Tube amp schematics 101, etc: NEW NEWER UPDATE: Biasing tutorial!

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Tube amp schematics 101, etc: NEW NEWER UPDATE: Biasing tutorial!

Post by øøøøøøø » Thu Jul 24, 2008 8:14 am

It was suggested in another thread that I make a little post about some of the basics of reading amp schematics.  To tell the honest truth, I kind of have a guerilla/mercenary way of operating with amps.  When I look at a schematic I don't have to always know the reason-for-existing of every single component, but it does help to know what the main component types are.  There are guys that know the function of everything in a circuit by looking at it... in other words "why" everything is there... but you don't have to be one of those for a schematic to be useful.  The main thing I use a schematic for is a reference.  There are always little bits of info you can get from a schematic that you can't get from other places.

Here are some basic symbols.  I stole most graphics from the web.  You will want to familiarize yourself with these, among others.

Resistor:
Image

A resistor opposes current.  This will produce what is called a voltage drop across the component.  According to Ohm's law, "The electrical resistance R is equal to the voltage drop V across the resistor divided by the current I through the resistor."

Potentiometer:
Image

All this is is a variable resistor.  Typically, signal goes in through the wiper (the arrow part that points toward the resistor symbol) and then something happens to it.  Often it is used as part of a voltage divider to work as a volume control, or whatever.

Capacitor:
Image

The symbol for capacitor looks like this.  A capacitor is any two conductors separated by a non-conductor.  They perform a variety of functions.  They actually store small amounts of charge for a short time under the right circumstances.  They also block DC voltage and allow AC voltage (like audio signal) to pass through.  In guitar amps, they usually have a few main functions.  For one, they're usually used in the EQ section as filters.  They are also used as "coupling caps" to allow audio to pass from one stage of the amp to another while blocking the previous stage's DC operating voltages.  A "filter cap" or "smoothing cap" is there in the power supply.  Power rectified from AC to DC isn't a true DC... often they just chop off the AC any time it swings negative, so you get this pulsating DC.  In other words, whenever the AC would've swung negative, it's just gone.  Filter capacitors charge in the "peaks" and discharge in the "valleys" of this pulsing DC, smoothing out the pulsation into something resembling actual battery-power DC.  A property of capacitors is that they oppose or neutralize changes in voltage.  When the voltage changes, the cap will release a small amount of energy to make the voltage go back to what it was, until the cap's charge is gone.  This is what filter caps do.  Another function you will often see in tube amps is a "cathode bypass cap" that allows audio to pass the cathode resistor used to bias preamp tubes and sometimes power tubes for more efficient output.

You can see that the one in the pic above has a little "+" sign next to it, meaning the particular one in this schematic is the polarized electrolytic type.  If there is no polarity marked and the cap is a small value, it's usually non-polarized.  If the cap is a high value but there's no polarity marked, it may be electrolytic and they assume you're smart enough to figure out where the positive lead goes.  Many Fender amp schematics don't mark polarity.  Basically, if polarity is unmarked on the schematic, the negative lead is always "more negative" than the positive lead.  In a circuit with positive DC voltage, that means the negative goes to ground or at the lower voltage potential.  In a bias circuit, which is a negative DC voltage, the positive end is grounded because zero potential (ground) is "more positive" than the negative number.

Inductor:
Image

Whereas a capacitor opposes changes in voltage, an inductor opposes changes in current.  In this way they are also used as a "choke" to filter out additional hum from rectified power supplies that capacitors didn't get.  They are also used in EQ circuits in a manner similar to capacitors, but behave a little differently.  Your wah pedal uses an inductor as a part of its sweepable "EQ" filter.  That's why just turning your tone control on your guitar or whatever doesn't have that 'peaky' sound your wah pedal has.  Usually the only place you find inductors in guitar amps is as a choke in the power supply.  The "Varitone" circuit on the Gibson ES-345 and 355 guitars uses an inductor.

