Have you ever looked at the guts of a guitar amplifier and wondered what all those parts do? Well, I'll walk you through the signal flow and discuss the components in this very simple but great sounding 1950's Fender 5F1 Champ guitar amplifier. Once you understand the simple 5F1 you'll be able to understand more complicated amps. 5F1 was Fender's internal model code for the 1950's tweed Champ. Although this page discusses Guitar tube amps everything here applies to audio stereo tube amplifiers too with the goal of distortion prevention in audio amps being the biggest difference.
Volume control on top, Circuit Board inside, tubes on bottom: V1 Preamp Tube on right, V2 Power Tube in center, V3 Rectifier Tube on left. The Power Transformer and Output Transformer are attached to the other side of the chassis.
: A tube amplifier chassiscontains lethal high voltage even when unplugged--sometimes over 700 volts AC and 500 volts DC. If you have not been trained to work with high voltage then have an amp technician service your amp. See more tube amplifier safety info here.
Push Pull (pp) El84 Tube Amplifier Schematic (ecc83 Input)
We'll start with the amplifier layout diagram. If things get too cluttered you can refer back up to this clean diagram. The guitar input jacks are at the upper right, the circuit board is in the center, the power transformer (PT) is on the left and the tubes and speaker jack are at the bottom. The output transformer (OT) is not shown but OT In are the output transformer primary wires and OT Out are the secondary wires.
I added component numbers to this layout that match the schematic diagram below. Compare this to the picture of the chassis above. I have had questions about the grounding scheme shown in this layout. Grounding the V2 power tube grid leak (R9) and the power tube cathode resistor (R8) and cathode bypass cap (C6) to the first filter capacitor's ground (C3) is best practice and should result in a quieter amp. Click the image to download the pdf.
Signal flow is shown above and below (orange arrows above, fat red line below). Signal from the guitar enters at upper right guitar Input Jack 1 or 2 and flows down to the circuit board and then to the preamp tube V1A at bottom right where the signal goes through its first stage of amplification. The signal then goes up to the circuit board and on to the volume control at top center, then back down to tube V1B (the second half of the first tube) for its second stage of amplification. From there the audio signal goes back to the circuit board then down to the power tube V2 for the third stage of amplification. V2's output goes out the blue wire to the output transformer (not shown) for a current boost, then from the output transformer via the green wire to the speaker jack (to the right of V2) and on to the speaker. I have added component numbers to the layout diagram above that match the schematic below.
Mze Electroarts Entertainment
The signal flow in red seems much simpler on the amp's schematic. Component numbers match the Layout diagram above. V1A is one half of tube V1, V1B is the other half. Voltages shown are approximate. Click the image for the full size schematic. Click here for the clean schematic.
The next few paragraphs will help you visualize the flow of electrons through simple circuits. Learning to visualize the flow of individual electrons was a breakthrough for me in understating tube amplifier electronics.
Electric guitars generate an alternating current (AC) audio signal. The guitar's pickups are small electric generators. Pickups have magnets (poles) that magnetize the metal guitar strings. The movement of the magnetic field surrounding the magnetized strings generates electricity in the pickup's coil. The coil is simply a thin insulated wire wrapped around a spool and when a magnetic field cuts through a coil of wire it generates an electric voltage (electronic pressure) and current (electron flow) in the coil's wire.
W Single Ended Class A Stereo Tube Amplifier
The black and white wires leaving the guitar pickup are the two ends of one long coil wire. A humbucker pickup is simply two of these coils connected end-to-end (in series).
The Pickup on the left is a wire coil that generates the guitar signal. The Tone Control bleeds high frequencies to ground. The Volume Pot is wired as a variable voltage divider. The Volume Pot bleeds guitar signal to ground to lower the guitar's output volume.
I've been asked many times, What is voltage? It's pretty simple really. Like electrical charges repel the same way like magnetic poles repel. So if you cram a bunch of negatively charged electrons together onto the metal plates of a battery their negative charges repel one other--they want elbow room. The tighter they are packed together the higher the pressure so I like to think of negative voltage as electron pressure.
How Tube Amps Work
A quick note about 'Conventional Current Flow.' In electric circuits negatively charged electrons actually flow from the negative '-' battery terminal to the positive '+' terminal. That's right, the electricity in your car flows from the battery's - terminal through the ground wire, through the car's body, through the radio's ground wire to the radio and then through the positive power wire back to the battery. The problem is that Benjamin Franklin guessed wrong on the direction of electrical flow so conventionally we think of electricity as flowing from + to -. People say electric current flows + to - (conventional current flow) but electrons flow - to +. With tube electronics it's easier to think in terms of how the electrons are really moving in order to understand them.
If you connect a wire across a battery's terminals the jammed together electrons in the negative terminal see the wire as a pipe with lots of room so they flow down the wire. When you have an 'excess' of electrons tightly packed together you have negative voltage. When you have a 'scarcity' of electrons, or electrons are pulled apart from one another, you have a positive voltage.
When I think about a wire with very high positive voltage on it I imagine the wire as an empty pipe with very few electrons in it with lots of 'elbow room' so electrons really want to flow into that wire. Ground or earth represents an unlimited supply of electrons at zero volts (or neutral voltage). Touch that high voltage wire wire to a ground and the electrons hanging out there will rush in to fill the void of electrons in the wire. Voltage is the force of electrons wanting to move from a conductor crowded with electrons to a conductor with fewer electrons and more elbow room . Current is the measure of how many electrons are flowing through a conductor--the more electrons flowing, the higher the current. Keep this in mind when thinking about amp circuits, high voltage is an extreme scarcity of electrons and ground represents an unlimited supply of electrons.
Construction Of A Guitar Amplifier
As a guitar string vibrates it moves one way and generates a positive voltage in the pickup coil, then as the string reverses direction the voltage is reversed and a negative voltage is generated . This occurs with every string vibration so an alternating current (positive-negative-positive-negative. . .) makes up the audio signal put out by the guitar. I'll repeat that because it's a very important concept, as the guitar string moves one direction over the guitar pickup coil it generates a negative voltage (excess electrons), then as the string reverses direction the electron pressure (voltage) and electron flow (current) reverses too and a positive voltage is generated (scarcity of electrons) and this repeats with every vibration of the string creating an Alternating Current (AC) electrical signal. This is why guitar audio signals are AC, or alternating current--as the strings alternate their direction of travel the signal voltage alternates between + and -. This tiny little AC signal is what the guitar amp will amplify until it's strong enough to move a speaker cone in and out. The speaker cone alternates in and out with the alternating current from the guitar's pickup coil. For every guitar string movement there is a corresponding speaker cone movement .
An AC guitar audio signal on a wire alternates between positive and negative voltage. A negative signal voltage packs electrons closer together (excess of electrons = negative voltage). The positive half of the AC guitar signal pulls electrons apart and creates a scarcity of electrons. Remember, voltage = electron pressure.
If you graph a guitar audio signal the pitch of the guitar string's sound is expressed as wave spacing (frequency) and loudness is expressed as wave height (amplitude). A high frequency sound will have tight wave spacing and a low frequency sound will have wide wave spacing. In the graph below the high E string is on the left and the low E is on the right. A quiet sound will have short waves and a loud sound will have tall waves.
Hifi Stereo Tube Amp Schematics
The direct relationship between string movement and electricity generated in
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