Power Supply & Rectification
The power supply is the foundation of any tube amplifier. A well-designed B+ rail provides clean, stable high voltage while managing ripple, regulation, and rectifier characteristics. This guide covers the complete signal path from AC mains to filtered DC.
B+ Supply Block Diagram
Every tube power supply follows the same fundamental chain
Full-Wave Topologies
Center-tap vs bridge — the two dominant approaches
Rectifier Tube Comparison
Forward resistance creates voltage sag — a key tonal characteristic
Indirectly heated, low sag, 250mA max — gold standard
Directly heated, moderate sag, 275mA max — classic power amp
Directly heated, high sag, 125mA max — vintage character
Near-zero sag, no warm-up, stiff supply
CLC Pi-Filter Design
Interactive calculator for capacitor-input LC filter with optional RC stage
Capacitor Input vs Choke Input
The first element after the rectifier fundamentally changes the supply behavior
- DC output approaches Vpeak of secondary (higher voltage)
- Higher ripple amplitude — diodes conduct in short pulses
- High peak charging currents stress rectifier tube
- Poor regulation — voltage drops significantly under load
- Standard approach for most guitar and hi-fi amplifiers
- DC output is Vavg = 2Vpeak/π (about 64% of peak — lower voltage)
- Much lower ripple — choke smooths before any capacitor
- Gentle on rectifier — current flows continuously, no spikes
- Excellent regulation — B+ stays constant over wide load range
- Preferred for high-power transmitter and broadcast applications
If the choke inductance falls below Lcrit, the filter reverts to capacitor-input behavior (current becomes discontinuous). The choke must maintain sufficient inductance at the minimum expected load current — this is why swinging chokes are used, with inductance that increases as current decreases.
Rectifier Sag Under Load
Compare B+ regulation across different rectifier types
Filament Power
AC vs DC heating, hum reduction, and elevated heaters
Most amplifiers use 6.3V AC directly from a dedicated transformer winding. Simple and efficient, but the alternating heater current induces 60Hz hum into the signal via capacitive coupling between heater and cathode.
Hum reduction: Use a center-tapped heater winding or an artificial center tap (two 100Ω resistors) tied to a DC reference — typically ground or an elevated voltage. Twisted heater wiring is essential.
For ultra-low-noise preamp stages (phono, microphone), DC heater supplies eliminate heater-induced hum entirely. Typically a dedicated bridge rectifier and RC or regulator circuit fed from the 6.3V AC winding.
Elevated heaters: Biasing the heater supply to +20V to +50V above ground reduces the heater-cathode voltage differential in high-B+ stages, minimizing leakage current and extending tube life. Essential when cathode sits at high DC potential (cathode followers, series regulators).
Directly-heated rectifier tubes (5U4, 5Y3, 5R4) require a dedicated 5V winding rated for 2-3A. This winding must be isolated from the 6.3V heater supply. The GZ34/5AR4 is indirectly heated but still uses the 5V winding. This 5V winding provides the automatic soft-start: the rectifier heater needs time to warm up before conducting, giving signal tubes time to stabilize before B+ appears.
High Voltage Safety
Lethal voltages are present — respect every precaution
A high-value resistor (100k-220k, rated for full B+ voltage) across the main filter cap slowly discharges the supply when powered off. Without it, caps can hold lethal charge for hours. Size for 2-5mA continuous bleed current.
Large filter caps draw enormous charging current at power-on. NTC thermistors (inrush limiters) in the primary reduce this surge. Tube rectifiers provide natural soft-start since they need warm-up time before conducting.
Opens the B+ circuit (typically center-tap to ground) while heaters warm up. Prevents cathode stripping in output tubes. Wait 30-60 seconds before engaging standby. Some designs use a time-delay relay.
Before servicing: power off, remove plug, wait 2 minutes, then discharge each filter cap through a 10k/5W resistor using insulated clip leads. Verify zero volts with a meter rated for the voltage present. Never trust the bleeder alone.
Always fuse the primary (mains) side. Use slow-blow fuses sized for normal operating current plus inrush margin. Secondary HT fuses protect the transformer if a rectifier shorts. Never increase fuse ratings to "fix" blowing.
The chassis must connect to mains earth ground with a proper 3-prong cord. Star grounding at a single chassis point prevents ground loops. Use a ground fault interrupter (GFI/GFCI) at the outlet for additional protection.
B+ voltages of 300-500V are lethal. Always discharge filter capacitors before touching any circuit. Work with one hand in your pocket. Never work alone.
Key Equations
Essential formulas for power supply design
Test Your Knowledge
Validate your understanding of power supply design before moving on.
What is the fundamental chain of a tube B+ power supply?