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TubeValve amps - quality and queries...

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(@simonhome-co-uk)
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Joined: 20 years ago
Posts: 677
Topic starter  

Hey,
I've got a hybrid amp at the moment n am thinking of upgrading to a full valvetube amp. I've been acquiring all these boutique pedals I figured it's about time I got an amp with some serious tone to match. Just a few questions:

People always talk about tube overdrivedistortion, but what about clean? Do tubes have the same impact on clean channels? I ask this because I'm currently using a distortion pedal over my amps clean channel. So will it sound any better doing that through a tube amp?

Does more tubes generally mean better tone? Or perhaps more volume? I've been looking at Laney amps
http://www.laney.co.uk/show_spec.php?prod=lc30-ii
This first one only has two:
Preamp Valves Premium ECC83
Output Valves Premium EL84

Where this one
http://www.laney.co.uk/show_spec.php?prod=l20t-410
has several more:
Preamp Valves 3x 12ax7
Output Valves 4x EL84

i mean...my current hybrid has one tube in the poweramp section, so would I be wrong in saying the one with more tubes will give the greater improvement?

Finally - classes...When they're described as class A, AB, B or C etc does that actually have any bearing on tone? I've read that bellow class A you get cross-over distortion, what does it mean in practice other than that (if anything)?

Any other commentsadvice would be appreciated! Thanks! (and yes, I will of course try out whatever it is before buying. But prior knowledge and advice and can only be a good thing :D ) - for the record i'm looking for something loud enough for small gigs, and is suited (bearing in mind i have pedals for versatility also) to things like Vai, Van Halen, Malmsteen, SRV, Hendrix etc... :twisted: :P


   
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(@rparker)
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I've got a Fender Blues Deluxe. I went to it from a Fender FM212R (solid state). The clean tone difference is incredible. I also tried out the Hot Rod Deluxe. The difference between that and the FM212R was also very different, but also different from the Blues Deluxe. In fact, for me, I'd say the clean tone is what sold me on the Blues Deluxe.

Roy
"I wonder if a composer ever intentionally composed a piece that was physically impossible to play and stuck it away to be found years later after his death, knowing it would forever drive perfectionist musicians crazy." - George Carlin


   
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(@yashicamat)
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Joined: 17 years ago
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I can't comment on the different valve amp classifications or for that matter having amps with a multitude of valves in them. However, what I can say is my little 5W Class A valve amp (an Epiphone Valve Junior) beats the socks off a pretty highly regarded valve hybrid amp (VOX Valvetronix AD50VT) for tone.

Hope this helps. :-)

Rob

If something's not worth doing it's worth forgetting about.
Epiphone Les Paul Std - Yamaha Pacifica 112XJ - Takamine EG340SC - Taylor Baby - Grainger Hammerhead 50 - Grainger Valve Five
http://www.youtube.com/yashicamatonline


   
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(@steve-0)
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I also have the Epiphone Valve Junior, and I gotta say it is simply amazing. I can get a wide variety of tones out of it as well, especially with my collection of pedals.

Now for the technical stuff: more tubes doesn't necessarily mean better tone, more tubes also doesn't necessarily mean more power. Some amps have tremelo circuits in them that are run on tubes, i'm not an expert on tubes but I've been lead to believe that generally speaking certain types of tubes have certain characteristics: some have marshall type tones, others have Fender like tones. That is obviously an oversimplification (non-marshall amps can have tubes that are present in marshall amps), but the main idea is that they have different characteristics that affect tone.

As for amplifier classes, that's even more technical. Basically, amplifiers are commonly classed as Class A, Class B, Class AB, and in a nutshell, these classes tell you what tubes conduct the incoming guitar signal and for how long they conduct that signal. So, for example, class A, means that all the tubes conduct for the entire cycle of the signal, because of this they are not very efficient.

"Crossover distortion" is a technical term referring to when one tube turns off and another turns on during the cycle of a guitar signal, which creates a sort of distorted output wave.

So now that you're probably confused and frightened :lol: you might wonder what all this means. Obviously the number of tubes, type of tubes and amplifier classes have an effect on tone. Unfortunately, I really have not played a variety of different amps with different tubes and different classes, so I can't really tell you.

