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Old 10-30-2009, 01:17 PM   #1
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Amp building resource

*NOTE: This will be updated to be more correct. This is not intended to be all encompassing. You are encouraged to be innovative. This is a resource for beginners, but everyone can always learn something, or contribute. If you see anything wrong, feel free to correct me. Also, if you'd like to add anything, I'd gladly quote you into the main part of the thread.

**ANOTHER NOTE: This is currently incomplete. Please don't ask "What about the output section?" Or "What about the power section?" I'm working on it. I'm just busy.

The point of this thread is to direct people who are interested in learning about building/designing an amp, but are sick of hearing "Learn how tubes work before you try it."

There are several great resources online. Here is a list that is subject to change:

www.ax84.com
www.freewebs.com/valvewizard
www.angelfire.com/electronic/funwithtubes
www.aikenamps.com (you'll need to click "TECH INFO" on the left)
blandguitar's Preamp Thread
Tube thread that shows reviews of various tubes.

Here's a link to the "secret" part of my site. It contains hundreds of free schematics (many of which I am yet to provide links for):
http://www.flightlessamps.com/Secret.html

Here are some people that I feel know what they are talking about when it comes to this (May be modified):

DLRocket89
XGamerGt04
kurtlives91
SomeoneYouKnew
CECamps



Here is the current layout of the thread:

1. Terminology

2. Preamp (from schematic)

3. Single-Ended Output (from schematic)

4. Push-Pull Output (from schematic)

5. Power supply (from schematic)

6. Biasing (from schematic)

7. Layout (From schematic to actual layout)

8. Misc.

First, some terminology:

Valve - same thing as a tube. Sometimes called a "thermionic valve." This is where the amplification of the signal occurs.

Cap - capacitor. It is two metals or conductors separated by a non-conductor. They pass AC signal easier than DC signal. They will also store a charge and release it rapidly. Said another way, the capacitor will resist change in voltage. These are used in the tonestack and power supply as well as other parts of the circuit.

Resistor - something that will impede the flow of electricity. These are used extensively through an amp's circuitry.

Inductor - A spiral of copper (or other material) that will resist change in current. It will store energy in a magnetic field. Typically used in the power supply. Sometimes used in the tonestack and other parts of the circuit.

Transformer - Two or more inductors with their magnetic fields coupled. A change in one inductor's voltage or current will be reflected onto the other. These are used in the power supply, reverb circuit, output section, and sometimes in the phase inverter.

Variable resistor - rheostat. A resistor who's value can be changed.

Potentiometer - Pot. A variable resistor that is center tapped. Can be wired as a variable resistor by connecting an outside lug to the center tap.

Diode - a device that allows current to flow in only one direction.

Triode (tube) - A tube consisting of a grid, anode (plate), and cathode. The 12AX7 is a dual triode, meaning there are two individual triodes within the tube.

Tetrode (tube) - A tube consisting of a grid, anode (plate), cathode, and screen.

Pentode (tube) - A tube consisting of a grid, anode (plate), cathode, screen, and suppressor. Most tubes considered to be tetrodes are actually pentodes with an internal connection between the suppressor and cathode.

Tonestack - EQ or Equalizer. Where frequency can be attenuated or boosted. Most guitar amps use a passive tonestack, which will only allow for frequency attenuation.

Bias - Setting idle current flow of a tube to be within the operating conditions of the amp. Can be either fixed-bias or cathode-bias.

Fixed-bias - THIS IS NOT self-biasing. This is applying a small negative voltage to the grid of the power tube. This is a more efficient way of biasing than cathode biasing. In a fixed-bias setup, the cathode of the tube is grounded (0v).

Cathode-bias - This is bringing the cathode to higher voltage potential than ground with a resistor and a capacitor. This is what is referred to as "Self-Biasing." This term is true to an extent. A cathode-biased amp can still have the bias adjusted, but may not have to.
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Last edited by end_citizen : 04-30-2012 at 04:00 PM.
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Old 10-30-2009, 01:18 PM   #2
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Part 2 - The preamp.

Part 2a - The triode gain stage

I start with this, because it is the easiest for me to understand. This is where most of your tone is created and sculpted. It also amplifies the signal, but not enough to be an entire amp. Typically it consists of a few triode stages and a tonestack. Here is the pinout of a typical preamp dual triode (12AX7, 12AU7, 12AT7, etc.)



I will deal with each triode separately.

Pin 6 is the first plate. This is typically where signal will leave the tube.

Pin 7 is the first grid. This is where signal will come into the tube.

Pin 8 is the first cathode. This is where the bias of the preamp tube is set. Since the preamp tube is cathode biased, there is no need to adjust the bias of the preamp tube when you change it out.

Pin 1 is the second plate. This is typically where signal will leave the second part of the tube.

