Tube Bias Calculator

Plate Voltage (V)

Max Plate Dissipation ($P_{p,max}$ in W)

Desired Bias Percentage (%)

Calculated Values

Target Plate Current ($I_{p}$):

Recommended Cathode Resistor ($R_k$):

Target Cathode Voltage ($V_k$):

This calculator provides a starting point for cathode bias circuits. The recommended cathode resistor is a simplified calculation based on a common assumption that the target cathode voltage should be approximately 5% of the plate voltage. For optimal performance, always measure the actual plate current and adjust components as necessary.

What Is Tube Bias?

Tube bias refers to the operating point of a vacuum tube in your amplifier. In simple words, it’s how much current flows through the tube when it’s idle.

Too cold (under-biased): The amp may sound thin, harsh, or lack warmth.
Too hot (over-biased): You risk tube overheating, distortion, or even tube failure.
Just right (optimal bias): You get rich tone, controlled heat, and longer tube life.

Biasing is not just about tone—it’s also about reliability and safety. That’s why every amp owner, builder, or repairer should care about it.

Why Use a Tube Bias Calculator?

Traditionally, setting bias required manual calculations with Ohm’s Law and plate dissipation formulas. While experienced techs can do it quickly, beginners often find it confusing.

A Tube Bias Calculator makes the process straightforward:

Enter plate voltage (V).
Enter maximum plate dissipation (W).
Enter the desired bias percentage (%) (usually 65–75%).

The calculator instantly gives you:

Target Plate Current (Ip) in milliamps.
Recommended Cathode Resistor (Rk) in ohms.
Target Cathode Voltage (Vk) in volts.

This saves time, avoids human error, and gives you a reliable starting point for your amp.

How the Tube Bias Calculator Works

The logic behind the calculator is based on simple electrical formulas:

Target Plate Current (Ip): I_p = \frac{P_{p,max} \times \text{Bias%}}{V_p} This tells you how much current should flow through the tube at idle.
Target Cathode Voltage (Vk):
Many designs assume cathode voltage should be about 5% of plate voltage.
Cathode Resistor (Rk): Rk=VkIpR_k = \frac{V_k}{I_p} This gives a recommended resistor value to achieve the desired bias.

Example: EL34 Power Tube

Let’s say you have an EL34 tube with:

Plate Voltage = 450V
Max Plate Dissipation = 25W
Bias % = 70%

Plugging into the calculator:

Ip Target ≈ 39 mA
Vk Target ≈ 22.5 V
Rk ≈ 577 Ω

This gives you a safe, solid baseline for setting up your tube.

Benefits of Using the Calculator

Saves Time: No need to crunch numbers manually.
Accurate: Reduces human error in calculations.
Beginner-Friendly: Even if you’re new to amps, you can bias tubes correctly.
Versatile: Works for different tube types—6L6, EL34, KT88, and more.