Transformer:
Image

More than meets the eye (sorry), these are basically two inductors that share the same magnetic core.  Their job is to change from one voltage to another (or multiple others).  Usually there are two types in an amp: The power transformer turns 120v or 240v wall power into all the different voltages needed for the amp.   The output transformer mates the power tubes to the speaker.  The "incoming" voltage is on the winding called the primary, and the new, outgoing voltage is on the secondary.  Voltage can be either stepped up or down, varied by whether there are more turns on the primary or the secondary.  The simplest type of transformer is like the one pictured above, which has a single secondary.  My picture didn't work so I had to find another one... imagine for this picture that you have 110V marked on the left side and 25V on the right side.  That would mean it's a step-down transformer that is being used to step 110v down to 25v.  Of course, it's a ratio that corresponds to the turns ratio of the primary and secondary.  So if you applied 220v to the primary, the secondary would be 50v in this case.  Get it?  The power transformers in amps usually have multiple secondaries (or taps on the secondary) that get several voltages, some stepped up and some stepped down.  In a typical BF Fender amp you will have a "B+" winding for the high plate volts... 300-450 or so volts depending on the amp.  You'll also have 6.3v for the tube filaments, 5v for the rectifier tube filament, and maybe another voltage of some sort for the bias.  There will be a wire for each end, and some secondaries may be "center-tapped," or have another wire for the center of the winding. That's why the power transformer has all those wires coming out of it.

Diode
Image

This guy's job is to let current flow in one direction only.  They are often used as rectifiers.  Amps with solid-state rectifiers will usually have four of them arranged in a ring.  There will also usually be one in the bias supply of a fixed-bias amp.


Earth/Ground
Image

"common," "ground," or whatever... zero voltage potential with respect to the rest off the circuit.


Switch
Image

Relatively self-explanatory, but there are multiple kinds of switches with multiple numbers of poles.  Some are also momentary (as opposed to toggle).


Vacuum tubes on scehmatics:
Image

Tubes like the one in the schematic above contain several electrodes.  The one above is a 6V6.  Pins 2 and 7 connect to the filament.  All the filament does is to get the tube hot so it can function properly.  Right above the filament (pin 8 ) is the cathode.  The cathode emits electrons, but only when heated by the filament.  The two dotted lines are screens or grids, and the thick line is the plate, which attracts electrons emitted by the cathode.  The grids and screens' job is to modulate the stream of electrons going from the cathode to the plate.

Some tubes have actually two tubes in one.  The 12AX7 is one of these... it's a "dual triode."  This means it has 2x units with 3 electrodes each.  A triode within a 12AX7 has a cathode, a grid, and a plate.  Audio signal goes in at the grid.  Then the low-power audio signal has an influence over the high power going between the cathode and the plate.  In this way, the little bitty guitar signal tells the big bad high-powered tube what to do.  The 12AX7 has two sections of triode within it.  So on a schematic when you see "1/2 12AX7" that's what it means... one triode from within the dual-triode tube.  Tubes drawn on schematic often look slightly different, but it's usually easy enough to figure out what's going on.  The filaments are almost never shown on the schematic, because they're the same for all tubes. 

Electrical networks

There are some very common things that you see over and over again in schematics... sort of "building blocks" that are made of a few components that accomplish a task.  These are called "electrical networks."

There are many many types.  One example is the "voltage divider," that I mentioned above when I was talking about the potentiometer.  It looks like this:
Image

Its job is to output a fraction of the input voltage.

Another common one in audio circuits are low-pass and high-pass filters.  A low-pass filter looks like so:
Image

Its job is to let low signals pass, and block higher frequencies.  Usually the resistor in the graphic above is a potentiometer, allowing you to vary the amount of treble attenuated.  Exactly what the EQ slope is is determined by capacitor value.

A high-pass filter looks like just the opposite:

Image

This allows highs to pass but blocks low frequencies.  Again, the resistor is usually a potentiometer.

There are many many more common electrical networks but number one I'm not an expert on most of them, and number two, it's sort of beyond the scope of this little post-blog,  :)


There is more I could say, but honestly, I'm tired of typing. :) 

I'd suggest that if you're into this,  download the NEETS US Navy training manuals.  They are really easy to understand and very thorough.  There are many volumes in the series.  There's one on vacuum tube amplifiers.  There's also one on magnetic recording devices.  If you're reading this post, you might want to start with the first couple of volumes though.  You can download them free in PDF format in many places, here is one: http://www.tpub.com/neets/

Hope this helps.
Last edited by øøøøøøø on Tue Sep 09, 2008 2:28 pm, edited 1 time in total.