Pin 2 is the second grid. This is where signal will enter the second part of the tube.

Pin 3 is the second cathode. This is where the bias of the second part of the tube will be set.

Pins 4, 5 and 9 are the heater or filament of the tube. This heats the cathode so that electrons can boil off of it and produce a current. If you are using a 6.3v heater supply (which is typical of most homebrew amps and almost all production amps) you connect 4 & 5 together. You then run one lead to 9 and the other to 4&5. If you use 12.6v, run one heater lead to 4 and the other to 5.

Now that we know some parts and what they are meant to do, let's look at a typical circuit.



R is a resistor
C is a capacitor

The way this preamp functions is a high DC voltage is applied to the plate. A resistor (R6) is placed "in series" between the high voltage supply and the plate. A small AC voltage (from your guitar) is applied to the grid. This will cause the voltage of the plate to vary with the exact same frequency as the signal applied to the grid. The bias of this tube is set at the cathode. The cathode should be at a voltage potential that is above ground (0v), above the grid voltage (when so signal is present), but below the plate's voltage. This will attract some of the AC signal coming into the grid towards the cathode, rather than the plate.

With this, we have a single gain stage with two inputs.

R1 & R2 are "Grid Stoppers." They prevent radio frequency interference from entering the first gain stage. At the beginning of the preamp, values of 32k - 150k are common. Later stages may use larger resistors (220k - 1M) to keep oscillation (squealing and such) down. However, large resistor can also cut down on treble

R3 & R4 are the "Grid Resistors." The lower this value is, the more signal sent to ground. Typically you don’t want much signal sent to ground this early in the circuit, so you use a high value resistor here. R3 is typically 1MΩ. R4 in this circuit acts as in conjunction with a switching jack. When you plug into the bottom jack, the switch opens, and only R3 is in the circuit. When you plug into the top jack, R3 and R4 are in parallel. This lowers the resistance, making this a "Low" input.

R6 is what is known as a "Plate Load Resistor." This between a high voltage power supply (usually designated a capital letter and a + sign) and the plate of the tube. If this were a 12AX7 or similar tube, this would be on Pin 1 or Pin 6.

R6 is typically either 100kΩ or 220kΩ. Typically, if you raise the value of the plate load resistor, your amp will have more distortion. This does not actually mean more gain. It will just cause the tube to run at a lower voltage, thus causing break-up to occur sooner. R6 will drop the high voltage from the power supply to a lower voltage that is applied to the plate. As a signal enters the tube via the grid, this causes the plate voltage to change. This oscillation is where your signal will leave the tube.


R5 is the "Cathode Resistor." This is where the bias of the preamp is set. As this resistor's value is lowered, the cathode becomes closer to ground. This will cause the tube to be "biased-hotter." This will increase the gain of this section. As the resistor's value is increased, the tube is "biased-colder" which will decrease gain, resulting in a cleaner tone. Typical values range from 820Ω-3.3kΩ.

C1 is the "Coupling Cap." This is a very important part of the preamp. Without this, you would destroy the next tube. Remember, a cap will pass AC voltage, but not DC voltage. The DC voltage on one side of the cap will be VERY large, but the AC voltage will be quite small by comparison. The size of this cap will greatly affect your tone. As this cap's value increases, so does the amount of bass that passes through it. Typical values are from .001µf to .1µf. As you move down the signal chain in the amp (from preamp to Output section) you will usually see that the coupling cap's values increase.

C2 is the "Bypass Cap." This allows an easier place for AC voltage to pass through. DC voltage still has pass through R5. This cap is not required, but increases the amount of gain you can get from the tube. Typical values for this are 1µf - 50µf. As this value increases, you will notice more bass response.



Part 2b

Alright we have a gain stage. What's next? I will cover the standard tonestack. This is the type you'd see on a Marshall or Fender amp. It consists of 3 capacitors, 3 potentiometers, and 1 or 2 resistors.

Here is an example of a Marshall tonestack:


Here is an example of a Fender tonestack:


As you can see, they look very similar. The difference is that the Mid pot is wired as a variable resistor in the Fender tonestack, whereas the Mid pot is left as a potentiometer in the Marshall tonestack.

C1 is your "Treble Capacitor." As the name suggests, it is the capacitor that affects your treble frequencies. This will be the smallest value capacitor. Typical values range from 47pf - .001µf.

C2 is your "Bass Capacitor." This will affect the bass frequencies of your signal. This capacitor does not actually have to be the highest value cap. Usually it is the same value as the mid capacitor or larger. Typical values range from .001µf - .1µf.

C3 is your "Mid Capacitor." This will affect BOTH your bass frequencies and mid frequencies. As the value is decreased, your bass response and your mid response will increase. As the value increases, your bass and mid frequencies will be "scooped" out. Typical values are the same for the Bass cap.

