Trying to Understand Preamp vs. Power Tubes and their Emulation

[jafo]

What makes the difference in how they sound and feel?

I know that for clean preamps at least, any decent one will do, even an op amp; for example, Santana has been known to use sand preamps, and a lot of multi-fx gear from Big Name Companies using a 12AX7 still sounds like crap. Myself, I use a tube preamp (a PreSonus TubePre) and don't get much "tube tone" -- if anything, I get a nicer, warmer tone with a preamp I modded from the first stage of a JFET Vulcan. Simple fact: putting a tube in the signal path does not guarantee tube tone/feel, and it isn't the triode. Even a nice BJT design can be wonderful.

Power tubes, however, seem to make the difference that guitarists love: squishy compression, sustain, warm harmonics... yadda yadda. So why is triode emulation the Holy Grail? Is it strictly the way that a triode can distort when it's abused properly? And if it's truly the pentodes which matter, and if a MOSFET is the closest thing to a pentode, shouldn't we be working on those instead of worrying about Fetzer Valves and Triodizers?

And which of these account for how more and higher harmonics appear when you play harder on a tube amp? Are these strictly from triodes? Softer limiting and softer distortion are certainly part of it, but JFETs, BJTs, and diodes in the feedback path can do this, yet seem kinda flat in comparison (albeit completely fine for cleans). I wonder if it isn't because the triode is more linear than all of these -- perhaps it's a combination of lower amounts and orders of distortion, plus gentle compression? And yet, back to the beginning of this muddled missive, it seems to be the pentodes and tetrodes which cause the magic to happen.

I wonder what a stompbox based on a 6L6 or EL-84 would be like. Obviously you won't get the full flavor without pushing a transformer and speaker, but still...

*shrug* I think I'm trying to ask too many questions at once. And am too concerned with tubes.

[DavenPaget]

A small signal pentode is the EF86/UF86/PF86 ( But i remember reading from the VOX AC30 article that they kind of unreliable )
Why doesn't someone try a similar pentode for a stompbox 

[jcgss77]

Didn't FrequencyCentral do just that?  A pentode-based distortion box?  Let me see if I can dig it up...

Edit: Here it is- http://www.diystompboxes.com/smfforum/index.php?topic=70524.0

The Pentaboost-very interesting

EDIT2: So what would happen if you adjusted the bias of the screen 3 on a pentode in the Pentaboost setup-you could control the flow of electrons to the plate-like speed them up or slow them down?  Need to get my hands on some to see what the effect would be...

[iccaros]

To quote R.G.
Adding tubes or other magic does not make a better sound. ok that is a bad paraphrase but the point is there.

as to the difference between preamp tubes and power amp tubes.. nothing .... and everything...

nothing in that a pentode and be a preamp tube, and triode can be a power amp tube..
everything in how that tube is built, better trans-conductance, less or more plate resistance.

Power tubes, however, seem to make the difference that guitarists love: squishy compression, sustain, warm harmonics... [\quote]  what is the definition of power tube.. a good power amp should be clean or a little overdrive. Way over driving some power tubes is bad.

It all comes down to the total design, not one or two parts.  the EQ setting and gain settings are more important than the other parts

Now one things, that can be designed around, is that tubes use a inductor (transformer) which changes loading based on frequency, so the OT has a lot to do with the sound of  a tube amp, along with speaker.

[mac]

It's the power tube + output transformer duo what many pros like.
Of course it depends on the design, can sound like crap.

mac

[R.G.]

Welcome to the Quest.   

Much smarter guys than me have been trying to figure out the answers to those questions out for at least 40 years that I know of. There are a veritable plethora of patent disclosures for "emulating tubes". If someone really knew, they'd already have produced it and become rich, I suspect.

[jafo]

Hmm, you're all at step 89 or so while I'm back at step 1... maybe I should redefine the scope of my questions!

I've been told a lot of contradictory things about the differences between preamp tubes and power tubes and would like to disentangle them; tube tone and feel have been ascribed to one or the other, and sometimes even to both. Leaving aside the issues of speakers, transformers, and any sort of emulation, what does a preamp tube contribute and what does a power tube contribute? What are the characteristics of each? How do they differ?

