Guitar Amps: Tube vs. Digital

[NateHevens]

Okay... so I've seen a ton of threads on "audiophile" technology, so I thought it'd be interesting to get a discussion on this here.

I played with a guitar for 9 years, but only started taking it seriously about 3 years ago, all with the outcome of sounding like I've only played guitar on and off for 3 or 4 months :( (I really have to learn to just play those scales no matter how much I hate them).

Anyways, I have a couple amps, both are tube. I've played through digital before, and although I wouldn't dare claim to actually hear any difference, I thought I could.

My preference for tube amps is mainly on the fact that I'm comfortable with them, just about all the guitarists I know swear by them, and I'm told time and time again that if you're gonna play the Blues, you gotta play it through the tubes. And I do love the sound they produce and the power behind them (in my experience, a 15-watt tube can out-blast a 30-watt digital, but then I will admit that the tube amp was in better condition and a couple years younger then the digital amp used, and it wasn't an actual test... I was playing live with a friend's band and I played through the tube amp and the friend played through the digital amp).

My question is this... do y'all think there's any marked difference? Is there a legitimacy to the claim that many guitarists make that tube amps are better then digital amps? I think I'll always prefer tubes... I'm just hoping that the preference isn't strictly because of comfort level and actually has a legit reason behind it... :D

And yes, I'm stupid... but I'm learning... :o

[RussDill]

Tube and various digital amps definitely have different properties and you'll get different sound, especially when at the edge of the envelope. More accurate doesn't always sound better, however, that being said, it is likely that digital amps exist that can reproduce the effect of a tube amp.

[kuroyume0161]

I've been playing for 30 years. Tube amps are definitely the preferred way to go for good sound qualities and tonal shaping (Marshall, Bugera, Bogner, Fender, VOX, et al). The caveat is that there is maintenance involved. First, you have to give them time to warm up before playing (only about a minute or so thankfully). Second, somewhat like lightbulbs, they burn out over time. Instead of just not working like lightbulbs, they slowly degrade (though the degradation time can vary) - the 'power' and/or the crispness dissipates. Then you have to replace them and have them properly biased which is rather expensive (even today with their revival).

Well, I prefer tube amps for the warmer, dirtier tones. Purely digital amps tend to be too clean and crisp for playing with any feeling (to me anyway). Currently, I have a Line 6 Spider Valve 212 (with 2x12" Celestion speakers, yeah baby!) which has both preamp and power tubes (Bogner designed) but has digital effects and amp simulation. It is sort like the best of both worlds. Not perfect but very satisfying to play. I get bluesy blues, raking treble, soft acoustics, and ground-thumping metal out of it. What I do is let the tubes speak through the amp and use a Line 6 POD XT Live do the effects and simulation. It is not a bad combination in my book.

All that said, it is all up to you. What you play, what you hear, what you like are going to determine what kind of amp (etc.) fits you as a player. There was a time when I was more into 80s technical guitar where a purely digital Peavey amp was the correct choice. Since then I have changed my tastes and required a fuller range from my equipment for various styles. Even my setup doesn't favor every possibility but it satisfies my needs. Also realize that each amp and guitar has its own idiosyncrasies. You'll spend years finding the combination that works for you. I've only reached that point within the past five or so.

My best advice is take your axe (or use the same one in the store) and plug it into every amp you can see. Try clean, dirty, distorted, overdriven. Change the guitar pickup selector switch. Play them low and loud. And remember that every 'room' is acoustically different. What you hear in a voluminous guitar store will assuredly sound different in your basement or bedroom or on-stage.

[NateHevens]

I've been playing for 30 years. Tube amps are definitely the preferred way to go for good sound qualities and tonal shaping (Marshall, Bugera, Bogner, Fender, VOX, et al). The caveat is that there is maintenance involved. First, you have to give them time to warm up before playing (only about a minute or so thankfully). Second, somewhat like lightbulbs, they burn out over time. Instead of just not working like lightbulbs, they slowly degrade (though the degradation time can vary) - the 'power' and/or the crispness dissipates. Then you have to replace them and have them properly biased which is rather expensive (even today with their revival).

Oh yeah. Maintenance is a witch with a capital "B" for tubes, but, IMO, absolutely worth it.