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Re: Reading tube amp schematics 101: by request

Post by øøøøøøø » Thu Jul 24, 2008 8:20 am

fixed the broken pics

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Re: Reading tube amp schematics 101: by request

Post by Orang Goreng » Thu Jul 24, 2008 12:09 pm

Great post, man! I see someone else already stickied it :).
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Re: Reading tube amp schematics 101: by request

Post by Maggieo » Thu Jul 24, 2008 12:11 pm

As they say in the Bible Belt: "Y'all are doin' the Lord's Work, my friend."
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I am not an attorney and this post is for entertainment purposes only. Please consult a licensed attorney in your state for legal advice.

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Re: Reading tube amp schematics 101: by request

Post by zhivago » Thu Jul 24, 2008 12:15 pm

Orang Goreng wrote: Great post, man! I see someone else already stickied it :).

that was me :)
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Re: Reading tube amp schematics 101: by request

Post by noirengineer » Thu Jul 24, 2008 1:00 pm

thank you for this post

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Re: Reading tube amp schematics 101: by request

Post by ||||||||||||||||||||||||| » Mon Jul 28, 2008 9:44 am

I suppose it's about time I figure some of this stuff out.  I'll be glad once I'm a little more familiar with it, but it's tough to start.

Thanks for the help!

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Re: Reading tube amp schematics 101: by request

Post by øøøøøøø » Tue Jul 29, 2008 7:50 am

List of terminology often used when discussing the guts of amps!

Figured I'd add to this little sticky thread today, because I have a minute.  :)  Remember that this stuff is all according to my understanding, and I'm totally self-taught... no electrical engineer here, just a dorky amp guy... so I'm not responsible if there are gaps and possibly small accuracy quibbles.  I do try to only post stuff I know for sure, though.   ;)

Here is a list of some commonly used terms that will make people look at you like "huh" if you say them and they don't know what you mean.  Sort of a little 'glossary.'  They are in no particular order, but I kind of started with the power supply.  I am focusing on more in-depth terms than the ones in the schematic-reading guide above.

B+, and less commonly C-, and even less commonly A+ voltage:  These terms are remnants from when tube amps ran off of batteries.  A huge, huge battery called a B battery provided the high voltage for the plates of the tubes.  This is a positive DC voltage, hence "B+."  The C- voltage was a negative voltage that biased the tubes, and it came from a C battery.  Finally, the A+ was a positive voltage that powered the filaments from an A battery.  So B+ equals plate voltage, and C- equals bias voltage.  Nowadays, we use a power transformer and rectify AC wall power instead of using batteries, but the terms have stuck around.

Rectifier: A rectifier turns AC (wall power) into DC (battery power).  AC voltage swings alternately positive and negative, and DC voltage is either positive OR negative in a steady state.  Rectified AC creates a "pulsating" DC.   How it pulsates depends on whether you have a half-wave rectifier or a full-wave rectifier.

Half-wave rectifier: Simply chops off the negative bits and leaves only the positive bits.  This leaves big gaps in between with no voltage.  Graphic:
Image

Full-wave rectifier: Takes the absolute value of the voltage swing, making what was formerly negative positive, so that the no-voltage gaps are very small.  Graphic:
Image

Filter capacitor: This capacitor's job is to charge and discharge repeatedly to smooth out rectified AC.  Remember from the descriptions above that rectification turns AC into pulsating DC.  You know from the first post of this thread that a capacitor can store electricity, and it also resists changes in voltage.  Those two qualities are related, actually: it resists changes in voltage because when the voltage swings high the cap will absorb some energy by charging, and when it swings low, the cap will release some of the energy it stored.  So in this way, a filter capacitor, sometimes called a "smoothing capacitor," fills in the gaps of the pulsating DC, making something more like clean battery power.

Center Tap: On a transformer, a "tap" is anywhere one of the windings is 'tapped' with another wire that comes off of it.  Sort of like the tap on your sink comes off of the main pipe.  A center tap is a tap in the exact center of a transformer winding. Depending on what the winding is for,  this can be useful for all sorts of things.  It might allow your filament winding to get a reference to ground, or it might allow you to get multiple output impedances from a single output transformer.

Tube filaments or Heaters: This is the part of the tube, not technically an electrode, that heats the tube up.  It's the part you see glowing inside.  It gets hot and boils electrons off of the cathode.  Occasionally, like on directly-heated rectifiers, the filament is the cathode.

