Heaters & Filaments
The question every tube amplifier builder asks: AC or DC heaters? Why does it hum? How do I elevate the heater supply? This guide covers everything from cathode chemistry to practical regulated DC supply design, based on established thermionic engineering principles.
Anatomy of the Filament
How thermionic emission works at cathode level
In most receiving tubes (12AX7, EL34, 6SN7), a tungsten wire heater is coiled inside a nickel cathode sleeve, separated by an Al₂O₃ (alumina) insulator. The heater raises the cathode to operating temperature (typically 700–850°C) without any electrical connection between them. The cathode’s oxide coating (barium/strontium oxide) is the actual electron emitter.
In directly-heated triodes (300B, 2A3, 45), the filament IS the cathode — no separate sleeve. The wire itself is coated with emissive material and serves dual purpose: heating element and electron source. This gives DHTs their characteristic intimacy of sound, but makes hum management more challenging since the AC heater current modulates the emission directly.
BaO/SrO coating on nickel. Low operating temperature (~750°C). Most common in receiving tubes. Sensitive to ion bombardment and cathode stripping from excessive Vhk.
Used in transmitting tubes and some power triodes (211, 845). Higher operating temperature (~1600°C). Much more robust, longer life, but requires more heater power. Bright orange glow at operating temperature.
AC vs DC Heaters
Two sources of hum, and when each matters
AC heaters introduce hum through two distinct mechanisms. Understanding both is essential to deciding whether DC conversion is truly necessary for your application.
The heater wire carries AC current, creating a 50/60 Hz magnetic field that couples into nearby signal wiring and tube electrodes. Twisted heater wires reduce this, and grounding the center-tap of the heater winding helps further. This is the dominant mechanism in most amplifiers.
Imperfect insulation between heater and cathode allows a tiny AC current to flow through the cathode circuit. At high impedance grid circuits, even nanoamperes create audible hum. This is the mechanism that DC heaters solve directly, and why low-level stages benefit most.
| Application | AC OK? | DC Required? | Why |
|---|---|---|---|
| Guitar power amp | ✅ | — | High signal levels mask residual hum |
| Guitar preamp | ✅ | — | AC with center-tap usually sufficient; some boutique builders prefer DC |
| Hi-fi line stage | ⚠️ | — | Depends on tube type and layout; elevated AC often adequate |
| RIAA phono stage | ❌ | ✅ | Very high gain (40+ dB), high-impedance input; nA leakage is audible |
| Microphone preamp | ❌ | ✅ | Extreme gain (60+ dB), low-level signals demand silence |
| Headphone amp | ⚠️ | — | Close coupling to ears makes hum obvious; DC preferred |
| Power stage (EL34, KT88) | ✅ | — | Power tubes are AC-heated in virtually all designs |
Cathode Elevation (Vhk)
Protecting the cathode from electrolysis
When the heater sits at ground potential and the cathode sits at a positive voltage (due to cathode bias), a DC voltage differential exists across the heater-cathode insulation. This Vhk voltage drives electrolysis through the alumina insulator, gradually stripping the barium oxide coating from the cathode. RCA recommended keeping Vhk within +40V for long tube life.
The solution: bias (elevate) the heater supply above ground, closer to the cathode voltage. A simple resistor divider from B+ or a zener reference can establish a DC offset on the entire heater bus. This reduces the voltage stress on the insulation and extends tube life dramatically.
| Tube | Vhk max | Heater V | Notes |
|---|---|---|---|
| 12AX7 / ECC83 | 180V | 12.6V | Dual Triode |
| 12AT7 / ECC81 | 180V | 12.6V | Dual Triode |
| 12AU7 / ECC82 | 180V | 12.6V | Dual Triode |
| 6SN7-GT | 200V | 6.3V | Dual Triode |
| 6SL7-GT | 200V | 6.3V | Dual Triode |
| 6922 / E88CC | 130V | 6.3V | Dual Triode |
| EL34 / 6CA7 | 100V | 6.3V | Pentode |
| EL84 / 6BQ5 | 100V | 6.3V | Pentode |
| 6L6-GC | 200V | 6.3V | Beam Tetrode |
| KT88 | 200V | 6.3V | Beam Tetrode |
| EF86 / 6267 | 100V | 6.3V | Pentode |
| 6DJ8 / ECC88 | 130V | 6.3V | Dual Triode |
| 5687 | 200V | 12.6V | Dual Triode |
| 12B4-A | 90V | 12.6V | Triode |
| 6V6-GT | 200V | 6.3V | Beam Tetrode |
Two equal resistors (typically 100kΩ each, 1W) from B+ to ground, center-tap connected to heater center-tap. Provides B+/2 elevation. Simple but varies with B+ regulation. Add a 100nF bypass cap to ground on the junction.
