. But not always.

LOL!!!!
Well, there is always that. :lol:

Kinda like havin' an old Fender tweed Champ or a white tolex Gibbo GA-5 turned up to 10.... 8)

I believe the best that I've heard that sound described is - Raunchy.
And that was meant in a good way! :D

The thing with those amps though that made them 'raunchy' sounding was that in addition to the sound of totally
saturated tubes (class A amps by the way)
The speakers were less than great, leading also to massive speaker breakup!!!!
No foamed in edges back then. :wink:

Ken

Yeah, all that analysis above totally leaves out the effect of speakers, though I gave a nod to them at the beginning. I'll leave that to someone more knowledgeable about them. But they make a HUGE difference in how amps sound!

Ricochet, thanks for the nice explanation. The experiment will have to wait until I do finally get myself a tube amp. I just picked up "The Tube Amp Handbook" which has a lot more discussion of amps. Seems to be a worthwhile book.

Solid state amp distortion's not as fundamentally different as some would have it, BTW. :D

Solid state amp distortion's not as fundamentally different as some would have it, BTW. :D

Yeah, I know that. The clipping is different though and the "squishiness" (delay and more non-linearity) that happens in a tube amp doesn't happen in solid state without lots of extra circuitry to make it happen.

I'm actually pretty familiar with electronics - degree in physics, lab electronics, built lots of digital stuff in grade school and high school, built a transistor radio, etc. However, I don't have any experience with tubes and while I've listened to a lot of music over the years I don't haven much connection between the sounds I've heard and the cause of that sound (tube pre-amp, tube power amp, "saggy" tube rectifier, distortion pedal, etc.). I'm trying to figure out that connection now so that I can get the sound that I want and not spend a lot of money wandering through a lot of different amps/effects. I know some of it is inevitable but I can try to reduce it.

Biggest difference is that most solid state circuitry's not operating in that mode where the bias shifts when it hits the limit of "normal" operation. (And some tube circuits don't, either.)

Thanks for the technical explanations, you can learn a lot from Ricochet. :D

I am just a simple guy, don't understand all this electronic jargon, but basically preamp distortion is harsh, whereas power amp distortion is round, rubbery, or puffy sounding (sorry for the horrible descriptions).

Here is a good clip that demonstrates preamp and power amp saturation for the Epiphone Valve Jr. Combo.

http://www.youtube.com/watch?v=K8twAHspOw4

First, the guitar itself has a lot to do with distortion. Gibson LPs usually have very high output pickups, so they will push an amp into distortion earlier than most single coil guitars like a Fender Strat or Tele.

At the 9 o'clock position you hear a nice clean tone.

At about 11 o'clock position this amp begins to distort. This is mostly preamp distortion, it is a little harsh sounding. A single coil guitar would probably still sound clean at this amp setting.

At about 2 o'clock position you hear some power amp saturation kicking in. Note how the tone is much thicker, and not as harsh. If you listen carefully you can hear the notes are slightly "rounded off". Most tube amps tend to "roll off" the higher frequencies as the power tubes saturate. This gives a warmer tone which many prefer to the generally harsher tones of a solid state amp. But some guitarists, especially Metal guitarists actually prefer the harsher, more cutting tone of a solid state amp.

At 4 o'clock position you have a combination of preamp, power amp, and speaker distortion. The speaker has a lot to do with the distorted tone as well. A speaker with low handling power will breakup earlier than a speaker with much higher ratings. Some guitarists want a low handling power speaker for this very effect. Some guitarists will use a higher rated speaker to attain more "clean headroom". It's all what you are after and the application you need.

At the end of the clip this guitarist puts a Boss distortion pedal through the amp at about 12 o'clock position. Pedals like this push the preamp section, so this is mostly preamp distortion. Notice how slightly harsh it sounds.

Today, most guitarists are really using preamp distortion. To get power tube saturation you usually have to really crank the amp which is too loud to do.

Here is an great clip of Neil Young. Neil never uses distortion pedals, he is famous for cranking his old Fender Deluxe to max settings. You can really hear power tube saturation in his playing. Listen to how fat his tone is, and lots of noise like feedback. It takes practice to master playing with this type of distortion. This is the tone of a cranked tube amp.

http://www.youtube.com/watch?v=K8twAHspOw4

Wes,
Hey!!!! :D

I tend to agree with the power tube overdrive thing.
Especially for that early 70's British Tube hard rock sound.
It seems that there is some debate over this vs. pre-amp tube distortion. (Which is actually making 'that' sound)
I think more discussion is need on that topic.

And, I agree that HB pups are more powerful and will eventually lead to a greater distortion....
I know that I played a 1974 Marshall w/a RI 50's LP standard w/PAF HBs and at the volume level that I had the amp at,
I could not get the LPs volume up enough to get the PAFs to really sing.
Lowering the amp volume so I could raise the guitars volume made things too muddy or too quiet.
I ended up switching to an mia Tele w/SC pups, turned the volume on the guitar up (because of the lower power of SC pups)
and got the tone I was looking for at the volume I needed for that situation.
Sort of a double edged sword. :D

I was really bummed - you posted both the same links for the Epi 5 watt amp.
I really wanted to see that Neil vid as 50's 12 watt tube amps are my favorites. :)

Hyperborea - I think we need to expand this discussion to have a bit about "tube sag".
I've heard that true class A tube amps (even w/tube rectifiers) do not have any sag.
Sag is suppose to happen in class A/B amps only.
That sure doesn't explain the ultra sag that 50's Gibson class A tube amps are famous for.

Any thoughts on that out there????

Ken