Amp primer

The preamp

Reminder : The articles in the Amp Primer section are designed to give the novice an understanding of what does what in a tube guitar amp and how the various components are usually connected together. A whole book could be written about the subject but the goal here is to keep it simple and easy to digest.

Ok, so it's finally time to start learning about tubes! A vacuum tube is a glass envelope with elements inside that are connected to metal pins that stick out of the bottom. The air is sucked out of it, hence the term "vacuum".

The simplest tube used in audio signal amplifiers is called a triode . The "tri" prefix indicates that there are 3 elements in the tube : the plate, the grid and the cathode.

The most common preamp tube used in guitar amplifier preamps is the 12AX7. It is a dual triode, meaning there are two triodes inside of it. It has nine pins, 3 of them are connected to the tube's filament, the other 6 are the plate, grid, and cathode of the two triodes inside the glass envelope. It helps when working on amps to commit to memory that pins 1,2,3 and 6,7,8 are the plate, grid, and cathode of each of the 2 triodes. The usual way that the filaments are connected is one leg of the 6.3V filament supply goes to pin 9 and the other leg goes to pins 4 and 5.

Below is the schematic of the preamp portion of a basic tube amp : The Fender Champ (5F1 circuit)

See the complete schematic : "Tweed" Champ 5F1

Hi-lo jacks

Guitar cords use 1/4 inch phone plugs like the one on the right. The diagram below shows the Hi-Lo jack wiring of the 5F1. It does not show a connection for the "ring" portion of the guitar cord's phone plug because they assume that you know that it is connected to ground by virtue of it being connected to the chassis.

The input jacks on this amp have a built in switch that is normally closed (which grounds the input grid, silencing the amp when nothing is plugged into it) but is open and is not connected when the plug is inserted into the jack. The switches are labeled "SC1" and "SC2".

High sensitivity

When an instrument is plugged into the hi sensitivity jack, SC1 is lifted from the tip connection of the jack and the guitar signal travels across both of the 68K resistors and arrives at the grid of the first gain stage in the amp.

A resistor in series with the audio signal and the grid of a tube is called a grid resistor or sometimes a grid stopper. ( For an explanation of the purpose of grid resistors see the article on Randall Aiken's site : Grid Resistors - Why Are They Used?) The 2 68Ks in parallel are the grid resistors in the circuit above (2 of the same value resistors in parallel have the resistance of half of one of them, so the grid resistor is in effect 34K.)

The 1 megaohm resistor, R3, is also connected to the tip connection, but is connected to ground. This is called the grid load resistor. It provides a reference to ground for the triode, and must be present for the tube to work.

Low sensitivity

When an instrument is plugged into the lo sensitivity jack, SC2 is lifted from the tip connection of the jack and the guitar signal travels across the grid resistor, R1 and arrives at the grid of the first gain stage in the amp. R2 is now the grid load resistor, as it is connected to ground through SC1.

First gain stage

The guitar signal, a small AC voltage, is input at the grid of the first gain stage and appears at the plate of the triode with it's voltage amplified approximately 100 times in a 12AX7 ( well, not really, here's why : TUBES: it’s RP, not MU, that's important! ).

The 100K resistor, R5, is called the plate load resistor, or plate resistor. It is connected to the power supply and delivers the high voltage (approximately 200V DC give or take 50V)

C1 is called the coupling capacitor, or blocking capacitor ( this one is a .02 microfarad rated to withstand up to 600V ). The amplified guitar signal travels across this capacitor, but the B+ high voltage does not because DC cannot pass through a capacitor. After going through the coupling cap, the signal goes to the 1 megaohm potentiometer, P1. This is the volume control.

a potentiometer is a resistor with an element called the "wiper" that slides along the resistive element as the shaft is turned. On the "pot" at the right, when the shaft is all the way counter-clockwise, there is no resistance between points A and B, and 1 meg of resistance between B and C. When it is all the way clockwise, there is no resistance between B and C, and one meg of resistance between A and B. When it is somewhere in between, the resistance between A and B + the resistance between B and C = 1 meg.

P1 in the schematic above is wired as a variable voltage divider. A voltage divider produces an output voltage that is a fraction of it's input voltage. In the circuit above, when the volume is all the way down, no signal flows to the grid of the second gain stage because the grid is at ground. As the volume is turned up, the wiper allows more and more signal to pass until at maximum volume the maximum signal is allowed to pass and the circuit effectively is like this:

If the amp were hard wired this way, with the signal coming off of C1 and going straight to the second gain stage, the 1 meg resistor would be the grid load resistor for the second stage. This illustrates an important point, that often there will be no discrete grid load resistor, but a volume pot between stages will provide the necessary reference to ground that the triode needs to operate, although this reference will be varied depending upon the setting of the volume knob.

Second gain stage

The signal comes off of the volume pot and enters the second gain stage ( which is the other half of the lone 12AX7 in this amp) at the grid. R6 is the cathode resistor, R7 is the plate resistor, and C2 is the coupling cap that blocks the high-voltage DC and allows only the AC audio signal to go to the grid of the 6V6GT power tube. The new element in this stage is the negative feedback loop.

