Voltage Regulation
From gas-discharge VR tubes to silicon Maida regulators — keeping B+ stable, quiet, and predictable for every stage of your amplifier.
Why Regulate?
What happens when you don’t — and when you must
Unregulated B+ carries 100/120Hz ripple directly into the signal path. In a preamp with 40dB gain, even 1mV of ripple becomes audible.
Under load variations (signal dynamics), B+ drops. In power amps, this creates compression — sometimes desirable ("sag feel"). In preamps, it shifts the operating point.
Mains voltage can drift ±10% through the day. Without regulation, B+ follows, shifting bias points and gain.
RIAA phono stages (60dB+ gain), microphone preamps, and any stage feeding a long signal chain. Power output stages often benefit from partial sag.
Shunt Regulators
The simplest approach: absorb excess current to hold voltage constant
A shunt regulator sits in parallel with the load. A series resistor drops the input voltage; the shunt element (VR tube or zener) clamps the output. Current not taken by the load flows through the shunt device.
Gas-discharge tubes with fixed striking voltages: OA2 = 150V, OB2 = 108V, VR150 = 150V, VR105 = 105V. Can be stacked in series for higher voltages (2×OA2 = 300V).
Limitations: fixed voltage steps, limited current (5–40mA), glow discharge generates wideband noise.
Silicon zener diodes: more flexible voltage selection, lower noise than VR tubes, tighter tolerance. Use 1N series or BZX types.
Key insight: a single high-voltage zener (200V) is noisier than multiple low-voltage zeners in series — see Statistical Regulators.
| VR Tube | Voltage | Imin | Imax | Notes |
|---|---|---|---|---|
| OA2 / VR150 | 150V | 5mA | 30mA | Most common, orange glow |
| OB2 / VR108 | 108V | 5mA | 30mA | Lower voltage variant |
| OC2 / VR105 | 105V | 5mA | 40mA | Older type, still available NOS |
| OD3 / VR150 | 150V | 5mA | 40mA | Higher current rating |
| 0A3 / VR75 | 75V | 5mA | 30mA | Low voltage, good for bias |
Series Regulators
The pass element controls all load current — more efficient, adjustable
A series regulator places a variable resistance (tube, MOSFET, or IC) in the current path. A feedback loop compares the output to a reference and adjusts the pass element to maintain constant voltage.
6080/6AS7G dual triode: low rp (~280Ω), high current (125mA/section). Classic choice. 6C19P is a good substitute.
IRF840 (500V/8A): modern, efficient, no heater power wasted. Requires careful gate drive and oscillation prevention.
Excellent for heater supplies (6.3V/12.6V). LM317: 1.5A, LM338: 5A. Low noise, thermal shutdown, current limiting built in.
| Approach | Voltage | Current | Noise | Best for |
|---|---|---|---|---|
| Tube (6AS7G) | ≤450V | 125mA | Low | Purist all-tube builds |
| MOSFET (IRF840) | ≤500V | 8A | Very low | High-current B+ |
| LM317/LM338 | ≤37V diff | 1.5/5A | Excellent | Heater supplies |
| Maida (LM317+MOSFET) | ≤450V+ | ≤300mA | Excellent | Preamp B+ |
The Maida Regulator — Deep Dive
LM317 bootstrapped for high voltage: simple, reliable, excellent regulation
The Maida regulator (Walt Jung & Richard Marsh) uses a standard LM317 to regulate voltages up to 400V+. The trick: the LM317 "floats" at high voltage — it only sees the small Vin–Vout differential across its terminals. A depletion-mode MOSFET (DN2540) provides startup current and drops the bulk of the voltage.