R1 is the "Slope Resistor" (Thanks XGamerGt04). As its value is increased, less of the signal is sent to the bass and mid controls. This will lower the amount of bass in your amp. As this value is decreased, your amp will have more bass. Typical values are from 20kΩ - 150kΩ.

R2 is your "Treble Pot." This is where you can sculpt your tone from the outside of the amp. You are familiar with it, so I won't tell you what it does. As the pot value is increased, your amp will have more high frequencies. It is usually an audio tapered pot. Typical values range from 250KΩ - 1MΩ pots.

R3 is your "Bass Pot." It is wired as a variable resistor. As the pot's value is decreased, so is your bass response. This is usually an audio tapered pot. Typical values are from 250KΩ - 1MΩ pots.

R4 is your "Mid Pot." This is wired differently in the Marshall tonestack compared to the Fender tonestack. It does the same thing in either case. It affects the frequencies that are in between the Highs and the Lows. It is usually a linear tapered pot. As the value of the pot is increased, the tone is less "scooped" but the less affect turning the pot "up" will have. Decreasing the pot value will lower your volume and cause more of "scoop." Typical values range from 10KΩ - 100kΩ pots.

Remember: the 3rd leg of the mid pot MUST BE GROUNDED for the tonestack to perform as intended. Not doing this will render your tonestack almost useless.

R5 is the "Load Resistor." The smaller the values, the quieter your signal will be. Typical values are from 512kΩ - no resistor at all –instead a potentiometer is wired in.

A note about these tonestacks. These are passive tonestacks. You can only cut frequencies. Turning everything up will just "cut" less of your signal.

Here is a fun tool to experiment with to see different tonestacks:
http://www.duncanamps.com/tsc/download.html




Part 2c. Another gain stage.

Remember when I said there must be a coupling cap after the plate of any gain stage? Well, what you'll find most of the time is that the tone stack is used as the coupling cap. The problem is that the tone stack can lose much of the signal that was amplified in the first gain stage. We add a second gain stage, sometimes called "The recovery gain stage."
Here is an example of a preamp that we have heard several times over. It is the basic 2 gain stage preamp with a Fender-type tonestack. This is actually the schematic I used for an Ampeg build I did. I just moved the tonestack (which I'll cover later).





All the parts function the same as stated above.

R12 is the Load resistor from the tonestack earlier. Now, it is a potentiometer though. This allows you to control the amount of signal going into the grid of the second triode. The more signal there is, the more the second triode will distort. This is typically a 1meg audio tapered potentiometer.



With the tonestack situated as above, you will hear more of a difference as you adjust the controls. If you move the tonestack after the second triode, you will hear less variation in sound. Also, you might want another triode gain stage due to the signal loss associated with the tonestack.
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Last edited by end_citizen : 09-02-2010 at 12:36 AM.
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Old 10-30-2009, 01:19 PM   #3
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Here is an example of a 3 gain stage preamp with the tonestack situated between the 2nd and 3rd triode:


If you notice, the last triode does not have a bypass capacitor on its cathode. This will decrease gain and distortion in the last triode. You will already have plenty of distortion from the first two triodes, but if you want even more (which may get muddy) you can add a capacitor there.


Part 3. Single-ended output



*Note. I will be showing cathode bias until I reach section 5.



Here's DLRocket89's single-ended amp build:

http://www.ultimate-guitar.com/foru...d.php?t=1136168



Now we have a signal, but it is not amplified enough to drive a speaker. We know from our experience with tube amps we need what are referred to as "power tubes" or "output tubes." I prefer "output tubes."



Just like last time, let's start by breaking down a tube. Here is the pinout of an EL-34:






Here is the pinout of a 6L6:



Pin 1 is the suppressor grid. Pin 1 not connected to anything in a 6L6. Instead, there is an internal connection between the suppressor grid and the cathode. The pin is sometimes ommitted. This is usually attached to the cathode, or grounded. It is does as it sounds like it'd do. It "suppresses" electron flow. This makes the tube more efficient. Since nothing is really done with the suppressor except grounding/attaching it to the cathode, there isn't much variation in tone you can get from doing something with the suppressor.

Pin 2 is the first heater pin.

Pin 3 is the plate (anode). Just like the plate in the preamp tubes, this is where signal will leave the tube. It is attached directly to the primaries of the output transformer. Sometimes two or three plates can be attached in parallel to the output transformer.

Pin 4 is the screen grid. This is usually attached to a high voltage source. The point of the grid is to discourage feedback to be caused by gain. You've seen tubes that have the Pentode/Triode switch. This is achieved by connecting the screen grid to the plate. When this is done, the screen grid and plate act as one unit. Thus making the tube less efficient and operate as a triode.

