What is a tube amp and what do I need to know?

Discussion in 'Gear & Equipment' started by 7 Dying Trees, Nov 24, 2007.

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  1. 7 Dying Trees

    7 Dying Trees Forum MVP

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    There's been a few questions on tube amps lately, and seeing as this quite frankly archaic technology still warms many a musicians heart i thought I'd see if I could knock up a quick guide!

    Why tube amplifiers?
    well, we don't really know, there's just something magic about them glowing glass tubes. Also something to do with power valves sounding great when cranked as they deliver natural compression, and do nice things to even harmonics. Some people would describe the sound as sweet, singing, you name it. What we do know is that some people prefer solid state, some people prefer valves, this isn't the place to debate it, but overall valves rule the guitar amp world with all boutique amps being valve based, and all the desirable amps being the same (mesa, rivera, soldano, vht, engl, etc etc).

    Suffice to say that once guitarists discovered that distorting these tubes produced a musical sound, well, life was never the same again, and solid state amps and modelling amps have been trying to and succeeding to varying degrees, but never nailing it perfectly, to emulate the sound of an overdriven/distorted valve for years. In most electronics the transistor caused the demise of the vacuum tube (even TV's, probably the other last vestige of the tube (cathode ray tube, or electron gun) are now transistor based), but in the music world wherte it is not about efficiency, but rather human hearing, the tube survives.

    How do I use the Standby Switch? How Do I turn my amp on and off?

    excellent description here:
    And for you all day players:
    Why does my amps impedance need to match the cabinet?
    well, basically, that output transformer i was talking about, well, it needs to know what impedance you are running into. Why? well...

    If it thinks the impedance is higher than it is, it is thinking it has to work harder than it has to, and pushes the correct amount of power out, but, your load is say 4 instead of 16n ohms that the amp thinks the amplifier is suddenly going to draw 4 times more power than the transformer thinks is going through, and eventually (or pretty soon) the output transformer and or the valves will all fry, taking your amp with it and rewinding transformers is expensive. very expensive. Revalving isn't cheap either, and nor is having to replace burnt fried and cooked components. So don't do it. (I've had this happen due to someone else plugging my amp into a 4ohm cabinet. It fried everything. He paid. He wasn't happy)

    The other way around (amp set to 4, cab=16, in other words the amp thinks it's see a lower load than it is) just means it's quieter and will wear down the transformer and shorten valve life (if anything, to be honest this is a gray area, but suffice to say that while not ideal it's not too terrible, and is the only way you can use marshall cabs with mesa power amps, and I have done this and it has been fine. it's not ideal, but should be ok-ish)

    Basically for the most efficient power transfer you need to match the amp to the impedance of the cab, or the combined impedance of the cabinets.

    In laymans terms:
    If you are using one cabinet:
    1)Find the ohm rating of the cabinet
    2)Set your amplifier to the same ohm rating as the cabinet

    If you are using more than one cabinet (ie, a stack)
    1)Make sure BOTH cabinets are the same impedance.
    2)Take the impedance, and divide it by 2, this gives you tyhe impedance the amp will be seeing, I know it's weird that they don't add up, but that's the way it goes :)
    3)Set your amp to the divided impedance. For instance, if you were using two 16 ohm cabinets, 16/2 = 8, so you set the amp to 8 ohms

    Now these days a fair few amps are starting to label the different jacks with what impedance should go into them, which is usefull, but remember, that astill doesn't mean you can plug a 4 ohm and a 16 ohm in (as that would be quite lethal to your amp, especially if you then set it to 16ohms...)

    Do I need to attach a load to my amp? What is a load anyway?

    well, a load is a name for putting a resistance the amp can see on it. This will almost always be a speaker cabinet, or a power attenuator, or specialised DI device, but 99% of the cases it'll be a good old fashioned box filled with speakers ready to make your poor unsuspecting victims ears bleed. In other cases it would be a dummy load (used when the guy at the amp shop wants to find why your amp isn't working, but wants his ears to live) which would be a specialised resistor that can handle high power (please don't use one from radio shack, in fact, DON'T EVER try and build your own dummy loads unless you really know what you are doing. Unless you have a death wish after listening to my chemical romance, in which case...)

    Do you need to attach a load? YES. Even if your amp is idiot-proof as if you don't get into the habit, one day you'll kill someone elses amp, and that's not fair as it will have been through carelessness. So, why is it bad?

    Well, you are making the amp see an infinite resistance that it is trying to feed, and all the current it's producing gets stored up and has no way to escape. Think of it this way: You've just drunk 5 gallons of beer, and now you are stuck in a car for 5 hours, can't roll the windows down, and can't get out. Not nice is it?

    What is a tube?
    A tube (also known as a vacuum tube, electron tube (inside North America), thermionic valve, vacuum tube or valve) is a glass tube with 2 to 5 "plates" inside it and a heating element within a near vacuum (the inside of the tube cntains very little to no air basically (it's not a perfect vacuum).