Well, I prefer tube amps for the warmer, dirtier tones. Purely digital amps tend to be too clean and crisp for playing with any feeling (to me anyway). Currently, I have a Line 6 Spider Valve 212 (with 2x12" Celestion speakers, yeah baby!) which has both preamp and power tubes (Bogner designed) but has digital effects and amp simulation. It is sort like the best of both worlds. Not perfect but very satisfying to play. I get bluesy blues, raking treble, soft acoustics, and ground-thumping metal out of it. What I do is let the tubes speak through the amp and use a Line 6 POD XT Live do the effects and simulation. It is not a bad combination in my book.

That's pretty nice. I have a Fender Blues tube and then a no-name basic beginner one that I only use for practice at home.

I use a DigiTech 3000 pedalboard with the tube currently and it does a good job.

I'm not a clean player by any means. I prefer the sloppy guitar of players like Page and Hendrix to the clean, technical stuff of Malmsteen and Halen. Actually, to be honest, I hate 80's shredding with a passion. All technique and no feeling. I'm utterly against that.

All that said, it is all up to you. What you play, what you hear, what you like are going to determine what kind of amp (etc.) fits you as a player. There was a time when I was more into 80s technical guitar where a purely digital Peavey amp was the correct choice. Since then I have changed my tastes and required a fuller range from my equipment for various styles. Even my setup doesn't favor every possibility but it satisfies my needs. Also realize that each amp and guitar has its own idiosyncrasies. You'll spend years finding the combination that works for you. I've only reached that point within the past five or so.

My best advice is take your axe (or use the same one in the store) and plug it into every amp you can see. Try clean, dirty, distorted, overdriven. Change the guitar pickup selector switch. Play them low and loud. And remember that every 'room' is acoustically different. What you hear in a voluminous guitar store will assuredly sound different in your basement or bedroom or on-stage.

I've been doing this for a long time, but yet to figure out my "perfect set-up". I'm a Blues player first and foremost, with my major inspiration being Led Zeppelin (my all-time favorite guitarist is Jimmy Page, followed closely by Jimi Hendrix). However, I have a serious love for effects. Because of that I've gotten heavily into Psychedelic, Progressive, and Space rock. I absolutely love Pink Floyd and Syd Barrett and the Doors and 13th Floor Elevators and so on. And although I do not like U2, I definitely like The Edge with his use of effects.

So what I need is a versatile rig that I can use to play dirty blues and acoustic blues, then turn around and pull out a psychedelic romp featuring tons of guitar effects and out-and-out experimentation. Sadly I'm broke, but one day I hope to have enough money to basically build my perfect rig, with a mountain of guitar effects (pedals and pedalboards and so on) with strong tube cabinets and so on...

[MRC_Hans]

For guitar amps, the discussion is not the same as for Hi-Fi. In a Hi-Fi setting, the amplifier is not supposed to add to or subtract from the sound, it is supposed to just amplify it as faithfully as possible. A Hi-Fi amplifier is run well within its envelope, so it is nowhere near saturation.

A guitar amp, OTOH, is part of the sound system, it is run to the edge of, or over the edge of its working envelope, so of course it adds to the sound picture. Since different technologies have different properties when driven to and over the edge, they will create different sounds.

So if your favorite sound is the one created by a tube amplifier, that is what you will want to use.

I assume it is possible with modern technology to emulate tube sound, which might be preferable in the long run, due to maintenance and reliability issues.

However, if your characteristic sound is the one from a good old Fender 30W amp, with two 6CA7 tubes and a built-in speaker, driven to full distortion, then I expect there is no substitute for the real thing. :cool:

Hans :)

[MRC_Hans]

Then you have to replace them and have them properly biased which is rather expensive (even today with their revival).

Here might be a bit of superstition you can do without. Tubes amplifiers are auto-biased, so there should be no need to 'have them biased' whatever that means. Unlike semiconductors, tubes can be produced with quite uniform characteristics, so you should be able to always substitute them 'off the shelf'. Still expensive, though.

Another maintenance thing you might look at, especially if your amp is vintage, is electrolytic capacitors. These things also have a limited life-span, and the way a guitar amp is driven, this may impact the sound picture, so you might want to have them replaced at some point.

Luckily, due to the proliferation of switch-mode power supplies, high-voltage electrolytics of good quality are easily available and relatively inexpensive.

