Amplifier Circuits

Part of Vacuum Tubes

Amplifier circuits translate a tube’s fundamental properties into practical devices that boost weak signals to usable levels.

Why This Matters

A vacuum tube amplifier circuit converts a small, weak signal — the output of a microphone, the output of a detector stage in a radio receiver, the signal from a telephone line — into a larger signal capable of driving a speaker, transmitting over a cable, or feeding the next stage of processing. The circuit surrounding the tube determines how much gain is achieved, how much the signal is distorted, and how stable the amplifier is under varying conditions.

Building communication equipment from vacuum tubes means building amplifier circuits repeatedly. Every radio receiver has at least one audio amplifier stage after the detector. Every microphone circuit needs preamplification. Every transmitter modulator contains an audio amplifier chain. Mastering the common amplifier configurations — common cathode, cathode follower, and differential — gives you the tools to build any audio or low-frequency circuit needed for community communication.

The tube amplifier topologies developed between 1910 and 1960 are well-documented, extensively tested, and thoroughly understood. They work with the tubes available from salvage — triodes, pentodes, and multi-section tubes — without any modern semiconductor components. A builder with basic electrical knowledge, a handful of tubes, and some passive components can construct a functional amplifier from first principles.

The Common Cathode Amplifier

The common cathode amplifier is the workhorse configuration, analogous to the common emitter transistor amplifier. The cathode is the common terminal between input and output (connected to ground through a bypass capacitor). The input signal drives the grid. The amplified, inverted output appears at the plate.

To build a common cathode amplifier: connect the plate to the positive supply through a load resistor (the plate resistor). Connect the grid to the signal source through a coupling capacitor, and bias the grid through a large resistor to a negative voltage (or use cathode bias — see below). Connect the cathode to ground, either directly or through a bypass capacitor in series with a cathode bias resistor.

Cathode bias is the preferred method for simplicity. Insert a resistor between the cathode and ground. Current flowing through the tube also flows through this resistor, producing a voltage drop that makes the cathode positive relative to ground. Since the grid connects to ground through the grid resistor, the grid is negative relative to the cathode — exactly the bias required. The cathode bypass capacitor short-circuits this resistor for AC signals, preventing negative feedback that would reduce gain.

Calculate the cathode resistor value: divide the required grid-to-cathode bias voltage by the expected plate current. For a 12AX7 operating at 1mA with a bias of −1.5V, the cathode resistor is 1.5V / 1mA = 1500 ohms. Bypass with a capacitor whose reactance at the lowest frequency of interest is much less than 1500 ohms. At 100 Hz, a 10 microfarad capacitor has a reactance of 159 ohms — adequate for most audio applications.

Setting the Operating Point

The operating point (or quiescent point, Q-point) defines the tube’s DC conditions in the absence of a signal. Choosing it correctly ensures the amplifier can swing symmetrically without clipping. The Q-point must sit near the middle of the tube’s load line.

Draw the load line on the tube’s characteristic curves (plate current vs. plate voltage for various grid voltages). The load line extends from the supply voltage on the horizontal axis (at zero current) to the supply voltage divided by the plate resistor on the vertical axis (at zero plate voltage). The intersection of this line with the characteristic curve at the chosen bias voltage is the Q-point.

For a clean, low-distortion amplifier, choose a Q-point where the characteristic curves are evenly spaced on both sides — indicating equal behavior for positive and negative signal swings. Near the bottom of the curves (low current, low plate voltage) the curves bunch together, indicating high distortion. Near the top (high current, high plate voltage) the curves may be cut off by the maximum plate dissipation line.

A supply voltage of 200-300V with a plate resistor of 100 kilohms works well for a single 12AX7 stage. At 2mA plate current, the resistor drops 200V, leaving roughly 100V at the plate. The load line intersects appropriate operating conditions for moderate gain and low distortion.

Pentode Amplifier Stages

Pentodes offer much higher gain than triodes because their additional screen grid shields the control grid from the plate. This shielding raises the plate resistance dramatically, allowing the pentode to approximate a constant-current source — ideal for driving high-impedance loads or achieving maximum voltage gain.

A pentode amplifier circuit resembles the triode common cathode configuration with two additions: a bypass capacitor from the screen grid to the cathode (or ground), and a screen supply resistor from the B+ supply to the screen grid. The screen bypass capacitor is critical — an unbypassed screen grid causes massive negative feedback that destroys most of the pentode’s gain advantage.

The screen grid typically operates at 50-75% of the plate voltage. Use a resistor voltage divider from the plate supply or a separate lower-voltage supply. Screen grid current is typically 10-20% of plate current, so calculate the screen resistor accordingly. A 100V screen supply from a 250V B+ through a resistor: if screen current is 0.5mA and screen voltage should be 100V, the resistor drops 150V at 0.5mA, requiring 300 kilohms.

Pentode amplifiers achieve voltage gains of several hundred from a single stage — a 100mV input signal produces 30-50V output. This makes them excellent for the IF (intermediate frequency) amplifier stages in superheterodyne receivers, where high gain with narrow bandwidth is needed. The high plate resistance of pentodes also makes them easy to couple to tuned circuits.

Coupling Between Stages

A multi-stage amplifier requires coupling the output of one stage to the input of the next. Three coupling methods are common: RC coupling, transformer coupling, and direct coupling.

RC coupling uses a coupling capacitor in series between the plate of one stage and the grid of the next, with a grid resistor to ground on the receiving stage. The capacitor blocks DC (preventing the high plate voltage from reaching the next grid) while passing AC signals. Choose the coupling capacitor so its reactance is small compared to the grid resistor at the lowest frequency of interest. For a 1-megohm grid resistor and a low-frequency cutoff of 20 Hz, the required capacitor is about 8 microfarads. A 10 microfarad value works well.

Transformer coupling uses an audio transformer between stages. The transformer provides impedance matching, DC isolation, and can provide voltage step-up if wound with a higher secondary-to-primary turns ratio. Transformer coupling was common in early radio equipment and telephone amplifiers. Its disadvantages are bulk, weight, and frequency response limited by the transformer’s inductance at low frequencies and winding capacitance at high frequencies.

Direct coupling connects the plate of one stage directly to the grid of the next without a capacitor. This eliminates low-frequency rolloff and is required for amplifying DC or very-low-frequency signals. The challenge is that the plate voltage (typically 100-200V) is too high for the grid of the next stage. Solutions include using a different tube in the next stage biased for high grid voltage, inserting a large resistor divider, or using a more complex long-tailed pair (differential) topology.