A zener diode (e.g., 68V) with a series resistor from B+ provides a stable, regulated elevation voltage. More precise than a resistor divider but slightly more complex. Ideal when cathode voltages vary significantly between stages.
Heater-Cathode Insulation
Why some tubes are worse than others
The Al₂O₃ insulation between heater and cathode is never perfect. At operating temperature, its resistance drops from gigaohms (cold) to megaohms, allowing nanoampere-scale leakage currents. In high-impedance grid circuits (1MΩ grid leak), these currents develop millivolt-level AC signals — directly audible as hum.
Notoriously poor heater-cathode isolation. Many builders have abandoned this tube for low-level applications despite its otherwise excellent linearity. DC heaters are mandatory with this tube in any gain stage.
The miniature 9-pin (B9A/Noval) generation introduced in the 1950s generally has reduced insulation compared to the larger octal types. The smaller physical dimensions mean shorter leakage paths. Octal tubes like 6SN7 typically outperform their B9A equivalents (12AU7) for heater isolation.
Use a megohm meter (insulation tester) at operating temperature — cold measurements are meaningless. Good tubes measure >100MΩ; suspect tubes 10–50MΩ; reject below 10MΩ. Test between heater pins and cathode pin with the tube warmed up for 5 minutes.
Symptoms progress from intermittent hum, to crackling noise (as the leakage path breaks down and reforms), to eventually a hard short that blows fuses or damages the cathode bias circuit. Replace the tube immediately at the first sign of crackling.
6.3V vs 12.6V — Series vs Parallel
The *SN7 family and wiring strategies
Many dual triodes exist in both 6.3V and 12.6V versions: 6SN7 (6.3V, 600mA) vs 12SN7 (12.6V, 300mA), or 6SL7 vs 12SL7. The 12-volt types have their heater sections wired in series internally. The 12AX7/12AT7/12AU7 family can run either way: pins 4+5 for 12.6V series, or pin 9 jumpered for 6.3V parallel.
Half the current means less electromagnetic radiation from heater wiring — less magnetic hum coupling. However, one end of the heater string sits at 12.6V above the other, creating a larger electrostatic voltage difference to surrounding conductors.
✓ Less magnetic hum ✗ More electrostatic hum
The standard approach: all heaters in parallel on the 6.3V winding. Simpler wiring, and with a center-tapped winding or artificial center-tap (two 100Ω resistors), each heater end sits only 3.15V from the reference point. Lower electrostatic coupling.
✓ Less electrostatic hum ✗ More magnetic hum
Long heater runs in rack equipment where electromagnetic radiation is the primary concern. Also useful when a 12.6V DC supply is easier to design (more headroom for regulation).
Standard choice for most amplifiers. Simpler wiring, lower voltage to ground, compatible with 6-volt-only tubes (6SN7, EL34). Use twisted pair from transformer to tube sockets.
Some designs use 12.6V DC for the sensitive first stage and 6.3V AC for the rest. This minimizes cost and complexity while eliminating hum where it matters most.
Regulated DC Heater Supply
Full design with interactive calculator
A regulated DC heater supply chain: transformer secondary → bridge rectifier (Schottky diodes for low dropout) → large filter capacitors (low ESR) → voltage regulator (LM317/LM338 for linear, or LDO). Schottky diodes (1N5822, MBR2545) drop only 0.3–0.5V vs 0.7–1.0V for standard silicon, preserving precious headroom at these low voltages.
Soft-start consideration: large filter caps present near-short-circuit to the rectifier at power-on. An NTC thermistor (5Ω cold, dropping to 0.5Ω hot) in the AC primary or a timed relay bypass protects the diodes from inrush damage.
Schottky preferred: 1N5822 (3A/40V), MBR2545 (25A/45V), SB560 (5A/60V). Forward drop 0.3–0.5V vs 0.7–1.0V for standard 1N400x. Critical at 6.3V where every 0.1V counts. Use a bridge of 4 discrete Schottkys rather than a monolithic bridge for better thermal performance.
Low ESR electrolytics: 4700µF–10000µF at 16V or 25V. Panasonic FM/FC, Nichicon HE, Rubycon ZLH series. ESR matters more than capacitance for ripple current handling. Parallel multiple caps for lower effective ESR. Add a 100nF ceramic bypass across each electrolytic.
LM317T: adjustable, 1.5A max, 2.5V dropout. LM338T: 5A version. LDO options: LT1085 (3A, 1V dropout), LT3080 (1.1A, 0.33V dropout). For >3A, use LM338 with current sharing or a discrete MOSFET pass element driven by a TL431 reference. Always add input and output caps per datasheet.