- Choose Vout (e.g. 300V)
- Set R1 = 240Ω (standard)
- Calculate R2 = R1 × (Vout/1.25 − 1)
- Choose Vin for 10–50V dropout
- Verify MOSFET power dissipation
- Add 10µF output capacitor
- Keep R1/R2 close to LM317 ADJ pin
- Wide traces for current path
- MOSFET heatsink isolated from HV
- Bypass caps on input and output
- Keep signal ground separate
Maida Regulator Designer
Compute component values for a Maida HV regulator
Statistical Regulators
Morgan Jones’ low-noise trick: many small zeners beat one big one
A single 200V zener diode generates significant noise. Morgan Jones’ insight: use 30–40 low-voltage zeners (5–6V each) in series. Each zener generates uncorrelated noise, so the total noise increases only as √N rather than N. Result: the same regulation voltage with noise reduced by a factor of √N (≈ 6× for 40 zeners).
High noise
Noise ÷ √40 ≈ 6.3×
40 × 5.1V
Practical benefits: extremely cost-effective (40 small zeners cost less than one precision HV reference), self-temperature-compensating (averaging effect), and the total voltage is adjustable by adding or removing diodes. Use 5.1V zeners (lowest noise point for silicon zeners).
Practical Designs
Complete regulation schemes with component values
Maida regulator: Vin=350V, DN2540, LM317, R1=240Ω, R2=57.4kΩ. Output cap: 10µF/450V. MOSFET dissipation: ~2.5W. Add 100nF ceramic bypass at LM317 input and output.
Raw CLC filtered or lightly regulated. For guitar amps: deliberate sag via tube rectifier (GZ34). For hi-fi: silicon bridge + CLC + optional Maida. At 200mA, Maida MOSFET dissipation is 10W+ — needs substantial heatsink.
Screen voltage must be regulated for consistent pentode operation. Series MOSFET regulator (IRF840) with TL431 reference. Typical: 250–300V at 10–30mA. Critical for push-pull EL34/6L6 output stages.
Separate winding + silicon diode + RC filter. Regulate with LM337 or zener shunt (1N4756 = 47V). Low current (<5mA) makes zener shunt ideal. Add 10k trim pot for bias adjustment.
- Always include a bleeder resistor (100k–220k, 2W+) across B+ to discharge filter caps safely.
- For regulated supplies, place the regulator after the main filter to reduce input ripple.
- Separate regulators for each channel minimize crosstalk in stereo designs.
- Soft-start (NTC thermistor or relay delay) protects regulators from inrush current.
- Use slow-blow fuses rated at 2× normal operating current for each supply rail.
All B+ regulation circuits carry lethal voltages. Always include bleeder resistors, discharge paths, and fuse protection. Never work on energized circuits.
PSRR and Ripple
How well each topology rejects power supply noise
Power Supply Rejection Ratio (PSRR) measures how much supply ripple appears at the output of a gain stage. Design target: total ripple at the speaker should be at least 120dB below full output power (≈10µV for a 1Vrms signal).
| Topology | Typical PSRR | Notes |
|---|---|---|
| Common cathode + R load | ~6 dB | Worst case — nearly all ripple passes through |
| Common cathode + CCS | 50+ dB | CCS forces constant current → excellent rejection |
| µ-follower | 50+ dB | Bootstrapped load rejects supply variations |
| Cascode + CCS | 60+ dB | Best passive topology for PSRR |
| SRPP | ~30 dB | Moderate — depends on load matching |
| Cathode follower | ~20 dB | Buffer stage, moderate rejection |
| Regulated supply (Maida) | 40+ dB | Reduces ripple before it reaches the stage |
In a multi-stage amplifier, PSRR values in dB add up: a CCS stage (50dB) followed by a µ-follower (50dB) gives 100dB of total rejection. Combining topology PSRR with a regulated supply (40dB) can achieve the 120dB target.
Inject a known AC signal (e.g. 1Vrms at 100Hz) into the B+ rail via a coupling capacitor. Measure the signal at the stage output with an oscilloscope or audio analyzer. PSRR = 20×log10(Vin/Vout). Test at multiple frequencies — PSRR typically degrades above 1kHz.
PSRR Chain Calculator
Model ripple rejection through your signal chain
Key Equations
Essential formulas for voltage regulation design
Test Your Knowledge
Validate your understanding of voltage regulation for tube amplifiers — from shunt basics to PSRR design targets.
For which application is voltage regulation considered essential (not optional)?