The negative feedback loop takes the signal from the output of the amp at the OT secondary (where the speaker is connected), sends it through the 22K feedback resistor, R9, and injects it back into the preamp at the cathode of the second gain stage. The result of doing this is a decrease in distortion in the output stage and improved stability (it also results in slightly lower output). Not all guitar amps have a negative feedback loop, but most of them do. The higher the value of the feedback resistor, the less negative feedback that is put back into the circuit. It is called negative feedback because the signal that is input back in is 180 degrees out of phase with the stage it is being injected into. If it were put back into the amp at the cathode of the first gain stage of the 5F1 champ it would be positive feedback and would squeal uncontrollably, like it would if you held a microphone right next to the speaker of an amp.

When an audio signal goes through a voltage amplifier stage, such as the two preamp stages in the 5F1, the amplified waveform will be 180 degrees out of phase with the input waveform.

If this audio signal was input at the grid of a tube...
Then this would be the shape of the 180 degrees out-of-phase output waveform that would appear on the plate of that tube, although the voltage would be amplified.

Another amp

Lets leave the 5F1 topology and look at the preamp of another classic amp. the 5F6-A Bassman.

See the complete schematic : "Tweed" Bassman 5F6-A

This amp has the Hi-Lo input jacks like the Champ, but there are two sets of them because there are two channels. It was common in the 50's and 60's to have 2 channels (or more) on a guitar amp because it allowed more than one instrument to use the amp at one time and it allowed manufacturers to offer more features without adding that many more components. The first gain stage of each channel uses 1/2 of a 12AY7. The 12AY7 is a nine-pin miniature dual triode, like the 12AX7, but it has less than half the gain of that tube. The cathodes of the 12AY7 share the same cathode resistor and it is bypassed with a 250 microfarad(U) capacitor ( I think the schematic has a misprint, it reads 250-6, but it should read 250-60, meaning the cap is rated at 60 volts). A cathode bypass cap is often seen on preamp tubes in guitar amplifiers. Their purpose is to increase the gain somewhat and they also can affect the frequency response of the stage. A 250U cap is pretty large, usually you see between .68U and 25U in most amps. This large of a cap makes the signal very bass heavy, but this was intended to be a bass amp after all.

The next thing in this circuit that is new to us is the .0001U (aka 100 picofarad) capacitor connecting the wiper and the input side of the bright channel volume pot. This is known as the "bright cap". It acts as a high-pass filter to add a brightness to the signal. A high-pass filter allows higher frequencies to pass while attenuating frequencies below a cutoff frequency, thereby emphasizing the higher frequencies. The higher the value of the capacitor, the higher the cutoff frequency and the "brighter" the signal will be. Common values for bright caps range from 100p to 500p, although smaller and larger values are sometimes seen. I like to put a switch in my amps that lets you choose between 100p, 500p and no bright cap, like I did in this amp: 6V6 Plexi. As the volume is turned up the bright cap has less of an effect, and when it is all the way up it is shorted and has no effect at all.

The two 270K resistors that connect each channel to the grid of the second stage are called the "mixer resistors". This is a typical value for a mixer resistor, Most popular vintage amps (tweed and blackface Fenders, 4 input Marshalls, AC30s, vintage Gibsons, Ampegs etc. ) have mixer resistors that range from 220K to 470K.

The mixer resistors feed the second gain stage, which is 1/2 of a 12AX7. this stage does not have a coupling cap on the plate of the tube, rather, the plate is wired directly to the grid of the third stage because the third stage is a cathode follower. The cathode follower gets it's name because the output is taken from the cathode instead of the plate, and the output signal is in phase with, or "follows" the input signal. The voltage gain of a cathode follower is unity, meaning there is none. So why is it used? It provides a low impedance signal to drive the tone stack. Many amps do not have cathode followers, but those that do typically connect them as in the circuit above : 1) There is no plate load resistor, the plate is connected directly to the power supply, 2) The signal input at the stage's grid comes directly off of the preceding stage's plate with no coupling cap between the two stages, and 3) The output signal is taken off of the stages cathode.

Final thoughts

The above examples should give a good understanding of how the common components in a tube guitar preamp are connected together and their basic function. If you are not clear on the meaning of the following terms : plate, gird, cathode, plate resistor, coupling cap, grid resistor, grid load resistor, grid stopper, cathode resistor, cathode bypass cap, voltage divider, potentiometer, wiper, negative-feedback loop, feedback resistor, bright cap, high-pass filter, mixer resistor, and cathode follower, then you should review the material because these items are the basic vocabulary that you will need in order to learn more. Yes, there are a number of things that I left out in order to keep it simple, such as reverb circuits, vibrato, effects loops, channel switching, etc. I will have articles on this site about those topics at some future date, but I just want to cover the basics first. Lastly, If there are any errors in the above lesson, please bring them to my attention, as it is important to me that this material be as accurate and error free as possible.

Now, quit foolin' around on the internet and go play your guitar!

Tube amps contain lethal voltages and should not be worked on by those unfamiliar with the proper safety precautions that should be taken when working on them.