For a 4-tube RIAA stage (2× 12AX7 at 300mA total): transformer 9VAC/2A secondary → 4× 1N5822 Schottky bridge → 2× 4700µF/16V Panasonic FC in parallel → LM317T set to 6.3V (240Ω + 820Ω divider) → 2× 100µF/16V output. Regulator dissipation: (9×1.414 − 1.0 − 6.3) × 0.3 = ~1.6W. Mount LM317 on small heatsink (TO-220 clip sufficient). PCB layout: keep high-current traces wide (2mm+), star-ground the output.
Reducing Heater Dissipation
The 5% rule for lower noise
Slightly underrunning heaters — typically 5% below nominal voltage (6.0V instead of 6.3V) — reduces heater temperature, which lowers the thermal noise contribution and reduces heater-cathode leakage current. The emission loss at 5% undervoltage is negligible for small-signal tubes since they operate far below maximum cathode current.
Benefits: reduced thermal noise, lower heater-cathode leakage, slightly extended tube life, lower grid emission (important in high-mu tubes like 12AX7). The cathode temperature drops only slightly since the thermal mass of the cathode sleeve provides significant smoothing.
Power output tubes under heavy duty (EL34, KT88, 6L6 in Class AB) need full heater voltage to maintain adequate emission during signal peaks. Underrunning power tubes can cause crossover distortion, premature cathode depletion under load, and reduced power output. Only underrun small-signal stages.
| Parameter | At 6.3V (100%) | At 6.0V (95%) | Effect |
|---|---|---|---|
| Cathode temp. | ~800°C | ~780°C | Minimal reduction |
| Emission capacity | 100% | ~92% | Negligible for small-signal |
| Thermal noise | Reference | −1–2 dB | Measurable improvement |
| Hk leakage | Reference | −20–30% | Significant improvement |
| Tube life | Reference | +10–20% | Extended |
Special Filament Designs
EF86 helical, DHT circuits, bright emitters
The EF86 (6267) features a helically-wound heater with alternating current directions in adjacent turns. This creates opposing magnetic fields that cancel each other out — a brilliant self-shielding design. Combined with an internal electrostatic shield between the heater and cathode, the EF86 achieves extremely low hum even on AC heaters, making it a favorite for microphone preamps and RIAA stages.
Transmitting tubes (211, 845, 805) use thoriated tungsten filaments operating at white heat (~1600°C). These draw significant filament power (10V at 3.25A for the 211). The filament glows bright white-orange and requires heavy-duty power supplies. AC operation with hum pot adjustment is traditional for these types.
For DHTs, the filament IS the cathode, so AC ripple directly modulates emission. Three approaches: (1) DC filament supply — lowest hum but expensive; (2) AC with hum pot — a 25Ω trimmer across the filament winding, wiper to ground, adjusted for minimum hum; (3) AC with center-tapped winding — simpler than hum pot, slightly less optimal. The 300B runs at 5.0V/1.2A; the 2A3 at 2.5V/2.5A.
Connect a 10–25Ω wirewound potentiometer across the filament winding terminals. The wiper connects to the ground reference (or to the cathode bias point in some topologies). Adjust with a signal present at low volume for minimum hum. Lock the pot after adjustment — vibration can shift the setting.
Relays, Standby & Safety
Hidden traps in heater switching circuits
Relays with conductive metal cores create parasitic capacitance between their coil (often driven from logic-level circuits) and their contacts (carrying mains or heater current). A relay rated for 250VAC may only have 1–2kV of insulation between coil and contacts — inadequate near HT circuits. Use relays with reinforced insulation (10kV+ rating) or interpose an optocoupler.
B+ standby (opening the B+ circuit while heaters remain on) is the standard approach — cathodes stay warm, ready for instant use. Heater standby (turning off heaters while B+ remains) is dangerous: with no emission, plate voltage rises to the full B+ across cold cathodes, risking insulation breakdown and cathode stripping on restart.
Apply B+ only after heaters have warmed for 30–60 seconds. Simple approach: a relay driven by an RC timer (47µF + 100kΩ = ~5s time constant, use multiple stages for longer delays). Modern approach: a 555 timer or microcontroller driving a relay. This prevents cathode stripping from cold-cathode high-voltage stress.
Cold tungsten heater wire has 1/10th the resistance of hot wire. At switch-on, heater inrush current can be 10x nominal — a 6.3V/2A heater string surges to 20A briefly. NTC thermistors (e.g., Ametherm SL32-2R515) in the primary limit inrush. Size the NTC so its cold resistance limits peak current to safe transformer and fuse ratings.
Vhk Safety Checker
Check if your heater-cathode voltage is within safe limits
Key Equations
Essential formulas for heater supply design
Heaters & Filaments — Full Quiz
Test your knowledge of heater design, Vhk, and filament types
What is the key difference between indirect and direct heated tubes?