Many people say it's all up to the preamp tubes. However, I've heard people get tuby magic with a solid-stage preamp, and crappy distortion with triodes, so I know it's not that simple. At the same time, I'm not convinced that it's purely the power amp tubes which count, because then people would be using EL-84s instead of 12AX7s all over the place... can you use just a pentode or tetrode and get sufficiently tuby?

*shrug* It's probably a happy synergistic love-fest of all the above. Most good things are.

[DavenPaget]

I recall listening to a Kustom KG100HFX through a amptweaker tightmetal ... It sounded like a Mesa MarkIII and a Peavey 5150
Probably some can trace out KG100HFX's schem if he has one , that is one sick head .

[defaced]

what does a preamp tube contribute and what does a power tube contribute? What are the characteristics of each? How do they differ?

I think isolating the question to just the active element of the circuit is kinda like trying to paint a portrait with one color.  I think to best research the answer to your questions you have to go back one more step and consider the circuits each kind of tube is typically used in and the subsequent results, what the results are when they're used in "atypical" circuits and what exactly that means for where you want to go (hands down the most important thing about your questions scope).  I suspect there are as many important differences between individual tiodes as there are between any given triode and any given pentode (and visa versa).  And that still hasn't even begun to consider circuits.  For instance, what does a 12AX7 sound like in a:
- clean preamp stage
- dirty preamp stage
- clean/dirty/fixed biased/cathode biased Class A output stage
- clean/dirty/fixed biased/cathode biased push-pull Class AB output stage.

So there's one tube, 10 stock circuits, I bet they all sound pretty different and we've not even scratched the surface of permutations of this one tube.

Now go get a long plate 12AX7 and repeat ;-)    

Many people say it's all up to the preamp tubes. However, I've heard people get tuby magic with a solid-stage preamp, and crappy distortion with triodes, so I know it's not that simple.

Don't trip over definitions you haven't defined yet.  What is "tuby"?  I can guarantee what I want out of a tube amp is entirely different than what a dude who plays classic rock wants out of one.  I want a mashed four gain stage preamp fed into a nice big clean power amp.  Mr. Classic rock wants a preamp with just a little bit of grit and a mashed power amp with plenty of power tube distortion.  The absolute last thing I want is power tube distortion, and the absolute last thing he wants is a completely mashed preamp.  IMO, both are tuby goodness, and both are as similar as a Jaguar and a Toyota, but they both haul your ass around to pick up beer.  This is the source of your contradictory information: there is no one single correct answer and "tuby" is an ambiguous terms that's abused in BS marketing copy. 

[zambo]

I think I agree with defaced. I also think that things sound more "tubey" when they are a bit louder. I think "tubey' Means warm bass and slightly crunching mids and complex highs ( if we've got to go to the wine tasting side of things....my amp is grassy with a citrus undertone...). all of these things can be done in solid state as well... ho hum...im babbling. I agree with Defaced.

[object88]

Not a lot to contribute here, but to look at the question of why we don't use EL84s vs 12AX7s everywhere, probably one of the big clues is the functionality of the tubes.  There are two things that can be "gained": "preamp" tubes increase voltage, and "power" tubes increase current.  That, along with all the surrounding components, maybe has something to do with the particular facets of the sound.

[jafo]

Well said and answered, defaced!

[iccaros]

defaced... spreading wisdom..  forget the rest,,, find an amp you like and study its design.. you can be suprised.. my favorit amp is a MArshall Mode 4.. People did not like them, but after 30 min  of working with mine, no pedals except Wah.., I got every thing from Chimey cleans, to 80's rock to full on Metal.. Just how I play and its Solid state for all of the power section with a single 12ax7 probably doing nothing in the pre...  But everytime I play live someone wants to trade or offers me money for it..

I have two amps I love, both 12ax7 and EL84. One is a Vox which has a "chime" when played slightly overdrive, makes me what to play my 62 Start and do some blues or even some "Grunge" like we used to in the 80's with JCM 800's just barely over-driven until you really hit the strings, kind of early Sound garden or Perl Jam, no Love Bone, that is a different tone.. .