Hans

[bluesjnr]

When you've had to lift and carry them in and out of venues a few times you'll soon notice a difference.

[a_unique_person]

I'm just wondering if you are confusing a transistor analog amp with a digital amp? There are real 'digital' amps out now, but most transistor amps are still analog. They give a different, more accurate sound, but a lot of people still like the distortion that a tube amp creates. That's ok, you want to play the sound you like to hear.

FWIW, I play bass, and tried lugging around a large amp, till I realised it was only a matter of time till I wrecked my back. My current amp is a Little Mark II, which is very light, and sounds great to my ears. The amplifier section is analog transistor, but the power supply is digital. This is very useful, as it means that an amp this small and light is rated at 500 watts. A small guitar amp can be very loud, but bass needs extra power to get the same effect.

You can also get ultra light amplifiers these days that have a digital amplifier section as well, which actually switches the sound signal on and off at supersonic frequencies. This makes them super efficient as well.

[MRC_Hans]

I'm just wondering if you are confusing a transistor analog amp with a digital amp? There are real 'digital' amps out now, but most transistor amps are still analog. They give a different, more accurate sound, but a lot of people still like the distortion that a tube amp creates. That's ok, you want to play the sound you like to hear.

The issue with analog transistor amplifiers is that the design technology is somewhat different from that of a tube amplifier. This is because tubes are intrinsically far more linear than transistors (not counting FETs). Therefore, tube amplifiers are traditionally built with linear stages, and a very moderate negative feed-back. In fact you can build a usable tube amp with no feed-back at all.

In contrast, transistors are basically non-linear, and keeping each stage linear is a bitch. Instead, as stages are cheaper, they are designed with plenty of surplus gain, and then you have a large negative feed-back, which not only gives you the gain you actually want, but keeps the whole thing linear. The end-result, if done properly, is a device that is far more linear than ye olde tube design, - as long as none of the stages are driven into saturation! Once THAT happens, however, all bets are off.

As for digital amps, well, actually, they are not digital at all (since it makes no sense to amplify a digital signal). What they really are is a switch-mode output stage. This gives you good efficiency, but otherwise, the rest of the amplifier chain is analog and thus subjects to the same issues as any other analog amplifier.

but the power supply is digital. This is very useful, as it means that an amp this small and light is rated at 500 watts.

So true, and don't let anybody tell you that a switch-mode power supply (again, it is not really digital) can change the sound in any way.

What a switch mode PS does is that it lets you do without the bulky mains transformers, and at the same time gives you far better efficiency, so you don't need cooling. It also saves energy, but that is perhaps a side issue in this context.

A small guitar amp can be very loud, but bass needs extra power to get the same effect.

The reason for this has really to do with speaker efficiency. Unless you were to build your speakers ridiculously large, the long wavelength of bass tones gives poor coupling efficiency to air-waves.

You can also get ultra light amplifiers these days that have a digital amplifier section as well, which actually switches the sound signal on and off at supersonic frequencies. This makes them super efficient as well.

Right! Although the term 'digital' is really a misnomer. 'Switch-mode' would be the proper term, but I assume somebody decided 'digital' sells better.

Hans

[Ove]

I'm just wondering if you are confusing a transistor analog amp with a digital amp? There are real 'digital' amps out now, but most transistor amps are still analog. They give a different, more accurate sound, but a lot of people still like the distortion that a tube amp creates. That's ok, you want to play the sound you like to hear.

FWIW, I play bass, and tried lugging around a large amp, till I realised it was only a matter of time till I wrecked my back. My current amp is a Little Mark II, which is very light, and sounds great to my ears. The amplifier section is analog transistor, but the power supply is digital. This is very useful, as it means that an amp this small and light is rated at 500 watts. A small guitar amp can be very loud, but bass needs extra power to get the same effect.

You can also get ultra light amplifiers these days that have a digital amplifier section as well, which actually switches the sound signal on and off at supersonic frequencies. This makes them super efficient as well.

