Tube Amplifiers Explained, Part 4: Big Picture of the Circuit
Updated: May 7, 2020
Part of a blog series Tube Amplifier Circuits Explained
We will go into detail on each part of the circuit, but for now, let’s get up on the balcony and take a look at the big picture. Here you will see the Analog Ethos AE1 Kit amplifier circuit that is used for this blog series.
Refer to the schematic, and you’ll see one section is the power supply and the other is the amplification portion with two stages, driver and output. The driver and output stages are duplicated in the actual amplifier, one for left and one for right channel, but only one is shown for simplicity. There is, however, only one power supply for the circuit.
A later section will go over the power supply in detail, but for now, just know that the purpose of this portion of the circuit is to take AC voltage from your wall outlet (“mains” voltage) and convert it to high voltage DC, referred to as “B+”. This is a historical term from when batteries were used as power supplies and this was the positive voltage. You may also see this referred to as High Tension or HT, also an older terminology. In our case, in comes the household 120V AC (US), and out goes the B+ of about 420V DC. You’ll see later why we need this high of voltage.
The arrow on the right-hand side illustrates that the B+ is an output of the power supply circuit. That same arrow is shown in two places in the amplification part of the circuit (driver and output stages), indicating that the B+ voltage is used to supply power to two parts of the circuit. See those spots? Just a convenience in showing the power supply and amplification circuits separately instead of a single schematic.
In the driver and output stage, you can see the diagram flows from left to right, with the input signal entering on the left, the 12AT7 tube is a triode vacuum tube that performs a first level of amplification ("input" or “driver” stage), then the EL34 is a pentode vacuum tube that performs a larger (“power” or “output” stage) amplification. We will also go over the details of these tube types and how they work. Finally, there is a transformer to convert back from high voltage to something usable for the output of the amplifier that will go to your 8 ohm speakers.
Next: my favorite part of the series, How a Vacuum Tube Works!