Then my Carvin, its can play really clean, but not the same, but put a Valve caster in front and I am in a Tool and Korn jam session, or a Govenor in front and I am can jam Static X, Anthrax ect..

They are so Diffrent, but the setup is almost the same, 100K plates, 1.5K cathode.. Its the minor differences and the power section, one is Class A (Vox) the other is AB1 PP. 

As for difference in tubes.. Voltage and current, is not  a good description, as all tubes control current with a voltage on the grid. That is what a tube does, see the Navy NEETS course..

In HI FI AMPs it is quite common to have a Pentode as a pre-amp, I have a design in front of me that used a EL84 as a pre-amps with EL34 Power amp..

then there is my Firefly, Stock its a good metal amp, no real good clean sound. Add a Ecc99 (still a dual triode) in place of the 12au7 and it get gets crystal clean, almost too clean for me... like a Qeensryche type clean, just add chorus and delay.  but crank it and you get a nice fat metal tone, better than stock, but don't go above half way or it gets buzzy..

My long and winded point, Tubes are defined as preamp, and power amp by tradition and some design.  Right now we use Pentodes and Triodes, but they do not matter, I have a nice amp with Tentrodes, that is diffrent than  a pentode or a beam power tube, but nice sound, it also makes a nice preamp..

[ask4tristan]

Im no expert on the issue but arent the power tubes one of the more defining aspects of an amp? Most amps use 12AX7s or 12AU7s in the pre but Ive seen tons of different power tubes. My Hilgen has 7591s, My Ampeg has 7027s, and my itty bitty Champ has a single 6v6 (but those are 6 really big watts). Then theres EL84s and a whole list of others. As a matter of fact, Egnater has an amp with 6v6 and EL84 tubes and a knob to chose between or blend them. Turn that knob from one end to another and its a whole wide world of difference.

Now, Im not denying the preamp tube any right to affect the sound but I believe the power amp tube holds the majority of the tone and color in its grip.

Now, Im just putting together the pieces and making an assumption. Dont ream me if Im wrong.

[iccaros]

Im no expert on the issue but arent the power tubes one of the more defining aspects of an amp? Most amps use 12AX7s or 12AU7s in the pre but Ive seen tons of different power tubes. My Hilgen has 7591s, My Ampeg has 7027s, and my itty bitty Champ has a single 6v6 (but those are 6 really big watts). Then theres EL84s and a whole list of others. As a matter of fact, Egnater has an amp with 6v6 and EL84 tubes and a knob to chose between or blend them. Turn that knob from one end to another and its a whole wide world of difference.

Now, Im not denying the preamp tube any right to affect the sound but I believe the power amp tube holds the majority of the tone and color in its grip.

Now, Im just putting together the pieces and making an assumption. Dont ream me if Im wrong.

When it comes to music there is no "wrong" ... Only opinion...

[DavenPaget]

Not a lot to contribute here, but to look at the question of why we don't use EL84s vs 12AX7s everywhere, probably one of the big clues is the functionality of the tubes.  There are two things that can be "gained": "preamp" tubes increase voltage, and "power" tubes increase current.  That, along with all the surrounding components, maybe has something to do with the particular facets of the sound.

I thought like transistors , only how much current it can pass matters , the thing is though , it's how it's used , when it comes to transistors . I don't know tubes .

[FiveseveN]

I thought like transistors , only how much current it can pass matters

Yes, it is the same thing. You can build small/tiny poweramps with tubes traditionally used in preamps (i.e. primarily for voltage gain): see the above-mentioned Firefly and even lower-power submini tube amps, headphone amps etc.

[Bill Mountain]

SWR made a bass preamp that was actually a 5 watt tube amp.  They just attenuated the output they way some people put a line out after the OT.  It supposedly made a nice overdrive sound.  Hughes and Kettner did the same thing.  I think they called theirs the BATT series.

What I'm getting at is that this is an easy way to get power tube distortion on a small scale.  Start with your jfets to save space for the pentode and OT.