    Anyway... When the heater is turned on (the valves starts glowing) it heats up the cathode. The cathode is made up of a metal that will emit electrons (negatively charged particles). Now, in the simplest form of a tube (a diode) there is only an anode. When the anode is positively charged, it will attract the negative electrons (positive attracts negative) and there will be a flow of electrons from the cathode to the anode, which we know in laymans terms as an eletric current.

    Now this isn't very usefull for amplification, we'd like to be able to control this current flow. Hence we can place a control grid in between the two plates (anode and cathode). If we make this plate negative, it will repel electrons, and positive we will attract electrons. By making this plates voltage vary (by feeding a guitar signal into it to look at it simply) we can take a small voltage swing, and prouce a large current out of it and convert that to a larger voltage swing (amplification). Now this isn't exactly how it works (as I forget, it's been a while) but it's the basic principles. A tube with 3 plates is known as a triode. tubes with 4 plates and 5 plates were known as tetrode (beam tetrode etc etc) and pentodes respectively. The reason for having more plates was to try and make the response of the tube more linear within a greater usefull range. To all you people it basically means bleeding more power out of them :D

    You can read more and research if you want to learn more, but basically tubes are finicky devices, there are no equations to describe them (there are too many variables, heater temperature, how perfect or imperfect the vacuum inside the tbe is, distance, and then the effect of the plates, oh, and then try adapting the equation to encompass 3 dimensional field magnetcs and electron flow and all that shit. trust me, it's not nice, otherwise someone would have figured it out by now) and most design is done by pen and paper


    What are the differences between tube types?
    Right, here's where confusion sets in. basically there arwe a fair few different types of tube. The little ones are mainly preamp tubes and are basically named as model names that the companies produced them used to use that basically describe the properties of the valve. You can look more of this up if you want, but that's for another thread or if someone wants to take the time I can add it in, here's a brief overview in any case:
    OK, I found a list which should satisfy even the most curious, and it can be found at the bottom of this post -> *(1)
    The huge ones that are about the size of light bulbs but with thicker glass and funny thick pins on the bottom are mainly power valves. These can come in triode,tetrode and pentode versions. Basically little ones are for the preamp, big ones for the power amp as a general rule.
    Also, in some amplifiers you'll find tube rectifiers, which are basically tube diodes that turn the AC voltage from the mains into a DC voltage for use in the amp.

    What are preamp tubes?
    The job of these is to provide voltage amplification, or in guitarists case to provide to much amplification to keep the signal clean, thus clipping it and producing what we call distortion. Model names that should be familiar (12ax7,ecc83,ecc81 etc) just describe the valve, but mostly these are dual triodes in one valve. These things can affect the sound a little bit, but mainly in the preamp is the actual circuit that provides the sound. You'll hear people recommending different types for V1,V2 etc in an amp, this basically means that mixing and matching different valve types of different gaisn and roperties produces desirable effects and slightly alters the response of the preamp. V1,V2 here normally correspond to valve positions within the amp. Someone else can elaborate on this :)

    What are power amp tubes?

    Power tubes take the output of the preamp section of an amplifier and basically add the "power" to it, and producing a large (very large) voltage swing in the order of about 300-800 volts depending on the amp, design, tube types used etc. Basically enough current and voltage to kill you. Which is why it's not recommended to go tinkering if you don't know what you are doing. Even if it is turned off large capacitances (stored charge) may remain, enough to deliver a shock that will stop your heart, which, could easily be yours with a misplaced screwdriver, voltage meter or slight mishap (and with no mains fuse to blow to save you).

    Anyway, power tubes are one of the things that can define the sound of an amp. Models you'll have seen are 6l6, el34, 6550 etc (and others). As a rough guide, el34, think marshall plexi sounds, 6l6 think fender chimey cleans, 6550's think of the modded jcm800's that zakk wylde et all use, were the el34 is replaced by a higher headroom valve (the 6550). For a description of the different sounds, well, someone can write another thread, but this is the general gist.

    Why do tubes need to be matched for a power amp?
    Power amps normally operate in class A/B (push pull) configuration, where one valve handles the negative swing of the signal, and the other the positive. You'll also hear about class A and B amps etc, but class A/B is the most commonly used. (Mesa's simulclass combines class A with class A/B and require matched quads)...

    Anyway, you want the negativer swing to amplify in the same way as the positive swing, hence matched valves. In amps with quads (4 valves) you'll want a matched set of 4 for the same reason.

    Anyway, see that huge second transformer? Well, valves don't produces current. Your speakers work by putting a low voltage, high current through them (ie, the signal is an increase/decrease in current, you know, the stuff that kills you, so don't test the output of your amp on your tongue like a 9v battery) so the "output transformer" takes the huge voltage swing, and turns it into a large current swing to drive your speakers.


