Universal Filament Regulator — Rev. 3.3

The advantages of using DC to power vacuum tube filaments and heaters are numerous. The most prominent advantage is elimination of a source of hum and inter-modulation distortion. The resulting improvement in sound quality is obvious. The mains hum is gone and the background fuzz caused by mains-related IMD is replaced by complete, dark silence.

The Universal Filament Regulator features:

  • Texas Instruments LM2267x-series switching regulator IC.
  • High efficiency eliminates the need for heat sinks.
  • Input voltage range: 4.5 – 23 V.
  • Output voltage range: 1.285 – 15 V.
  • Optimal input voltage range: 0.25 · Vout – 0.75 · Vout.
  • Output current: 0 – 3 A (LM22673) or 0 – 5 A (LM22679).
  • Programmable current limit (up to 3 A for the LM22673; 5 A for LM22679).
  • Soft start: Output voltage ramps gradually to the full value in about five seconds.
  • Ideally suited for directly heated triodes, such as 2A3, 300B, #26, and 801A, but equally useful with indirectly heated tubes.
  • Board dimensions: 2.10 × 1.90 inches (53 × 48 mm).

Preassembled and fully tested modules as well as unpopulated circuit boards are available for purchase. The board is accompanied by detailed design documentation including the full schematics, bill-of-materials for commonly used tubes (incl. 2A3, 300B, #26, 801A, as well as 6.3 V indirectly heated tube types).


An example of a fully assembled filament regulator module is shown below. To assemble this board, you will need the ability to solder surface mounted components. The smallest ones are size 0805. The IC can be a challenge to solder. For details, please see my step-by-step guide: Soldering ICs with Exposed DAP.
Universal Filament Regulator Rev. 3.3 Assembled
The assembled height is approx. 0.65 inches (16.5 mm). For stacking multiple boards, 5/8″ (16 mm) spacers work perfectly as shown below. The 0.13″ diameter mounting holes are sized for M3 (or #4) screws.


The goal of this project is to implement a PCB which will allow the user to implement a vacuum tube filament supply using Texas Instruments’ LM2267x-series of switchmode regulator ICs. The board is to be universal in the sense that the user, by proper choice of a few external components, can design the regulator to power filaments in tubes ranging from directly heated tubes such as 26, 2A3, 4P1L, 300B, etc. to indirectly heated tubes such as 6J5, 6L6, and 12AX7.

The advantages of a DC filament supply are fairly obvious. Using DC to power the filament eliminates a significant source of hum. With DC filament supplies, the choice is between constant current and constant voltage. Theoretically, the two types should provide the exact same performance – assuming the current/voltage sources are reasonably close to ideal. The characteristics of a well-designed, regulated supply approach those of an ideal source – at least within the audio band. I have personally never been able to tell a difference in sound quality between an amp with filaments heated by a constant current and one with filaments heated by a constant voltage. A constant voltage supply is considerably less expensive to implement, so that’s where I ended up.

To generate a regulated constant voltage, two choices are available: linear regulator and switchmode regulator. A traditional linear regulator is by far the easiest to implement but is horribly inefficient. In my DG300B stereo amp, using linear regulators would result in over 20 W dissipated in the filament regulators alone. That’s more than the amplifier can deliver to the speakers! My Universal Filament Regulators operate at over 90 % efficiency, hence only dissipate about 2 W total for the three Universal Filament Regulators used in the amp.

After looking at a handful of different switchers, I came across the LM2267x-series. These ICs come in various flavours that differ in options and output current. I ended up settling on the LM22673 as this offers a set of options that are very suitable for a universal filament regulator:

  • Programmable current limit (up to 3 A for the LM22673; 5 A for LM22679).
  • Output voltages from 1.285 V to well beyond 12.6 V.
  • Soft start.
  • Few external components required.
  • Package options exist that allow for humans to solder these ICs onto a circuit board.
It should be noted that even though the fundamental design is universal, some design effort needs to be applied to adapt the regulator to the specific application. I will cover some of the commonly used application (5.0 V for 300B and 6.3 V for indirectly heated tubes first with others to follow). By far the easiest way to calculate the external components is to use the WebBench tool provided by Texas Instruments. Also make sure to read the LM22673 datasheet.


Revision History

  • Rev. 3.1: First production version.
  • Rev. 3.2: Added footprint for tantalum output capacitors.
  • Rev. 3.3: Added protection against momentary input voltage reversal and accidental swap of input and output connections. Added option for remote voltage sensing.