Pin 5 is the grid or "control grid". Just like the grid in the triode, this is where signal will enter the tube.

Pin 6 has no connection.

Pin 7 is the other heater pin.

Pin 8 is the cathode. This is the ground connection of the tube. Unlike the triode, this can be connected directly to ground, rather than have a capacitor & resistor connected to it. More on that later.

The simplest way to amplify the signal is through a "Single-ended output section." It is quite inefficient, so it is not very powerful. The disadvantage of this is volume (to an extent) and clean headroom. Typically, a single-ended output section will produce no more than 30w. Usually you won't use more than two tubes.

Here is a schematic for a small single-ended output section



The pinout is displayed for an EL-34. This is the exact output schematic I will be using in my build thread that will accompany this thread

As you can see, there isn't a whole lot going on here. The blue wire from the transformer (all transformers I've seen use this color code) goes to the plate of the output tube. The brown wire goes to a high voltage source.

The suppressor grid is grounded.

The screen grid has a resistor (R7) in series to a high voltage source. R7 is there to prevent the tube from malfunction when the amp is cranked. It also keeps the screen at a slightly lower voltage than the plate. Typical values for R7 are from 470Ω - 1kΩ.

There is grid stopper (R8) on the grid of the tube. This helps prevent oscialation which will be discussed later. In the case of this schematic, R8 is 5.6kΩ. This seems to be a typical value.

This is a cathode biased setup. The cathode has a resistor (R9) and capacitor (C6) that are in parrallel. This is then placed in series between the cathode and ground. C6 should be a value that allows all frequencies of the guitar (lowest is ~40hz) to be bypassed. 100uf would be fine.

As the value of R9 increases, the tube is biased "colder." This will give a cleaner sound, a little less volume, and a longer tube life. As R9 decreases, the tube will be biased "hotter." Giving more volume, more crunch, and a shorter tube life. Typically you pick somewhere in between. I'll cover the math in Section 5.

R5 is safety feature. It is there in case you forget to plug the amp into a speaker, you'll notice before it burns up. I left the value of this, because this is the only value I ever use.

Of course, VR1 is your master volume control. I put a cap on the first leg of the pot. This is the coupling cap of the preamp. I just put it there to remind you that you need a coupling cap.

A note on transformer impedance: On a tube data sheet, you’ll find typical load resistances (RL) for different voltages. On a 6L6, for example, 300v will give you an RL (load resistance) of 4500 ohms. You may not have the correct transformer. However, you may be able to find something with a primary impedance of around 9000 ohms. Since this is twice the actual impedance, you can put a load 1/2 times the impedance it is rated for. The impedance will now match.

An example to help clarify: A 6L6 at 300v has an RL of 4500 ohms. You have a transformer with a primary impedance of 9000 ohms. If you wanted to use the 4 ohm output tap on the transformer, you’d use a 2 ohm speaker load to match the impedances. If you wanted to use the 8 ohm tap, you’d use a 4 ohm speaker load, and so on.

Here is how to use two or more tubes in a single ended amp:


You’ll have to connect the second control grid to the master volume, or simply connect it to the other control grid. These both are electrically the same thing. The two separate cathodes can be connected to different resistors and capacitors, but this complicates the layout. One advantage of this using two resistors and capacitors is you can bias the two tubes differently for a different tonality.

A note on impedances: When you connect two tubes in parallel like this, it is the exact same formula as connecting two resistors in parallel. This formula is 1/R=1/R1+1/R2. So, if you have two 6L6s in parallel running at 300v, you would have 1/R=1/35000+1/35000. This equals 2/35000. You then flip it over for 35000/2 which equals 17500. You’ll find whenever you connect two identical resistances or impedances, it will just be half the initial value. If you had a 6L6 and an EL34 both at 300v in parallel, you would have 1/35000+1/3500 = 11/35000. So your primary impedance should be around 3181 ohms.
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Last edited by end_citizen : 05-01-2012 at 11:43 AM.
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Old 10-30-2009, 01:20 PM   #4
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Part 4 – Push-Pull Amplification:
Now I will cover push-pull amplification. Usually any amp that uses two, four, or any other even number of output tubes will be a push-pull amp. This is a more efficient way of amplifying and most (if not all) high power amps are push-pull. This will give you much more headroom before the output tubes start breakup, and gives the potential for more volume.

The basics of push-pull amps is two identical, but opposite signals are fed into two (or four, six, etc.) output tubes. This causes the two tubes to amplify the in opposite directions. So one tube will “push” current through the output transformer, and the other will “pull” current through the output transformer.

The way these two identical signals are created is using a “phase inverter.” The most common phase inverter in modern amps is the “long tail paired phase inverter” or “LTPPI.”