I would just go out and play a bunch of amps and see what you like. I really like my Epiphone Valve Junior, although it's a little disappointing not to have a built-in effects loop, but it's still a great amp and well worth the money I paid.

Steve-0


   
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(@simonhome-co-uk)
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Joined: 20 years ago
Posts: 677
Topic starter  

As for amplifier classes, that's even more technical. Basically, amplifiers are commonly classed as Class A, Class B, Class AB, and in a nutshell, these classes tell you what tubes conduct the incoming guitar signal and for how long they conduct that signal. So, for example, class A, means that all the tubes conduct for the entire cycle of the signal, because of this they are not very efficient.

Thanks, for the replies. Keep em comming!

But when you say 'less efficient' what does that mean in practice?

Also volume wise how would a 30watt tube amp do for gigs and in comparison with a hybrid? - i own a Vox Valvetronix 50 watts.


   
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(@ricochet)
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Here's a pure copy-and-paste of an explanation I wrote sometime back on amp classes of operation as they apply to guitar amps:

Class A is a mode of operation where current is always flowing in the plate circuit of an amplifier tube. All "preamp" tubes run this way. A Class A tube can operate by itself, called a "single ended" amp. If you see an amp that only has one power tube, you know it's Class A.

A Class B amp (not used in guitar amps) is biased so that current flows in the plate circuit of the tube for half of the cycle of the alternating wave being amplified. It's got nearly zero current flowing at idle, and current flows during the positive half of the signal. That won't work with a single tube, it'd only pass half the signal and it'd be very distorted. So they're always installed in pairs, one tube amplifying the positive half, the other the negative half of the signal.

A Class AB amp is between a Class A and Class B. Current flows in each tube more than half the time, but less than full time. Part of the time it's "cut off." The other tube fills in. The closer it operates to Class B conditions, the more efficiently it operates and the more power it can put out within the tube's maximum power dissipation limits.

It's B.S. when people start talking about "Class A watts" as louder than "Class AB watts." Not true. Some Class A amp salesman must've come up with that one because a competitor's Class AB amp was rated at more power. Class A amps are the least efficient and waste the most power as heat. Also, within Class A operating limits, a Class A amp uses exactly as much power when it's quietly idling as when it's putting out full volume. All of that power is wasted as heat when it's idling. Paradoxically, when it's driven hard the output power is drawn from that wasted power, and the amp actually runs cooler than when it's idling. A Class AB amp runs cooler when it's idling and gets hotter as it's pushed harder. That's hard to understand without drawing a set of graphs of voltage and current (which can be found in texts like "The Radiotron Designer's Handbook"), but it's true. With a given set of tubes, you can get a lot more output power out of Class AB operation than Class A, which is why it became popular. Class A allows simpler and cheaper amp construction for low-powered amps.

Class A single ended amps do go into distortion in a different way than double ended amps, which are usually Class AB.

A single ended amp ideally would be biased exactly in the middle between the grid voltage that "cuts off" the plate current and the voltage (a little less than -1V) where the grid starts to draw some current from the electron stream. The more positive it gets, the more current it draws. Usually the grid is driven by the plate of the previous stage through a coupling capacitor, with a large resistor going from the grid to ground. When the grid draws current on the positive peak of the cycle, it limits the positive swing of the cycle. It won't easily go more positive, because the negative electrons flowing into that side of the capacitor neutralize the positive charge the signal's putting on it. It charges up the capacitor so that it goes more negative than the signal would've driven it when it goes back negative. It's the same as turning up the bias on the grid to a more negative value. The accumulated charge can't leak away quickly, because it has to flow through that resistor. So what happens as the amp is overdriven is that you first get a soft "clipping" on the positive peak of the signal, but a harsher clipping on the negative peak as the tube's driven into cutoff. That's an asymmetrical waveform, so it contains a lot of second harmonic and other even harmonics. Push it into crazy overdrive, and you get more or less sharp clipping on both peaks, which gets to be more symmetrical. Symmetrical waveforms have predominantly odd harmonics. A single ended power amp stage will rarely be pushed to the point of symmetrical clipping, though preamp stages often are.