[petemoore]

   Tube doesn't sound.
  Power supply is needed for tube or transistor to 'work' and sound...
    So here's my take: Until these output tubes are 'premium' and the power supply is excellent, and the OT iron has superb output transfer characteristics...hardly worth messin' with 'tube distortion'.
   Which brings us to the 'meat 'n potatoes' of output tube/speaker interactions.
    When the tube moves the speaker cone the speaker cone 'loads' the output differently, frequency response changes, amount of compression increases greatly [speaker cone can only go in/out so far...no farther], distortions of the linear signal increase/decrease 'per wave cycle'. Actually per 'partwave cycle'...the pressure of the speaker/air/suspension has an effect on current, basically the speaker is also a power source and 'fights' the output. These changes in OT/Speaker conditions change the sound, so when the speaker cone position changes even slightly, the load out the output follows the curves. Very fast at the 'speed of signal'...press harder on the strings [strike a hard chord] and lots of current tries to go through the speaker which impedes the current, some frequencies more than others, impedance rises a lot when the speaker tries to jump from it's Xmax limits for iinstance.
   Everything else is mere 'line signal' processing...nothing wrong with it but the process takes longer than a computer [digital amp emu's] can calculate so only 'portions of distortions' can be mimic'd without the speaker cone mass, suspension, air and other factors to make the split-nano-second changes which occur 'naturally' when a speaker is connected to output tube amp. That iincludes everything tube/transistor and other so far...as RG said...serious head bangin' has been happnin' on 'tube without tube sound' [even tube sound using tube] for plenty of time. The results are in in my case, output tube/speaker thing is 'ok', better when 'premium'd' [tubes irons and speakers all 'super']...then you put your dirtbox on it anyway ! !
  After fiddling around with cheap OT's and PT's, I figured out that to 'crank nice' IME, it takes 'the' speaker, premium tubes and power supply, any one little 'bottleneck' or spikes in the supply make me wanna turn it back down into 'more like transistor amp'...raw tube power can be plain ugly and unruly...hardly worth the trouble since I'd generally prefer a strong clean output amp because I'd be working the signal processing to get all the dirt and eq and comp etc.
  Basically the cranked premium tube amp is a dirt effect. A somewhat cranked 'cheap' tube amp starts off distorting but soon gets to the raw uglies if really cranked to distort much, a little bit of 'nice tube distortion' is all I really use, but 'bottleneck artifacts' is where I start avoidance of further tube loading. Change the irons for 'premium' and the tubes and...then the speaker and then...it's a nice super usable effect.
   PReamp distorters...I had better 'luck' with Jfets or opamp ''pre-distorters'' really, seems a long way around but have tried M, F, Peavey and most of the other preamps to know better, I just go with the clean preamp to push the output tubes to distort any more...just like when...back then, simple platforms with premium guts did the trick, it's a very nice 'vehicle' when it's built up to be so.

[jafo]

Lotta good stuff to read and decipher. If I'm interpreting these responses correctly, the basic difference is that preamp = buzz and power = squish, which is a vast oversimplification (and once again, leaving out issues of transformers and speakers) and due more to conventional usage than anything else.

So, let's see what I'm after... generally, that's a warm, rich, resonant, singing sustain without buzz, or some growl that can become bark or bite by digging in. Cream-era Clapton, middle-period Keith Richards, Doors-era Robbie Krieger, even Hendrix when playing clean. Per defaced, I'm in the classic rock camp. A little grit from the 12AX7 (sometimes less, sometimes a lot more), and a lot of push from the power tube. I have no real preference in power tubes, whether it be EL34, EL84, 6V6, or whatever. I also prefer even-order harmonics, which means a lot of Class A devices.

So, what's a good way to emulate a cranked power tube? More specifically, a pentode or tetrode -- 6V6, EL-34, or EL-84? MOSFETs are supposed to sound/act very much like a pentode, so should I try stringing a few together, perhaps building a Box of Rock or an Obsidian Overdrive? I don't want too much crunch or buzz, so perhaps only two stages instead of three? Or would a 4049 design be more in line with what I'm after? I don't need or even particularly want a perfect emulation -- just something that has that sort of magic.