Here’s a schematic of a LTPPI

R1 and R2 are just anode resistors. They do the same thing as before. 82k-220k values are common.

R4 is the cathode resistor. This sets the bias of the tube, which controls the headroom of the phase inverter & the output section. In LTPPIs, this value is usually quite low. 470 ohms to 820 ohms are quite common.

R3 and R5 are your grid resistors. These are ground references, but they are NOT connected to ground in this case. They are connected to the end of the cathode resistor. This is the “tubes ground” if you want to think of it like that. 1M is (almost) always used.

R6 is your tail. This will set how balanced the two signals actually are. Values between 10k and 50k are typical. The higher the resistor, the better the balance and stability. The lower the tail, the more the less balanced the two signals are. This may cause interesting harmonics to be produced in the output tubes.

How it works (kind of):
Signal comes in from the preamp into the first triode. This changes the amount of current being drawn through the tail, which changes the voltage of the second grid in the opposite direction. Thus, two opposite, but (more or less) equal signals are created. One is at each anode of the two triodes. You then use a coupling cap to send the signal to the next stage of the amp.

Here’s the output section of the amp:


An entire schematic for a push-pull amp with medium to high gain would look like this:



Hopefully you can piece the schematic together if it is not viewable. Here's a link to a bigger image of it.
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Last edited by end_citizen : 09-12-2010 at 11:46 AM.
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Old 10-30-2009, 01:20 PM   #5
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Now I will cover one the power supply. All those “B+” symbols on the previous examples each refer to the power supply.

The need for a supply comes from the fact that the line voltage from the outlet in your wall is AC. Not only is it AC, but it is also about 120v (in the U.S.). In an amp, you need at least two different voltages; one for you the heaters (usually 6.3v AC) and one for the plates/screens of the tubes. To obtain these different voltages, a power transformer is used.

Here’s a picture of a power transformer, and its schematic equivalent.


The black wires of the transformer are connected to your on/off switch that is connected to your power outlet. The red wires are now producing a much higher voltage (probably around 300vAC). The red and yellow wire is what is known as the “center tap”. It controls the voltage that the alternating current alternates around. It is usually connected to ground, but it is sometimes connected to a zener diode. This lowers your B+ voltage by the zener’s voltage.

The green wires are your 6.3v heater wires. They go to the heaters of all tubes that use 6.3v heaters. These are also center tapped. The center tap is usually connected to ground. Sometimes your heater supply won’t have an actual center tap. It is best to create an “artificial center tap” which I’ll cover in the layout section.

The yellow wires are your 5v heater wires. These can be used to run a rectifier tube’s heaters, or they can be rectified to create a power supply for relays, LEDs, etc.

Now that we know what’s going on with the transformer, let’s make a power supply. The first thing we need to do is rectify the AC voltage from the high voltage on the transformer. There are two ways to do this; either a rectifier tube or silicon diodes.

Here’s a diode next to its schematic symbol.



And here’s the schematic symbol of a rectifier tube. In this case, it is a 5Y3.



Rectifying turns AC signal (which swings up and down) into pulsating DC (which swings up and up). Here are three examples of rectifiers:

This is what is commonly referred to as a “full-wave” rectifier. All these are actually full wave rectifiers, but this is the one that got the name:


This is called a “bridge rectifier” or “Graetz bridge rectifier.” It is best used for transformers without a center tap.



This is a tube rectifier. It is just like the full-wave rectifier except it uses a tube. Remember that these need a heater supply. This will usually either by 6.3v or 5v.
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Last edited by end_citizen : 07-27-2010 at 02:12 PM.
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Old 10-30-2009, 01:21 PM   #6
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Once you have all the electricity flowing in one direction, you need to filter it from pulsating DC to a steady DC voltage. The way this is done with a series of capacitors, resistors, and (possibly) inductors. Let’s look at a schematic of a power supply:



The part we’re most interested in right now is what comes after the rectifier.

SW2 is the standby switch. As you can see it removes the high voltage from the rest of the circuit. This allows you to idle the tubes with just the heater voltage to them. Applying high voltage to the tubes before they have warmed up properly could lead to what is known as “cathode poisoning.” It helps protect your tube, basically.

C1 is known as your “reservoir capacitor.” This usually supplies the power to your power tubes’ anodes. You need to make sure that this capacitor is rated at no less than the voltage that it will see. So, if you are expecting 400v, get a cap rated for at least 400v. The value of the cap will affect the overall filtering of the power supply. 22uf is typical in smaller amps, but some bass amps actually use values up to 400uf (even greater for solid state designs). If you’re using tube rectification, you can NOT exceed the maximum rating for the tube (with a GZ34/5AR4 it is 60uf).