Coupling capacitor: This is a capacitor that links two stages of an amp together.  There are high volts DC on the plate of a tube, but there is also amplified audio signal, which is AC.  The coupling cap blocks the high DC voltage and feeds the AC audio through to the next stage, where it can be further amplified. 

Impedance: Impedance is the total resistive force of a circuit, combining electrical resistance and inductive reactance.  Recall from the first post of this thread that while capacitors resist changes in voltage, inductors resist changes in current.  Well, inductors or coils have a property called  inductive reactance.  Any coil will likely have some inductive reactance.  Even foil coupling capacitors, since they have layers of foil rolled up like a cigarette, act like a coil and have some inductance.  Speakers, with their voice coils, have significant inductive reactance.   That is why the DC resistance of an 8-ohm speaker is usually something like 6.7 ohms.  The other 1.3 ohms comes from inductive reactance, and is frequency dependent.  So an 8 ohm speaker will have a different impedance depending on what is playing through it at the time. 

Stray capacitance, capacitive coupling, etc: Remember from the post above that the definition of a capacitor is "two conductors separated by a non-conductor."  This is why they only pass AC voltage.  The AC voltage due to the motion of its swinging positive and negative, can have its influence across the non-conductor-- but the direct current cannot pass through it.  Well if your imagination is active, you can imagine that any two wires in an amp running next to each other will form a crude capacitor.  You'd be right.  This is why wire layout, sometimes called "lead dress," in a tube amp is so critical.  If you run an audio wire right along side a filament supply wire, some of the 60Hz AC from the filament wire will jump over and "infect" the clean audio signal in the audio wire.  This is why filament wires are in twisted pairs and placed far away from signal wires, and cross them at right angles if possible-- to minimize these stray capacitances.  The farther the two conductors are away from one another, the smaller the capacitance.  Past a certain point, it is small enough to only pass ultrasonic frequencies, and at this point it is insignificant unless it causes a....

Parasitic oscillation: Usually caused by stray capacitances, this is a sort of internal feedback inside an amp that is often corrected by physically moving wires around.  Sometimes the oscillation can be at an ultrasonic frequency, in which case the only noticeable effect is a weak, anemic sound resulting from the oscillation sapping the power from the amp that you need for the musical signal.  In other words, it robs power.  Can usually be fixed by moving wires or shortening wires that connect to grids of tubes.

Fixed bias: Fixed bias does not mean "non-adjustable bias."  To the contrary, the only circuits with bias adjustments are "fixed-bias!"  Fixed bias simply means that the bias voltage to the power tubes is set and supplied by an external power supply (C-, you may recall) and does not change no matter what happens in the amp, until the voltage is adjusted by turning the pot or changing the resistor.  This allows the output tubes to be more efficient than cathode-bias, described below.  Most fixed-bias amps have adjustable bias voltage, and it should be properly set.  Bias adjust is notably absent from a few fixed-bias amps, including Mesa Boogies and some brownface and larger tweed Fenders.  Bias adjust was on all fixed-bias Fenders by the blackface era.

Cathode bias, AKA "self-biasing.": Rather than supplying an external voltage C- supply, the bias in a cathode-biased amp is generated within the tube itself by use of a resistor between the tube's cathode and ground.  It gets a little technical, but current drawn through the resistor causes the cathode to become positive with respect to the grid.  So the bias voltage will always vary depending on how much current is drawn through the resistor.  There is no bias-adjust because it's auto-adjusting.  It has its own kind of sound, a spongier envelope, and is less efficient than fixed bias designs (so not as much power from the same kind of tubes as fixed bias).  Power tubes are occasionally cathode biased, and virtually all preamp tubes are cathode biased.

Cathode bypass cap: A capacitor that allows AC audio to "bypass" the cathode resistor.  Sometimes cathode resistors are bypassed, sometimes they are unbypassed.  The 25µf electrolytic caps in the preamps of Fenders are cathode bypass caps.  Increasing the value lets more bass through.

Power transformer: Turns 120VAC (or 240VAC in Europe) into all the different voltages needed in the amp... B+ C- etc... by stepping them up or down.

Output transformer: Transforms the high-voltage low-current signal from the power tubes into the low-voltage high-current signal needed by the speakers.