A double ended Class AB amp will behave differently. As the positive peaks draw current to the grids and charge up the coupling capacitors, the negative bias voltage on the grids rises. There's only very soft clipping on the peaks, but as the bias level starts getting higher, each tube begins to go into "cutoff" before the other one's come out of it fully. Instead of each half of the signal blending seamlessly into the other, there begins to appear first a slight notch or "glitch" at the crossover point, then a flat line segment between the positive and negative signal halves where no current's flowing. Nice and symmetrical. This is called "crossover distortion." It's all odd harmonics, primarily third harmonic. That's the "sweet power tube distortion" that fans of cranked amps with paired power tubes love.

There's some misinformation on the Web to the effect that "tubes sound better than transistors because they produce even harmonics, which sound good, while transistors produce odd harmonics that sound harsh and unmusical." That's totally untrue on every count. I've just shown how tubes can produce both even and odd harmonics, which predominates being dependent on the operating conditions. The same is true for solid state devices. And both even and odd harmonics are musical. The second harmonic is an octave. Kinda sounds like playing a twelve string guitar when it's the predominant harmonic. The third harmonic is a perfect fifth in the next octave up. The fourth harmonic is another octave, two octaves up from the fundamental. The fifth harmonic is a perfect major third, two octaves up. If you listen carefully as you increase the gain distortion on a single note with a single ended preamp, you'll hear it go from clean to a 12-string like sound as the second harmonic comes in, a "power chord" as the third harmonic becomes more prominent with more symmetrical clipping, and finally you can detect the sound of a major chord as the fifth harmonic becomes audible.

"A cheerful heart is good medicine."


   
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(@gnease)
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Linearly biased tubes are predominantly square law devices. As such, a single , class A biased tube produces mostly even order harmonics plus some lower level odd harmonics. When two such tubes are placed in a well designed and executed push-pull (class A-B ususally) configuration, there is a high level of even order harmonic cancellation at the summed outputs (via transformer) of the tubes, leaving -- even at low distortion levels -- those lower level odd harmonics, which now become the dominant distortion. That even harmonic cancellation due to the push-pull topolgy continues to hold true even at higher distortion levels as long as the two push-pull output tubes are reasonably matched in characteristics and bias.

So here's where I'll differ with R:

Class A-B crossover distortion can be a mix of even and odd harmonic distortion -- depends on the balance of the design and biasing. It does not make sense to me that crossover distortion is the "sweet power tube" distortion, as the level is relatively low compared to a cranked output signal. Instead, I believe the soft saturation due to power supply clipping at the peak extremes (sometimes aided by supply sag in some "rectifier" amps) is the true source of sweet power distortion. That causes even a pure fundamental input (sine) to flatten at peak swings, squaring the waveform => high levels of odd harmonics. Crossover distortion may add some interesting coloration to the signal, but not smooth, sweet saturation.

And there is a reason odd harmonics get a bad rap in music reproduction, though not necessarily instrument amplification, as the latter is more a YMMV situation: Play a minor third interval or chord through an amp with predominantly odd harmonics (staturated A-B output). Not so nice in reproduction, as the odd harmonics of this inteval does not fall "in key." A little bit of minor third interval through an odd order harmonic device can produce the loose, crunchy sound some players favor, but a lot generates fairly annoying beat notes. It's one of the reasons for playing power chords (no thirds) through highly saturated class A-B amps.

And FETs (field effect transistors) used in many power amp stages are - like tubes - square law devices. However, many of the strange coupling and bias design imperfections and quirks Rico describes help create the signature tube tone. Due to different biasing schemes for FETs, those now familiar imperfections do not exist in an FET power amp. That sometimes lead to the claim that FET power amps are sterile sounding.

-=tension & release=-


   
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(@simonhome-co-uk)
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Joined: 20 years ago
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Topic starter  

Class A amps are the least efficient and waste the most power as heat. Also, within Class A operating limits, a Class A amp uses exactly as much power when it's quietly idling as when it's putting out full volume. All of that power is wasted as heat when it's idling. Paradoxically, when it's driven hard the output power is drawn from that wasted power, and the amp actually runs cooler than when it's idling. A Class AB amp runs cooler when it's idling and gets hotter as it's pushed harder.

:| :shock:
lol all this tech-speak goes way over my head. As I asked with the other comment: what does this actually result in, in practice when playing the amp? :?
And would I be better off (tonally) with one class over another when playing at low bedroom volumes? Seeing as they generally sound better when you crank em...