L1 is the filter choke. It is an iron core inductor. This more readily lets DC current through while blocking AC, but it needs to handle the current that will flow to C2-C5. The higher the value of the inductor (measured in Henries) the better filtering it will offer. As a general rule of thumb, if you multiply the value of the filter choke in Henries by the value of the reservoir cap in uf, you should exceed 200. (L x C > 200) L1 will inherently have some resistance. This is usually stated along with all the other information associated with the choke. This resistance will allow you to calculate the voltage drop across the choke using ohm’s law (V=IR).

C2 is another filter capacitor. This one typically is the power supply for the screens of the power tubes. You still want to keep the voltage ratings in mind.

R2 is a dropping resistor. It lowers the voltage going to the next power section and "decouples" this power stage from the previous ones. In a push-pull amp, this would normally be the capacitor supplying power to the phase inverter. In a single ended amp, this would supply power to the last preamp tube. Once again, using ohm's law (V=IR) you can roughly estimate how much voltage will be dropped across this resistor. The phase inverter is usually run at higher B+ values than the other preamp tubes (around 350v DC). Remember when calculating the voltage drop the the current from the phase inverter and the preamp tubes will be flowing through this resistor.

C3 is another filter capacitor. Much like the other two, it supplies voltage for a certain part of the circuit. In this case, this will supply the third tube in the circuit. It would probably be the phase inverter. If the amp is single-ended, then it’d go to a preamp tube.

R3 is the next dropping resistor. This does the same thing as R2. This usually goes to the preamp tubes. So, you could get by with a 2 watt resistor on this one. For extra security you may use a 5 watt.

C4 is yet another filter capacitor. This will supply voltage to the second tube in the circuit.

R4 is the final dropping resistor. This will see the least current of all the dropping resistors, so a 2 watt resistor should be perfect.

C5 is the final filter capacitor. This supplies voltage for your first tube.

So, that’s what’s going on in a power supply. You may see some circuits that use the same filter cap for a few tubes. I consider this bad practice, and I don’t think I’m alone on this issue. You may be able to get by with it in some situations, but the extra load on the capacitor may cause the two tubes to interfere with each other.



Now time to look at biasing your amp. There is a lot of misunderstanding about this feature of tubes (and transistors) among people being introduced into electronics.

As I (may have) said earlier, tubes work as follows: The plate is held at a high voltage, the cathode is held at a low voltage. Current flows from the cathode to the plate. How much current that flows is determined by the voltage of the grid. The grid’s voltage should not exceed the voltage of the cathode.

There are two types of biasing used in guitar amps. The first and simpler version is the called “cathode biasing.” This is sometimes called “autobiasing.”

The basic idea of it is that the cathode is at a positive voltage, and the grid is at zero volts. This is achieved by putting a resistor between the cathode and ground. Current flowing through the tube has to flow through the resistor as well. This causes a difference in voltage between the cathode and ground. The grid is held at zero volts through a large resistor. Since there should be no current drawn by the grid, the grid should be the same voltage as ground (which is zero volts).



C1 and C2 are just the coupling caps coming from the phase inverter.

R3 and R4 are grid resistors. They simply keep the output tubes from picking up radio frequencies. The do more than that, but that’s the gist.

R1 and R2 are what keep the grid resistor at zero volts.

R5 and C3 are where this particular amp is biased. R5 is what causes the voltage difference between the cathode and ground. This varies with what tube you are using. I’ll give you a list of what is typical for which tubes below. C3 functions like the bypass caps on a preamp tube. It keeps the voltage on the cathode near constant. This allows the amp to give full power at all frequencies.

The advantages of cathode biasing are the ability to just plug the tubes in and they’ll practically bias themselves and the tubes tend to last longer this way. The disadvantage is that it is less efficient. You won’t be able to get as much power (which equals headroom) out of a cathode biased amp.

The other way of biasing an amp, called “fixed bias,” is what is most often found in more powerful amps. Most push-pull amps use fixed biasing. Like I said earlier, a tube operates as follows: The plate is held at a high voltage, the cathode at a low voltage, and the grid is held below the cathode. The plate voltage changes when the difference in the grid and cathode voltage changes.

Fixed biasing puts the cathode at zero (or near zero) volts. A negative voltage is applied to the control grid.
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Old 10-30-2009, 01:22 PM   #7
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This is what an example of what a fixed bias supply looks like:



There are numerous different circuits to generate a negative bias voltage, this one is used by AX84. They all consist of at least one filter capacitor, a diode, a voltage divider, and some current limiting resistor.

C1 and C2 do what the filter capacitors in the power supply do. They keep the voltage more constant. NOTE: C1 and C2 have their anodes (the positive end) facing ground. This is because ground is more positive than the negative bias voltage. Reversing the polarity can result in fire!