I play Bass myself and i have allways preferred transistor amps but that is because i DON'T want a distorted sound. Playing Bass i want a clean sound. Guitar otoh. should be played on a tube amp, preferrably a Marshall but hey, i'm old enough to have headbanged to "Deep Purple in Rock" when it came out. :D

My bass amp is a Roland 100w Cube

[Aitch]

I play Bass myself and i have allways preferred transistor amps but that is because i DON'T want a distorted sound. Playing Bass i want a clean sound.
Ditto!
Guitar otoh. should be played on a tube amp, preferrably a Marshall
Depends. If you only play in one style, yeah; but if you need different tones, a clean amp and an array of pedals is probably a better bet.
but hey, i'm old enough to have headbanged to "Deep Purple in Rock" when it came out. :D
Age has its compensations! :)
My bass amp is a Roland 100w Cube
I have a Laney 50W combo - comes with a free hernia! :boggled:

[Coolhat]

My current amp is a Little Mark II, which is very light, and sounds great to my ears. The amplifier section is analog transistor, but the power supply is digital. This is very useful, as it means that an amp this small and light is rated at 500 watts.
I use the very same amp, it literally fits in the side pocket of my gigbag!

But back to topic: I got the impression that the "digital" in the OP was referring to digital modeling of traditional tube amps. This technology has made great leaps in recent years and I would very interested to see how many guitarists would actually be able to tell a digital model apart from a real tube amp in a proper blind test.

In my experience players shun digital modeling gear mostly because of the user interface. They find all the buttons, LCD displays and myriad options too confusing, and hence are not able to tweak the sound to their liking. This is a shame, because modern modeling amps can sound fantastic. The preset sounds are invariably awful, but if you have the patience to start from scratch, you can dial in almost any sound you like.

[MG1962]

Well here is the solution I came up with when I played. A 40w Peavey transistor amp slaved through a 300w Vox tube rig

That let me have the warm fat sound of the tubes, and when I wanted it, the precise clean living sound of transistor amps for other times

Obviously you can buy pedals to reproduce the distortion from valve amps, but to my ear it never sounded just right

[Aitch]

Well here is the solution I came up with when I played. A 40w Peavey transistor amp slaved through a 300w Vox tube rig

That let me have the warm fat sound of the tubes, and when I wanted it, the precise clean living sound of transistor amps for other times

Obviously you can buy pedals to reproduce the distortion from valve amps, but to my ear it never sounded just right

As a matter of interest, have you ever tried something like this (http://www.theguitarfiles.com/product_id-2701.html)?

[Foster Zygote]

My question is this... do y'all think there's any marked difference? Is there a legitimacy to the claim that many guitarists make that tube amps are better then digital amps? I think I'll always prefer tubes... I'm just hoping that the preference isn't strictly because of comfort level and actually has a legit reason behind it... :D

And yes, I'm stupid... but I'm learning... :o

Emphatically yes. But this isn't some woo-woo audiophile truism that I spout because I read it in some magazine and I think saying makes me sound knowledgable. The main factor right now, in my experience, is that the tonal modelling still lacks the sophistication to accuratemly simulate the nuances of tube circuitry. Yet this situation will, I think, change pretty soon. I've already heard some modelling amps that come pretty darn close to the sounds of the amps they mean to mimic. Some programs are better than others even on the better units, but some can sound quite convincing, and in the thick mix of a big ensamble the difference might be unrecognizable. But in a good room, with the amp cranked up to its sweet spot and playing through your favorite guitar plugged straight into the amp, a good analog amp is hard to beat. But already I'd take some of the better modelling amps over some of the less inspiring tube amps.

[Vic Vega]

I practice through a Roland Micro Cube, which is a solid state amp with COSM effects that really works well for practice. When I play live or rehearse with my band, I use a tube amp (a Fender Blues Deluxe).

There is no question that tube amps provide a warmth and touch sensitivity that modeling amps have not gotten quite right, especially for overdriven tones that are not outrageously distorted.

Supposedly some of the new modelers like the AxeFX have gotten pretty close, but I've never tried one.

[Foster Zygote]

I play through a 30 year old Roland JC-50 loaded with a Weber Blue Dog AlNiCo. My only effect is a Fulltone OCD. I also use a latch foot-switch to engage the amp's on-board distortion set at minimum when I want a good bit of boost and just a little bit more dirt. I've had a number of guitarists say that they thought I must be playing through a nice tube amp when they first heard me... until they saw my rig. Of course, this is still analog circuitry and no digital modeling is involved.

[Foster Zygote]

This is very useful, as it means that an amp this small and light is rated at 500 watts.