    What is biasing?
    biasing is getting the correct base current to flow through the valve. This all has to do with that valves are large physical devices that just are impossible to make the exactly the same over and over again, so there will be differences between thwem.

    Note: this says it better:
    [FONT=Arial, Helvetica, sans-serif]
    [/FONT]


    Why do people say tube amps are louder than solid state amps?
    Good answer to the question below. They are not louder if measured, however, the ear is more sensitive to certain frequencies, and it just happens that, as explained below, tubes just hit those sweet spots in the human ear. (Also has a good explanation as to why a transistor distorting sounds crap, but a valve sounds more musical)
    I'll write more if people give me suggestions, but it's an ok start to a huge topic...

    Right, for the people that want to read more on the subject as it is huge (thanks to the people that found them!):

    Class A - Exposed and Explained by Randall Smith - How a tube amp works - Thanks to JohnnyCNote

    euro-e - More tube descriptions, although this is by a guy selling them, so pinch of salt ;) - Thanks to budda



    *(1)
     
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  2. nitelightboy

    nitelightboy I poop in shoes. Contributor

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    Lots of good info!!!! Thanks James.
     
  3. Apophis

    Apophis Banned

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    Nice article
     
  4. 7 Dying Trees

    7 Dying Trees Forum MVP

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    well, i've just added a list of tubes and highlighted the ones guitarists would be possibly interested in red. May have missed some though...
     
  5. Alpo

    Alpo ... Contributor

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    Might just be me, but that red is kinda hard to read. :2c:
     
  6. WarriorOfMetal

    WarriorOfMetal The Dragonrider Contributor

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    looks good....i agree about the red being hard to read, too.


    also, i noticed you didn't put in anything about power-up/power-down procedures for tube amps, which could also be useful info for t00b n00bs
     
  7. 7 Dying Trees

    7 Dying Trees Forum MVP

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    Changed it to white
     
  8. the.godfather

    the.godfather Mr. Flibble

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    Some good info there, this is worthy of being 'stickyed' surely? :yesway:
     
  9. 7 Strings of Hate

    7 Strings of Hate Mid-Level Asshole Contributor

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    nice james, but like WOM said, it would probably be better to tell the power up and power down procedures, the do's and dont's and things like that. I think most of us tube noobs know that the science of how a vaccum tube works, and thats all we need to know about that part :) but the tube list did help
     
  10. 7 Dying Trees

    7 Dying Trees Forum MVP

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    Added it in. Interestingly, in another thread, some people were recommending flipping it onto standby and then turning it off. Well, looks like that's the wrong way of doing it as well as the caps don't discharge, and something I never knew properly either! Well, kind of cool, as I am learning stuff by doing this!
     
  11. WarriorOfMetal

    WarriorOfMetal The Dragonrider Contributor

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    i sometimes hit a chord on my guitar and then flip the power switch....it helps discharge the caps. then i flip the standby switch to the standby position after the amp goes silent. otherwise i just turn both switches to the off position (in the case of the standby switch, meaning to the silent/standby position) at the same time.
     
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  12. Alpo

    Alpo ... Contributor

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    :yesway: Mucho better.
     
  13. 7 Dying Trees

    7 Dying Trees Forum MVP

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    Just added a whole load more, also found a nice description of why tube amps are percieved to be louder, and why the distortion sounds more musical :)
     
  14. technomancer

    technomancer Gearus Pimptasticus Super Moderator

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    Sweet :yesway: Now we need a mod to sticky this biznatch :agreed:

    Oh, double check the end of the section on load matching, as it seems a bit scrambled there.
     
  15. WarriorOfMetal

    WarriorOfMetal The Dragonrider Contributor

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    yeah, there are some typos that you might want to go through and fix.
     
  16. 7 Dying Trees

    7 Dying Trees Forum MVP

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    Well, if someone can do a better description, then post it and I'll edit it in :) Mine is a bit weird, so please, someone write a better explanation!

    THread is meant to collate information into one little guide to read that answers a few basic questions and gives some explanations, so it'd be cool for other people to add in edit and contribute!

    And anything else anyone wants to add, or if you see typos, please tell me, as I am not too good at seeing them!
     
  17. Leon

    Leon {##[====:::. Contributor

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    :metal:

    i nominate this for Post Of The Year!
     
  18. The Dark Wolf

    The Dark Wolf Contributor

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    Stickied. ;)


    I notice one GLARING typo. Stanby. Missing a D. :lol: Might wanna clean up.


    Nice post, James.
     
  19. ibznorange

    ibznorange Chief Officer/RHLC © Contributor

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    lol good work!
    there have been more questions on this than usual lately so yeah.
    nice bit of info man!
     
  20. budda

    budda Guiterrorizer Contributor

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    good thread!

    i would have used the tube comparisons from eurotubes instead of that beast of a list (how many of those tubes actually go in guitar amps? condense maybe?) but still a good read.
     
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