The diode facing the rectifier is what keeps this a negative voltage. Turn this around and you will destroy some tubes.

R1, R2, and VR1 make up a somewhat complicated voltage divider. VR1 allows you to vary to the voltage going to the tubes. This is where you actually set your bias.

R3 keeps your power supply caps separate and limits the current flowing in this circuit.

R4 is also there to limit current in the bias circuit.


And the output section would now look like this:



The biggest difference is that resistors R16 and R17 are now connected to V- instead of ground.

Notice that the cathodes of your output tubes are grounded in this case. You can also put a small resistor (Usually 1 ohm) between the cathode and ground to measure voltage and convert this to current. This is helpful when biasing the amp.

Advantages of fixed biasing include the ability to control how close the tubes are to break up. This may result in a more desirable to you. Also, if you're using an Octal tube in the KT family (KT88, KT77, KT66, 6L6, 6v6) you can adjust the bias accommodate a different set tubes from the same family. Just make sure your power transformer can handle the tube swap in terms of power rating. Also, adjust the impedance in accordance with the different Ra-a rating of your new tube.

There are dangers of fixed biasing. You can cook your tubes if you don't have the bias set properly. Also, you will have to spend time explaining to people why "fixed bias" is NOT service-free as Fender would have you believe. Also, when recapping a bias circuit, you must bear in mind the polarity is opposite of all other caps in the circuit.


Many have asked, "How do I properly bias my amp?" If you have a fixed bias amp, this is a relevant question. The same tube may act very different at 350v when compared to 250v. So, I tend not to rely on the old adage that "Just get the same brand you had in there before. You'll have no problem" mentality.

First, you must know the power rating of your tube. I'll use a 6L6 for example. A 6L6 is rated for a maximum plate dissipation of 30 watts. If I know the plate voltage to be 287 volts, the maximum amount of current to flow through the plate is 104 milliamps. I get this by the formula P=IV. P is power, I is current, V is voltage.

It's easy to measure voltage, but current is a bit tricky. One popular method is to put a low tolerance 1 ohm resistor between the cathodes of the output tubes and ground. Each millivolt across the resistor is equal to one milliamp through the cathode. The problem with this method is that the screens also draw current as well. So not all the cathode current is flowing through the plate as well, but most is.

Another method is called the "Transformer shunt" method. This method can be very dangerous if proper precautions are not taken. You set your meter to read current. Place one lead at the output transformer's center tap and place the other lead at the plate of the tube(s) you are measuring. The danger with this is that when a meter is set the read current, the two leads are shorted together. So if you're positive lead is placed at 560v, you're negative lead is also at 560v. The problem with this method is that it assumes that all of the current is flowing through the meter rather than the output transformer. The resistance through the meter is low, but not zero. This will ensure that some current is still flowing through your output transformer.


The purpose of the two preceding paragraphs is to warn you that you should not take you're meter's word to be infallible. Always err on the side of caution. I tend to bias all Fenders that come through my shop at 75% efficiency unless the owner wants their tube to run hotter.
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Old 10-30-2009, 01:22 PM   #8
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So we will finally move into the more interesting of topics. You’ve read some very basic info about how tubes work and what makes what sound. Now we will use this knowledge to put an amp together. I don’t have any builds going on as I write this section so I will work from a program called Visio. It is a Microsoft program and is typically used for different purposes than layout for tube amps.
The circuit will have two gain stages with a tonestack in between. Push-Pull with a long tail paired phase inverter.

Here are some general rules of amp building:
1. Keep your grid wires as short as possible and keep the grid wires away from the plate wires. Preamp stages are bad about picking up stray signals.
2. Try to ground each section in blocks. This means the filter capacitor, the cathodes and the grid resistors. This minimizes potential ground loops as well as making the layout more aesthetically pleasing.
3. Don’t run multiple grounds to the same point. This is very likely to cause annoying low frequency hum that is associated with a ground loop.


I know. I'm jumping around now. School is almost out, and then I should be finished with the Visio layout.

Here's list of places to get amp parts. Let me know if I'm missing anywhere worth mentioning. (Non-US people I need your help. I don't know where to buy outside of the states.):

Antique Electronics, not the best price on everything, but they've got everything you need to build an amp yourself.

Watt's Tube Audio, not the widest range of goods, but they sell custom sized turret board material and have some of the coolest knobs.

Tube Depot, mostly just tubes. They have a selection of various parts as well.

Ted Weber, known for their speakers. They have tons of amp parts, but the quality is not always the highest.

EdCor, just transformers. Great quality and good prices, but everything is made to order. So expect at least two weeks lead time before you get your transformer.