Yeah, a 500 Watt tube head would look like the pint glass rack at your favorite pub.

[thrombus29]

Here might be a bit of superstition you can do without. Tubes amplifiers are auto-biased, so there should be no need to 'have them biased' whatever that means. Unlike semiconductors, tubes can be produced with quite uniform characteristics, so you should be able to always substitute them 'off the shelf'. Still expensive, though.

Another maintenance thing you might look at, especially if your amp is vintage, is electrolytic capacitors. These things also have a limited life-span, and the way a guitar amp is driven, this may impact the sound picture, so you might want to have them replaced at some point.

Luckily, due to the proliferation of switch-mode power supplies, high-voltage electrolytics of good quality are easily available and relatively inexpensive.

Hans

Not all Tube amps auto Bias, My early 90's Randall needs to be manually Biased. Is there much difference in the sound? Who knows, and this isn't real evidence but, When I replaced the Original Randall tubes with G&l's I didn't bias it and the tubes (4 6l6) ran super hot and died in 6 months. When I got some Sovtek's to replace them, I had them biased and the tubes lasted about 3 years under the same conditions.

I don't play guitar out (Bassist now) any more but the solid state/tube question matters mostly for the style, the blues dudes I play with mostly play Fender Blues Jr's or Princetons, and boost with old Ibanez TS9's for lead crunch. When I did the Metal thing I was just running the Randall in the hot channel with no variation, I could have done that with a good solid state amp if they had the sound I was looking for in 92.

As for Bass, I have been playing for 4 years now, and have never played through a tube rig that was any better than a solid state rig. I use Ampeg b2r going through a ACME Lo-b. Whatever circut that they use in their solid state stuff is just as good as the tube stuff, with better attack, and convenience.

[ktesibios]

So true, and don't let anybody tell you that a switch-mode power supply (again, it is not really digital) can change the sound in any way.

What a switch mode PS does is that it lets you do without the bulky mains transformers, and at the same time gives you far better efficiency, so you don't need cooling. It also saves energy, but that is perhaps a side issue in this context.

Hans

Actually, power supply characteristics do have an effect on the sound of a guitar amp. Because virtually all output stages (for any amp in the 50W or higher class) operate class AB1, the current drawn from the power supply varies with output level. In a conventional transformer-rectifier-filter supply, this causes the supply output voltage to "sag" as the amp is driven harder. The timing of this sag depends (mostly) on the source impedance of the power transformer, the rectifier's V-I curve and the value of the main filter capacitors.

So, when the guitarist plays a note or a chord, the power supply voltages in the amp start to change in response to what he is playing. This in turn alters the characteristics of the output stage as the plate and screen supply voltages change. The plate supplies in the preamp section will also sag, but more slowly, as these are usually derived from the main HV rail via a string of dropping resistors and decoupling caps, which charge and discharge at rates determined by the component values in the B+ string.

The result is that the amp's response to the onset of a note changes as a function of time. The attack interval of a note (not just the amplitude envelope but also how the relative levels of the overtones change as the note develops) is very important to our perception of tone (for a discussion of this see Fritz Winckel's Music, Sound and Sensation (http://www.amazon.com/Music-Sound-Sensation-Modern-Exposition/dp/0486217647).

So I would not blithely assert that substituting a switchmode PSU (which is likely to be very "stiff") for a conventional supply in a guitar amp would not affect its subjective characteristics. Of course, a power supply engineer could probably deliberately design in a suitable load regulation characteristic...

[MG1962]

As a matter of interest, have you ever tried something like this (http://www.theguitarfiles.com/product_id-2701.html)?

God no - Its been 20 years since I played with any seriousness. Then the old pedals used to have two knobs - volume, and overdrive. You figured everything else out for yourself after that

Reading this thread I was suprised to learn valve amps even still existed. Thats how old school I am

[NateHevens]

I want to apologize. By "digital" I meant solid-state or otherwise "not tube". I've gotten so used to just saying tube and digital that I forget that there's all different kinds of "digital", and most of them aren't actually fully digital.

I've played through Line 6 before and the Spider Valve with effects is something I'm seriously interested.

I always thought the tube amp had a warmer, crunchier tone. I've already said I prefer the dirty sound to the clean sound, and I don't think the modeling amps have quite made it as far as mimicking that nice Blues sound of a good tube... although with each new technological upgrade for the modeling they do come closer.