Negative Feedback: This is a design technique that flattens frequency response, and here's how it works.  It takes a little signal from the output transformer and inserts it back into an earlier stage of the amp with reversed polarity.  In other words, the fed-back signal is "out of phase" with the signal where it is getting injected back in.  Why do they do this?  To flatten frequency response.  Tube amps are known to accentuate midrange frequencies.  When the signal is negatively fed-back, the loudest bits get cancelled the most.  So if there's a lot of midrange and not a lot of high end, the midrange will get cancelled out a lot and the high end will only get cancelled out a little, yielding a "flatter" net frequency response.  Tweed amps do not usually use negative feedback, and blackface/silverface amps almost always do, so there's a reference for you.  It is easy to disconnect the negative feedback in an amp to see if you like how it sounds without it.  Amps with no NFB will have a "looser" and "rawer" sound that is more interactive with the speaker, more midrangey, and has more apparent gain/volume.  I have the NFB routed through the now-defunct ground switch in my BF Deluxe, an easily reversible mod should I ever want to put it back stock.  Now I can turn it off or on at will.  Oversimplification: I tend to like distortion tones without NFB and I tend to like totally clean tones with it.  In other words, I turn the NFB 'on,' or 'stock,' for jazz.

Presence control: One of the most misunderstood knobs on an amplifier.  Most people assume it is just another tone control, a sort of "super-treble."  It's not.  What it is is a tone control for the negative feedback loop.  As you turn the presence control up, treble is removed from the negative feedback signal, meaning that less treble gets cancelled in the NFB loop, resulting in a 'revelation' of some treble that the NFB loop would've taken away.  In other words, the presence control doesn't add anything, it simply reveals treble that would ordinarily be lost in the negative feedback circuit.  If negative feedback is disconnected, the presence control would be totally non-operational.

All for now.  Tired of typing.  :)
Last edited by øøøøøøø on Tue Jul 29, 2008 8:05 am, edited 1 time in total.

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Re: Reading tube amp schematics 101: by request

Post by quarterpound » Tue Jul 29, 2008 4:46 pm

Wow, makes me almost wish this forum had feedback. But not really. You're the man! This came just as I'm getting into DIY amps, so... I'm really quite lucky! ;)

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Re: Reading tube amp schematics 101: by request

Post by fuzzking » Wed Jul 30, 2008 6:39 am

this is good stuff. really appreciated.  ;)
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Re: Reading tube amp schematics 101: by request

Post by Orang Goreng » Wed Jul 30, 2008 10:41 am

Yeah, this is a great thread, Brad...thanks a lot!

I'm getting a decommissioned Marshall in October. It's unclear what's wrong with it, and as it's one with a PCB (a JTM 610) it may not be an easy fix. I'm thinking I may gut it completely and eventually build my own amp inside it. A project for the not-so-near future though.
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Re: Reading tube amp schematics 101: by request

Post by øøøøøøø » Wed Jul 30, 2008 3:19 pm

vacuum tubes for dummies: How do they work?

This will explain in English (hopefully!) how vacuum tube amplifying devices work.  Vacuum tubes are NOT complicated devices... in fact they are very simple devices!

Let's start at the beginning... what is amplification?  In a broad sense, amplification is to take something and magnify it.   If you have a 10x magnifying glass and hold it over a printed word, you have amplified the size of the word by a factor of 10.  You can say you have caused the word to "gain" size relative to your eyes.  In an auditory sense, to amplify is to make a sound louder.  This is fairly obvious.  You cause a "gain" in loudness.  In an electronic sense, to amplify is to increase voltage.  If you have an input signal that is 1 volt, and an amplifying device outputs 50 volts, you have a gain in voltage of 50x.  Note that gain doesn't mean "distortion!"  This is a common misnomer.  "Gain" simply means a signal is getting stronger.  If this stronger signal happens to overload a subsequent stage of the amplifier, then we will get distortion.  But gain is not the same thing as distortion, and does not always cause distortion!   Gain simply means an increase in strength.

What we are concerned about in a musical instrument amplifier is the part where the auditory and electronic uses of the word "amplification" meet.  In other words, we want to make a sound louder, and we want to use analogous (remember this word) electricity to help us get there.  I'll explain further.  The term "analog" refers to the fact that an alternating current represents a good model of the compression and rarefaction of air that produces sound.  In English:  Alternating current is a wave, and sound is a wave.  If we make the alternating current wave rise and fall in a way that mirrors the rise and fall of the sound wave, we have an electrical representation of the sound-- an "analogy."  That's what "analog" means.  We can then use electronic gagedtry to manipulate the sound wave in all sorts of ways.