UK Dealers thanks to GABarrie

www.ampmaker.com - Highly recommended retailer, sells kits as well as components and gets all his transformers custom wound from a British company and they sound really good

www.valvepower.co.uk - Very basic Marshall 18W based cage amp kits, also sell the components individually

www.bluebellaudio.com - Not the cheapest (note: prices do not include VAT) but a wide range of chassis's, hammond transformers (only UK source for hammond chokes I know of outside of mouser) and other bits

www.solsound.com - Semi-reasonable prices, but good source for reverb tanks, the only place I've found a non-pcb reverb driver and where I get my grill cloth and tolex

www.watfordvalves.com - As you might guess they focus on valves (tubes for those of you state-side) but they have a few other bits on there, including reverb tanks and a good range or high end capacitors

www.hotroxuk.com - Valves and Speakers, fully set up for both digital and analogue testing of valves. Your best chance for the more obscure valves, particularly the pentode/triode ones

www.karltone.co.uk - Valves (mostly JJs) and speakers, also fully test their valves. The real value lies in their tested sets on their eBay shop (http://stores.ebay.co.uk/karltone-guitar-accessories)
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Old 10-30-2009, 01:57 PM   #9
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Someone correct me if I am wrong, but I am pretty sure that R1 in the Fender and Marshall type tone stacks is referred to as the Slope resistor as it changes the frequencies the mid control affects.
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Old 10-31-2009, 03:43 PM   #10
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I'll enjoy reading this later. Stuck for me.
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Old 11-03-2009, 01:54 AM   #11
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Nice, very easy to understand.
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Old 11-03-2009, 05:38 PM   #12
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*personal sticky*
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Old 11-04-2009, 03:52 PM   #13
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Might want to note on the last diagram the lack of a grid stopper resistor on the first triode.

The lack of grid stopper coupled with the high gain setup of the Trainwreck design create a somewhat unstable circuit. Most designs you will see a grid stopper there. The grid stopper along with the tube's Miller capacitance create a low pass filter. This prevents the tube from oscillating in a frequency you can here, if that happened.

To find cutoff frequency on cathodes...
F = 2Pi(R)(C) [resistance in ohms, capacitance in Farads]

edit: C1 treble cap, Vox uses a 47pF there. Think the range should be lower. Vox dosent use a typical "stack" though...
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Old 11-04-2009, 04:10 PM   #14
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Thanks kurt, I realized I completely left out the grid stoppers and I was trying to think of an easy way to explain what they do.
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Old 11-04-2009, 04:18 PM   #15
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This is exactly what I have been think of finding over the past few days.

Would you be bale to add some places to get amp kits in the uk?


Ta!
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Old 11-04-2009, 07:59 PM   #16
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Sorry I didn't get back to you w/ commments, life has been nuts lately. I'll get something back to you soon though!
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Old 04-17-2010, 10:31 AM   #17
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Here's most of the links I've acquired on valves, thought I'd share them as a little bit of a resource on tubes info. As far as where to buy tube parts, Invader Jim has been kind enough to update the resource thread with a ton of sites for amp/pedal making parts.

Safety Tips
How Vacuum Tubes Work
Tube Data Sheets
More Tube DataSheets
Geofex Tube Amp Info
World Tube Audio Portal Links to many a sites.
Output Transformerless Valve Design
TubeCad SRRP Stages This was a little above my head when I was first getting into tubes, but should be nice.
Technical Books Most old books are in public domain ( can be downloaded legally) Thought I'd share this.
Audio Articles
Tone Lizard
TubeFreak
Determining the Plate to Plate Impedance Sorry mods, this is the only info I could find for determining the plate to plate z if the data sheet doesn't cover it.
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Old 04-17-2010, 05:24 PM   #18
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^i acquired better tube datasheets just searching on google (ie. search "12AT7 datasheet") i get PDF datasheets from sylvania or phillips. they're very useful and i think its explained better than the duncanamp tube search engine.

they have graphs i can enlarge and print out too! bigger, clearer load lines!


this website is great for tube datasheets too:
http://www.drtube.com/tubedata.htm
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Old 04-17-2010, 05:31 PM   #19
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http://www.triodeel.com/tubedata.htm

Also a good tube data site. Scans of actual manuals, I always use it. Blandguitar's first tube data link also looks very good.

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Old 04-29-2010, 06:33 PM   #20
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Shot, google is great, yet so few people utilize its usefulness.

Mike, you can call me Jared.

I've got a question concerning output impedances. Say I decided to use two different types of output tubes. Let's assume I'm using 2 6L6GCs and 2 6V6s, am I able to divvy up the output impedance as I would resistors in parallel? Meaning 1/R= 1/R1 + 1/R2.... ect for the plate to plate impedance?

edit: I also just got matched tubes in, the numbers read 17/2.4 what does that mean? Is that the transconductance or something else?
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