What Is a B+ in Numbers: Vacuum Tube High-Voltage DC Rails Explained

B+ is the high-voltage positive DC power supply rail that feeds the plates (anodes) and screen grids of vacuum tubes in an electronic circuit. In numbers, a typical B+ rail ranges from 200V DC for small-signal preamp tubes up to 500V DC or more for high-power output tubes, delivering currents measured in milliamps rather than amps. If you are designing, repairing, or modifying tube gear, understanding the exact B+ values is the difference between a warm, dynamic amplifier and a melted output transformer.

The Core Numbers: Typical B+ Voltages by Tube Type

The exact B+ voltage required depends entirely on the tube's physical geometry, its maximum plate dissipation rating, and the intended operating class. While solid-state circuits standardize around 3.3V, 5V, or 12V rails, vacuum tube B+ voltages are highly specific to the envelope.

Source data derived from standard operating conditions found in the Duncan Amps Tube Data Sheet Library.

Worked Numeric Example: Calculating B+ Voltage Drop

To understand what B+ actually does in a real circuit, let us calculate the operating point of a standard 12AX7 common-cathode gain stage, the building block of almost every tube preamp. We will use real bench values.

The Circuit Parameters:

• B+ Supply Rail: 250V DC
• Plate Resistor (Rp): 100kΩ
• Cathode Resistor (Rk): 1.5kΩ
• Grid Leak Resistor (Rg): 1MΩ to ground

The Math:
Assume the tube settles at an idle plate current (Ip) of 1.2 mA (0.0012 A). Because the screen grid is internally tied to the cathode in a triode, the cathode current (Ik) is effectively equal to the plate current for this calculation.

1. Voltage drop across the Plate Resistor (Rp):
   V_Rp = Ip × Rp = 0.0012 A × 100,000 Ω = 120V.
2. Actual DC Voltage at the Tube Plate:
   V_plate = B+ - V_Rp = 250V - 120V = 130V.
3. Voltage drop across the Cathode Resistor (Rk):
   V_Rk = Ik × Rk = 0.0012 A × 1,500 Ω = 1.8V.
4. Grid-to-Cathode Bias Voltage (Vgk):
   The grid is held at 0V DC via the 1MΩ grid leak resistor. The cathode is elevated to +1.8V. Therefore, the grid is negative relative to the cathode: 0V - 1.8V = -1.8V DC.

This -1.8V bias point sets the tube's transconductance and determines how much AC audio signal it can amplify before clipping. If your B+ sags from 250V down to 220V under load, the plate voltage drops, the current drops, and the bias point shifts, fundamentally altering the harmonic distortion profile of the stage. For a deeper look at how these resistor ratios interact with the tube's internal resistance, the Valve Wizard common-cathode guide is the definitive bench reference.

Where You Meet B+ in Practice

You will encounter B+ rails primarily in three domains: guitar amplifiers, audiophile hi-fi gear, and high-power RF/ham radio transmitters.

What B+ changes in a real installation:
The B+ voltage directly dictates the headroom and maximum output power of the amplifier. In a push-pull Class AB guitar amplifier using 6L6GC tubes, raising the B+ from 420V to 480V increases the maximum voltage swing across the output transformer primary. This yields a louder, cleaner amplifier with tighter low-end response. However, it also pushes the tubes closer to their maximum plate dissipation limits, generating significantly more heat and reducing tube lifespan.

Power Supply Sag:
In vintage-style guitar amps, the B+ is often supplied through a tube rectifier (like a 5AR4/GZ34) and a relatively high-resistance power transformer. When you hit a loud chord, the power tubes draw a massive transient current. This current causes a voltage drop across the rectifier and the transformer's internal winding resistance, causing the B+ to momentarily 'sag' by 20V to 50V. This sag compresses the audio signal, creating the 'spongy' feel prized by blues and rock players. Modern hi-fi amps, conversely, use massive solid-state rectifiers and choke-input filters to keep the B+ rail absolutely rigid, prioritizing linear fidelity over compression. Read more on the mechanics of power supply sag at Aiken Amplifiers.

Common Confusions: B+ vs. A+, C-, and Solid-State VCC

When reading vintage schematics or talking to older technicians, the nomenclature can trip up builders used to modern silicon. Here is what people commonly confuse with B+:

• A+ (or just 'A'): This is the heater or filament supply. It is typically 6.3V AC (or 12.6V DC/AC for series-strung tubes). It heats the cathode to enable thermionic emission. It has nothing to do with the high-voltage audio path.
• C- (or 'C bias'): This is the grid bias supply. In fixed-bias power amps, this is a separate negative DC voltage rail (often -30V to -60V DC) applied directly to the control grids of the power tubes to set their idle current. It is distinct from the positive B+ rail.
• VCC / VDD: The solid-state equivalent of B+. However, VCC in transistor or op-amp circuits rarely exceeds 15V to 48V. Applying solid-state troubleshooting logic (where a 'high' rail is 12V) to a tube chassis is a fast track to a fatal shock.