   
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(@ricochet)
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Question 1: Class A single ended and Class AB push-pull amps sound distinctly different when cranked. See all the verbiage above, but it'll only make sense when you listen. They sound different. There are fervent fans of each. I like both.

Also, a Class A amp is WAY more expensive to build to a higher power level than a Class AB amp. Takes more tubes, or bigger, more expensive tubes, and requires a much bigger, more expensive output transformer if it's single ended. That's why higher power amps normally are Class AB and Class A amps are usually the 5-10W models.

Question 2: No, it doesn't make a bit of difference when you've got the volume turned way down low.

The tonal qualities of an amp don't come just from the class of operation of the power amp stage in it, or from the tube types. The "preamp" design has a huge influence, and so do the speaker(s) used. And as amp builders and modders know, small tweaks of component values can make very noticeable changes in an amp's sound.

"A cheerful heart is good medicine."


   
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(@dotfret)
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Joined: 16 years ago
Posts: 12
 

The classifications of amps (A,B,C) relate to the way they consume power. A class A amp will consume the same amount of power whether it is idling or amplifying an input signal. A class B amp consumes a certain amount of power when idling, then uses more power when a signal is applied. Class A/B behaves like a class A amp at low volume, then more like a class B amp when the volume is increased. These classes apply to solid-state amps, too - they just get talked about more with valve amps.

The target of amplifier designers in the past was to minimise distortion. Mostly, old class A designs were aimed at low distortion. Class A/B designs were a cost effective compromise, to give low distortion while providing a high output. Most class A valve designs have a single output valve, and are referred to as SIngle Ended (SE). The larger A/B amps have two or four output valves, which work in push-pull (PP) configuration. You do not have to know what this means, I just want to get away from the A,B,C terminology, which is less relevant in this context than SE/PP.

So now we live in an age where people actually want that distorted sound for their guitars, and it gets designed into the circuitry. Charlie (WEM) Watkins used to say it was not distortion, but "coloration" of the sound - and that is a nicer way to say it!

To get the coloration, some amplification stages have to be driven hard, to the point where they are contributing to the sound. Standard valve amp designs can stand a lot more of this than transistor designs, and will add more distortion than transistors. Valves generally add the "odd" harmonics, as mentioned above, rather than the "even" harmonics, which transistors will give - the valve distortion is generally perceived as more pleasant than transistor distortion to the human ear.

Amplifiers generally have two sections - the preamplifier and the power amplifier. The preamplifier incorporates the tone and volume controls, and amplifes the signal to a suitable level for the power amplifier to accept. The power amplifier makes the sound a lot louder, and delivers it to the speakers.

Coloration may be generated by overdriving part of the pre-amp (easy to design in), or overdriving the power amp output valves ( hard to design in). Most commercial designs which give added coloration do it by overloading a triode in the preamp. To get that classic late 60s tone, it is usually necessary to overload the output valves by using an overdrive pedal on the input (which is quite stressful on the amp).

The output valves are usually beam tetrodes or pentodes, and the coloration they provide is of a different character than that provided by the pre-amp triodes.

Most guitar amps designed to give coloration do it via the pre-amp valves, but a lot of the small SE designs have their output valve contributing to the sound as well. With the larger PP amps, the output valves contribute less, proportionately, to the sound, but they can be induced to add more by devious (and not usually recommended) means. Suffice it to say that those amps that Jimi Hendrix used were not exactly standard models, and they still spontaneously combusted during performance on a regular basis. Me myself I feel nervous using an overdrive sometimes ...

However, careful use of Field Effect Transistors (FETs) can produce designs with "odd harmonic" coloration, which emulate a valve sound - ordinary transistors cannot do this. So why are valve amps still so popular?

The sound is one thing - the valve sound is preferable to most ears - but the real advantage of a valve amp over a solid state sound is the IMMEDIACY of the sound, and most people do not realise this. A large transistor amp has many more stages than a valve amp, and each one delays the signal by a few milliseconds. Because a valve amp has less stages to go through, the sound comes out of the speaker less than 50ms after you hit the string, with a big transistor amp the delay can be 150ms or more. You can get a similar sound from SS components, but that delay is off-putting. Organists become used to even bigger delays (350ms between pressing a key and the sound coming out is not unusual), so it is no surprise that guitar players get used to the delay without even realising it exists - but a valve amp "feels live" in comparison to a SS amp, and that is why most people prefer them.