We need something to turn the sound wave into electricity, and then something else to turn it back into sound after we're done having our electrical way with it.  Such devices are called transducers.  Microphones, guitar pickups, speakers, electrostatic panels, piezo elements, etc. are all transducers.  They usually have some sort of diaphragm as a means for capturing or emitting vibrations, and some electromagnetic or electrochemical apparatus attached to said diaphragm that is capable of turning mechanical vibrations into electrical impulses (or electrical impulses into mechanical vibrations, in the case of a speaker). 

So once we have turned the sound vibrations into electrical impulses with a transducer, we may set about amplifying these electrical impulses so that when they are turned back into sound waves by another transducer (i.e. speaker), they are louder than the ones that went in.  We do this by increasing the voltages.  Simple enough, even if I've made it sound complicated.  It's really not.

English summary:  In an amplifying system, we turn sound into electricity, make that electricity stronger, and then turn that electricity back into (now louder) sound. 

If we are going to increase the strength of our electricity, the law of conservation of energy states that we need the extra power from some external source.  Fortunately, most houses in the western world have sources of electrical power right inside the wall.  In an amplifier, the goal is to take power from the wall, and bend this power so that it looks like a larger version of our guitar sound.  That's what the amplifier does.  It uses a small input voltage (your guitar) to change the shape of a large, externally-supplied voltage (wall power stepped up with a transformer).  The amplifier allows your tiny guitar voltage to make the large voltage "in its own image" and send the amplified copy of itself to the speakers.

How do we do this?

In a variety of ways.  But in a tube amplifier, we do it with an amplifying tube!!

Now after all that setup... which identifies the objective of an amplifier... we get to the good part.  How does the tube do it?

First: The anatomy of the tube.  In every tube there are little parts that serve different functions.  These parts generically are called "electrodes," and they all serve specialized functions.  The type of tube depends on how many (and which) electrodes are in the tube.  A diode (like a rectifier tube) contains two electrodes... an anode and a cathode.  A triode contains three-- an anode, a cathode, and a control grid.  A tetrode contains an anode, a cathode, a control grid, and a screen grid.  Finally, a pentode contains all of the above plus a negative suppressor grid. 

So what do all of those types of electrodes do?

The anode, often called the plate, attracts electrons very strongly when it has a positive voltage applied to it.  Remember, electrons, being negative in charge, are very very strongly attracted to a positive charge.  Opposites attract.
The cathode,  when heated up, emits lots of electrons.  It's coated with a chemical that spits them out when it gets hot.
A control grid sits in between the anode and the cathode, and when it has a charge applied to it, can affect how readily the electrons can flow from the cathode to the anode (plate).
The other two electrodes, screen grids and negative suppressor grids, help to focus the electrons toward the plate and to repel stray electrons, respectively.  These two help make the tube more efficient, if present.

All you need for the most basic amplifying tube, however, are the first three electrodes:  The cathode, the plate, and the control grid.

Now, how does it amplify?

If we take the tube filament and heat it up, then the cathode will get hot and emit electrons.   If no control grid were present, those electrons would go very very fast to the plate (anode).  Remember that the plate has a positive charge on it, and opposites attract.  Voltage is a measure of electrical pressure while current is a measure of amount of electricity.  So if the electrons are moving very fast with lots of urgency, then they have high voltage.   If there were a large number of electrons, it would be high current.  It pays to remember the difference between the two, but I digress.

Anyway, so without a control grid, we would have these electrons moving from cathode to plate very urgently.  High voltage

Now stay with me!

Once those electrons made it to the plate in their urgent manner, we would need a way for them to return to the cathode to complete the cycle.  We do that with a plate load resistor that goes back to the cathode through the power supply.  You can measure the electricity at this resistor.  As the amount of electrons flowing from the cathode to the plate increases, the voltage at this resistor increases.  As the amount of electrons flowing from the cathode to the plate decreases, the voltage across this resistor decreases.  So basically, the resistor gives us a way to 'tap into' the electrical changes going on inside the tube.  Stay with me... it gets good, and even if you are starting to glaze over you WILL understand the next part, I promise.  ;)

So let's recap that last part in English.   We have electrons going from cathode to plate, and now we have a resistor attached to the plate that allows us to have an external manifestation of how much voltage is flowing from cathode to plate and back.  Got it?  Good.