Modeling amps are actually nice for different things. I've played through them in the stores before. One I do like is the Line 6 Spider Jam because of all the backing track presets it has... I jammed over the Psychedelic one for over an hour once, and a few of the Blues jams for even longer.

My Fender Blues has been a saint to me, although, sadly, as it was used when I got it, it's coming close to replacement time (it's previous owner practically abused it... lovingly, but still... and I used it a lot... I did replace the tubes recently, but it's showing it's age in other areas now, as well... it's not young), so I've been shopping around for new amps trying to figure out exactly what I want. I don't want it to be too big, but it also has to be powerful. I have yet to find a tube with an option to plug in a CD Player, which I see in a lot of solid-states, and that's kinda sad because it'd be cool to have that option, although it's not a necessity.

The simplicity of the tube amps is also a major draw, I must admit. Although I love having the versatility many solid-states provide, I can also get that with my pedalboard. With the amp, I'd rather just have to turn up the volume and maybe distortion when I need it and then just leave it alone.

As far as pedalboards, as I said before I currently have a GNX 3000, but Vox has this awesome tube pedal-board and I'm seriously considering investing in that. I think a tube amp through a tube pedalboard will be better then a tube amp through a solid-state pedalboard, but I haven't done any experimentation to find out, yet...

Anyways, the discussion has been amazing so far, and I'm learning a lot of new things I didn't know, which will definitely help in picking out a new amp. So thanks, everyone.

[fsol]

In my experience players shun digital modeling gear mostly because of the user interface. They find all the buttons, LCD displays and myriad options too confusing, and hence are not able to tweak the sound to their liking. This is a shame, because modern modeling amps can sound fantastic. The preset sounds are invariably awful, but if you have the patience to start from scratch, you can dial in almost any sound you like.

This is one of the reasons I stay well away from modelling amps. They look way too much effort. I have a JCM900 50W head and it has tone controls, volume and gains. The amp is set to the sweet spot for overdrive and I can clean up the sound with the volume on my guitar if I need to. OK, so I usually have 8 or 9 pedals between guitar and amp... but those are all simple "single effect" boxes...so my excuse still works. Umm...I think.

[Coolhat]

I practice through a Roland Micro Cube, which is a solid state amp with COSM effects that really works well for practice. When I play live or rehearse with my band, I use a tube amp (a Fender Blues Deluxe).

There is no question that tube amps provide a warmth and touch sensitivity that modeling amps have not gotten quite right, especially for overdriven tones that are not outrageously distorted.
Micro Cube vs. Blues Deluxe is not really fair comparison though, since the former has only a fraction of speaker area, cabinet size and output power of the latter. So it is bound to be less dynamic and sensitive.

[Vic Vega]

Micro Cube vs. Blues Deluxe is not really fair comparison though, since the former has only a fraction of speaker area, cabinet size and output power of the latter. So it is bound to be less dynamic and sensitive.


I wasn't comparing them.

[Aquila]

My question is this... do y'all think there's any marked difference? Is there a legitimacy to the claim that many guitarists make that tube amps are better then digital amps? I think I'll always prefer tubes... I'm just hoping that the preference isn't strictly because of comfort level and actually has a legit reason behind it... :D


For me it depends what I'm using the amp for. For live or just jamming with my electric or acoustic-with-pick-up guitar in my room I use a small Squire amp but for recording, I've got about 300 presets on my Roland studio which can give me a host of different sounds at the touch of a scroll down menu.

So convenience is one factor. Another thing to consider, is by the time a recording of a tube-amp gets on to a CD or MP3 it's digital anyway. Do you remember when CDs first came out and some vinyl die-hards refused to buy them because of the perceived loss of warmth?

I know this is slightly off topic, but as far as "warmth" is concerned, I still won't use the new energy saving light bulbs - the quality of light they emit is disgusting to me - it seems "cold" and gives me a headache. . Give me the old bulbs or candlelight.

[MRC_Hans]

Not all Tube amps auto Bias, My early 90's Randall needs to be manually Biased. Is there much difference in the sound? Who knows, and this isn't real evidence but, When I replaced the Original Randall tubes with G&l's I didn't bias it and the tubes (4 6l6) ran super hot and died in 6 months. When I got some Sovtek's to replace them, I had them biased and the tubes lasted about 3 years under the same conditions.