   
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(@gnease)
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dotfret-

you have your odd and even harmonic situations reversed. valves and FETs are considered square law devices, and used in nearly linear or slightly non-linear class A mode produce predominantly even order harmonics due to small signal transfer (gain) characteristics -- which look somewhat parabolic. bipolar transistors (often simply 'transistors' have exponential (invlog) transfer characteristics and produce both even and odd harmonics, though at much lower levels than a valve if biased and used in class A feedback amps as is typical. However, a lot of this goes aways depending upon the by-stage topology of multiple transistors or valves, which often result in cancellation of even harmonics (e.g., push-pull stages, usually A-B suppress even harmonics). OTOH, symmetrical rail (power supply) clipping produces predominantly odd harmonics.

as to the delay of solid state amps being the big problem: could be (though the post attack delay of a LP alone has quite an inherent bloom!), but there's another problem when the delay through the main path is combined with multistage feedback. as mentioned above, transistor amps usually employ feedback to linearize their invlog transfer (gain) characteristic. if done on a stage-by-stage basis, the effect is not so bad. but once upon a time, input to output feedback was common in solid state audio amp designs to rid the amp of a lot of non-linear nastiness. unfortunately, the main path delay with the input to output feedback created a situation that became known as transient intermodulation distortion (or TIM distortion -- google it). that produced very bad sounding transient (attack) characteristics. as a result of the "discovery" of TIM distortion decades ago, the application of feedback in most audio amplifier design changed -- especially solid state models ... that is, except for the cheap ones :wink: .

-=tension & release=-


   
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(@simonhome-co-uk)
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Joined: 20 years ago
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Topic starter  

Well I've done it! I'd had my eye on the Laney LC-30 II, tried it out in a shop along with the VC-30 and was sufficiently blown away by the LC that I got one off ebay for £230! :P :P :P (sorry Mr.ShopMan)
Its a real interesting amp. The drive channel is very light when plugged into the LO-jack (even through my EMGs) such that I found a reeeally nice clean sound through it. And my Keeley DS-1 gets an incredible heavy ODdistortion tone when using this drive channel as the clean instead of the intended clean channel. Kinda like a dirtier early Malmsteen.
Thanks for the replies guys, Ill let you know when it's arrived. :mrgreen:


   
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(@gnease)
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Enjoy!

-=tension & release=-


   
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(@prndl)
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The only real way to chose an amp is to play it, and see if the tone is what you want, although a great tube amp will initially make you sound terrible until you adjust to it. Pretty soon you'll be able to coax honey tones from your fingers.

Don't forget to bring a few pedals, because tube amps may or may not like them. For instance, one of my amps hates distortion, but loves the TS-808 clone, which pushes it into metal glory. What's odd, is the opposite is true for the other tube amps.

As you can see from the prior posts, there's a lot of nonsense and hype in tube guitar amps. It's a lot easier if you stick with tried and true amps. The choices are based on price, wattage, and desired tone.

I'm a big fan of older amps. Fender has many choices for the musician: Champ, Princeton Reverb, Deluxe Reverb, 5e3 Deluxe - all fantastic amps. For a first amp, there's a lot of other great choices: Valve Jr, Blues Deluxe, etc.

I'd begin with a search for local music shops and tube amp repair techs.

1 watt of pure tube tone - the Living Room Amp!
http://www.naturdoctor.com/Chapters/Amps/LivingRoomAmp.html
Paper-in-oil caps rule!


   
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(@jamminejohn)
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I prefer Tweed combos.
The less EQ and amount of tubes in the preamp works for me.
I prefer cathode bias,class AB amps. I have fixed bias also with 6L6's.
I like low gain/high current preamp tubes like 12AY7 & 5751.
I like 6V6's. 5R4's 5V4's
IF I use a pedal it is an overdrive,analog delay,phaser. All analog.
I use all american tubes that were pulled out of old equipment.
I prefer Tele type guitars.

JJ


   
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