Now if we add a control grid in between the cathode and plate, we actually have a way to control how many electrons get through!  The control grid is physically very close to the cathode, so it has a lot of influence over the electrons emitted by the cathode.  If we apply a negative charge to the control grid, then electrons are repelled back toward the cathode and fewer reach the plate (and with less urgency).  If we apply a positive charge  to the control grid, then the electrons are attracted to the plate with urgency.  Remember, opposites attract, like charges repel.  One asterisk to this is that to make positive voltage relevant when applied to the control grid, we have to supply bias voltage* to the grid to give it a 'starting point.'  More on this later...

So what if we hooked up our guitar to this control grid?  Whenever the signal from the transducer (pickup) swung positive, there would be a positive charge on the grid.  Whenever the signal swung negative (remember, audio is alternating current!), we would have a negative charge on the grid. 

This would allow our guitar signal to control the urgency with which the electrons flowed inside the tube.  In other words, the small voltage on our guitar "controls" the large voltage going from the cathode to plate.  This is why it is called a control grid.  We have a very high voltage flowing from the cathode to the plate, and a very teeny tiny voltage modulating, or causing changes in, that large voltage.

To say it much more simply and directly: Your guitar puts out a tiny voltage.  Putting that tiny voltage on the grid controls the flow of electrons between the cathode and plate so that it looks like the input signal, only much much 'larger,' or more powerful.

*Now, in order for both the positive AND negative voltage swings to mean anything, we have to start with a small negative voltage on the control grid.  This is called the bias voltage, and here's why we have to have it.  In absence of negative voltage on the grid, electron flow from cathode to plate is at maximum.  So a positive voltage on the grid is the same as no voltage on the grid (and the tube will also burn up quickly from dissipating so much heat).  If we apply a relatively steady-state negative voltage on the grid, then all changes to the control grid's charge are relative to that constant bias point.  Now when the positive voltage is applied to the grid, it makes it "less negative" than the bias point, and when there is negative voltage applied on the grid, it makes it even "more negative" than the bias point.  This allows the tube to reproduce audio.  Got it?

Concise recap of bias explanation: Negative voltage is always on the control grid, keeping the rapid flow of electrons from cathode to plate at bay.  Otherwise the electron flow would 'run away' to maximum.  This is the "idle" point, with no signal supplied.   The bias sort of keeps the electrons in check with a repulsive negative voltage that keeps it at a sort of equilibrium.  Now if we apply a positive or negative voltage, there's actually a starting point and electrons can go faster OR slower, instead of just slower.  Got it?  In other words, by taming the flow of electrons, we give them a point in the middle from which they can go faster OR slower.

So, heat the filament, boil electrons off the cathode, apply positive DC voltage to the plate to attract said electrons, give the electrons a return path to the cathode via a plate load resistor, add a control grid between the cathode and the plate, bias the control grid with a negative voltage, apply signal to the control grid, measure amplified output at the plate load resistor.  Congratulations, you have used a vacuum tube to amplify a musical signal!

Hope this is understandable.  If not, feel free to ask questions.

 
Last edited by øøøøøøø on Wed Jul 30, 2008 5:27 pm, edited 1 time in total.

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quarterpound
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Re: Reading tube amp schematics 101: by request

Post by quarterpound » Wed Jul 30, 2008 8:33 pm

øøøøøøø wrote: Hope this is understandable.  If not, feel free to ask questions.
Q: Why do you love us so much?  :'(  :'(  :'(  :-*

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øøøøøøø
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Re: Reading tube amp schematics 101: by request

Post by øøøøøøø » Wed Jul 30, 2008 8:40 pm

quarterpound wrote:
øøøøøøø wrote: Hope this is understandable.  If not, feel free to ask questions.
Q: Why do you love us so much?  :'(  :'(  :'(  :-*
A: Boredom

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Re: Reading tube amp schematics 101: by request UPDATE: new info- "how tubes wo

Post by JJ Gabor » Tue Aug 05, 2008 11:25 am

Very cool thread.

I have just started building pedals.  This is helping even with that!!

Your high pass filter schematic above might solve my overly bassy sounding fuzz construction.

Thank You

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