I won't exclude the possibility that some idiot designed a tube amplifier that needs manual biasing, but I would simply avoid the brand. There is no sensible reason for it, except to make money off users.

Hans

[MRC_Hans]

Actually, power supply characteristics do have an effect on the sound of a guitar amp. Because virtually all output stages (for any amp in the 50W or higher class) operate class AB1, the current drawn from the power supply varies with output level. In a conventional transformer-rectifier-filter supply, this causes the supply output voltage to "sag" as the amp is driven harder. The timing of this sag depends (mostly) on the source impedance of the power transformer, the rectifier's V-I curve and the value of the main filter capacitors.

No, it depends on the filter capacitors.


So, when the guitarist plays a note or a chord, the power supply voltages in the amp start to change in response to what he is playing. This in turn alters the characteristics of the output stage as the plate and screen supply voltages change.


Admitted, but the effect should be very marginal.

The plate supplies in the preamp section will also sag, but more slowly, as these are usually derived from the main HV rail via a string of dropping resistors and decoupling caps, which charge and discharge at rates determined by the component values in the B+ string.

No. Unless the preamp is driven into saturation (very unlikely), variations in the B+ will not affect the sound.

The result is that the amp's response to the onset of a note changes as a function of time.

Very unlikely. To function at all, a power supply filter has a recovery time of less than about 200milliseconds. You are unlikely to hear such variations.

The attack interval of a note (not just the amplitude envelope but also how the relative levels of the overtones change as the note develops) is very important to our perception of tone (for a discussion of this see Fritz Winckel's Music, Sound and Sensation (http://www.amazon.com/Music-Sound-Sensation-Modern-Exposition/dp/0486217647).

That may be so, but remember that only when an amp is driven into saturation does the power supply voltage affect the sound characteristics.


So I would not blithely assert that substituting a switchmode PSU (which is likely to be very "stiff") for a conventional supply in a guitar amp would not affect its subjective characteristics.


Don't get me wrong: If you have the sound of a vintage tube amp, overdriven, only a deliberate emulation can exactly mimick it. However, a sound construction, run in a reasonable envelope will not differ audibly due to PS technology.

Mind you, I have a distinctly non religious approach to this. I belive that if you can't measure it, it doesn't exist.

Hans

[ktesibios]

I won't exclude the possibility that some idiot designed a tube amplifier that needs manual biasing, but I would simply avoid the brand. There is no sensible reason for it, except to make money off users.

Hans

Umm, I have to disagree about this. The only "self-biasing" output circuits in common use are "cathode-biased", in which the bias voltage is derived from the voltage drop across the output tubes' cathode resistors. This apparently has some disadvantages for higher power smplifiers. The Vox AC30 is the most powerful MI amp I know of which uses cathode biasing; the power amp used in the Leslie 122 cabinet, which delivered 40W from a pair of 6550s also used cathode biasing.

It's not just "some idiot"- most if not all tube guitar amps of 50W or greater output rating use fixed bias- a negative voltage generated by a separate part of the power supply and applied to the output stage control grids. In a few cases this is non-adjustable (or in the case of one Fender amp I've encountered, the only adjustment is a "DC balance" adjustment which is set to produce equal quiescent plate currents in both sides of the output stage). In most cases it's adjustable and the tech sets it to produce the correct quiescent current in the output stage. A few amps provide convenient test points where the cathode current can be read with a voltmeter, measuring the voltage drop across a set of 1 ohm cathode resistors.

Now I think it would be almost trivially easy to implement a truly automatic bias system- professional gear like the SSL 9000 series consoles have long been using "servo biasing", in which the DC voltage at an amplifier stage output is sampled, integrated and used to apply a bias voltage at the stage's input so as to force the DC output offset to 0V (if you want to see an example, Google "Jensen twin servo preamp"). A similar circuit which senses the voltage drop across two low-value cathode resistors and adjusts the grid bias so as to force the sense voltage to equal that produced by the desired quiescent cathode current shouldn't be too hard to cook up.

Unfortunately that would mean having transistors and, most likely, monolithic opamps in your amplifier, which might make the guitar players recoil in horror even though the bias servo would not actually be in the signal path.