Heater Design
Part of Vacuum Tubes
The heater is the thermal engine of an indirectly-heated vacuum tube, providing the energy to bring the cathode to electron-emitting temperature without coupling electrical noise into the signal circuit.
Why This Matters
Every receiving tube runs hot enough to glow faintly in a darkened room because its cathode must be heated to emit electrons. The heater is the electrical resistance element inside the cathode sleeve that does this heating. It must reach 700-900°C to adequately heat the cathode, do so within a reasonable warmup time, maintain a stable temperature despite supply voltage variations, and do all this without introducing hum or interference into the sensitive signal circuits nearby.
The heater circuit design affects the performance of the entire equipment. A poorly designed heater circuit introduces 50 or 60 Hz hum into the audio output — a persistent buzz that cannot be filtered after the fact because it enters the signal circuit rather than the power supply. A heater supply with poor regulation starves some tubes and overheats others. And heater design directly determines the warmup time, during which the equipment cannot be used.
For builders working from salvage or manufacturing new equipment, understanding heater circuits allows proper design of transformer windings, heater series-string calculations for battery operation, and isolation techniques for hum-sensitive input stages.
Heater Specifications
Commercial receiving tubes are designed for either 6.3V or 12.6V heater supplies, with 6.3V being more common. Power tubes and transmitting tubes may have special heater voltages (5V for rectifier tubes, higher voltages for high-power transmitting tubes).
The heater draws a fixed current regardless of B+ supply voltage or operating conditions. Common values:
- 12AX7: 6.3V, 0.3A (or 12.6V, 0.15A)
- 12AU7: 6.3V, 0.3A (or 12.6V, 0.15A)
- 6L6: 6.3V, 0.9A
- 5Y3 rectifier: 5.0V, 2.0A
- 807 transmitting tube: 6.3V, 0.9A
The total heater current for an equipment determines the heater transformer secondary rating. Add up all heater currents (accounting for parallel vs. series connection) and add 20% margin for the transformer rating.
The heater dissipates its rated power (voltage × current) entirely as heat inside the cathode sleeve. A 6.3V, 0.9A heater dissipates 5.7W — this heat must be conducted or radiated to the cathode and then away from the tube into the air. Chassis layout must allow adequate air circulation around tubes to prevent overheating.
Heater-to-Cathode Hum
In circuits where the cathode is not at ground potential (common cathode stages with cathode resistors, and especially output stages where the cathode may be at several volts above ground), the heater voltage creates a varying electric field between heater and cathode. If the heater runs on AC, this field induces a 50/60 Hz voltage onto the cathode, which appears as hum in the output.
The hum induced depends on the heater-cathode capacitance and resistance (insulation resistance of the alumina coating between heater and cathode) and the heater voltage relative to cathode. For sensitive input stages, the cathode may sit at only 1-2V above ground, while the heater voltage swings ±4.5V relative to ground (for a center-grounded heater supply). The induced hum can reach millivolts — enough to be audible in a sensitive audio amplifier.
Solutions:
Heater center-tapping: connect the center point of the heater winding to ground, or to the cathode bias voltage. This reduces the average potential difference between heater and cathode. A center-tap resistor divider (two 100-ohm resistors from heater supply ends to a center point connected to ground) provides this without requiring a physical center-tap on the transformer winding.
DC heaters: power the heater from a DC supply instead of AC. The constant heater potential eliminates the alternating electric field. DC heaters require a rectifier and filter for the 6.3V supply — a 10A rectifier and a 4700 µF electrolytic capacitor provides quiet DC heating. Essential for the most sensitive input stages.
Physical separation: position the most sensitive tubes (input stages) as far as possible from the power transformer and from each other. Heater leads carry AC current and radiate electromagnetic fields — keep them short and route them away from high-gain signal circuits.
Twisted heater wiring: twist the heater supply leads together (or use specifically paired heater wire). This ensures that the magnetic fields from the two heater conductors cancel in the region outside the cable, reducing magnetic induction into nearby circuits.
Series Heater Strings
Portable and battery-operated equipment, or equipment designed to run from a single AC supply without a heater transformer, can connect tube heaters in series. If all tubes have the same heater current (or are chosen for compatibility), the heaters can be strung in series across a higher voltage.
A series string of four 12AX7 tubes (each 12.6V, 0.15A in 12.6V mode) runs across 50.4V at 0.15A from a suitable supply. This approach was used in transformerless AC/DC radios (the all-American five) where the five heaters in series totaled to approximately 117V (the AC mains), allowing the receiver to plug directly into AC without a power transformer.
Series heater strings require that all tubes in the string have the same heater current. If one tube has a higher current rating, it will drop less voltage (because a fixed current flows through its lower resistance); if lower, it drops more voltage. The exact heater voltages each tube sees depend on the actual resistances, which vary between tube types. Check against specifications and calculate the actual voltages before committing to a series string design.
An open heater in a series string disables all tubes in the string — the circuit is broken and all tubes go cold. For equipment reliability, minimize series string length. Individual transformer windings for each tube or small groups, while requiring more complex transformers, improve reliability and allow easy fault finding.
Heater Warmup and Rush Current
Cold tungsten or nickel heater wire has much lower resistance than hot wire. As the heater element warms from room temperature to operating temperature, its resistance increases and the current decreases. At startup, the heater draws 3-5× its steady-state current for the first few seconds. This “inrush current” stresses the heater element and the power supply.
The inrush current is not normally a problem for transformers designed with adequate ratings, but matters when calculating fuse ratings (use a slow-blow fuse in the heater circuit to allow inrush without blowing) and when using DC heater supplies (the filter capacitor must be large enough to absorb the inrush without the voltage drooping excessively).
Indirectly-heated cathodes require 30-60 seconds to reach full operating temperature after the heater is energized. During this time, the tube provides little or no emission and the equipment cannot function normally. The warmup period is normal and expected. Rushing to apply B+ voltage before heaters are warm can cause cathode damage in some tube types (especially in mercury vapor tubes like the 866A, where applying high voltage before the heater reaches full temperature causes arc damage). For mixed equipment, apply heater power first, wait for warmup, then apply B+ through a delayed switching relay or simply by waiting before turning on the B+ supply.