How to Drive a 12.6V Heater From a 6.3V Supply 11 March 2008 (revised 6 Oct 2016) Mike@MDBVentures.com Visit us at http://www.MDBVentures.com - Great prices on great tubes! There are two simple ways to supply 12.6 volts to a tube filament. One way is to use a filament transformer that provides 12.6 volts output. This is the normal and usually preferred way to supply a heater. Sometimes the 12.6V power is just not available or possible to obtain. If a 6.3 volt filament supply is available, you can grow a 12.6 volt filament supply from the 6.3 volt filament supply using a voltage doubler. The components used will depend on the heater current requirement. The following describes a circuit to provide power to a 12.6V 0.15A filament. This circuit will draw 0.3A from the 6.3V supply. The "+" is a circuit connection. The "o" is the supply connections. The 0.1uf suppressor capacitors are optional. They should be ceramic. Their purpose is to supress any parasitic oscillations. This is not normally a problem, but can occur in some situations. The 1N4001 can be any 1 amp recifier diode. The 1.5ohm resistor is used as a combination fuse, surge supressor and voltage adjustment. You can adjust the value between 1ohm and 2ohm to tweak the output value. However, tube heaters can easily deal with voltage tolerances of +-20%, so you can leave it at 1.5 ohms for most situations unless you are picky. If you want to drive the heater harder, you can short out the resistor which will boost the voltage output to 14V. That can boost the output of a weak tube for a while (until it wears out). For higher drive currents, you will need to increase the capacitors from 470uf to a higher value. As a rule of thumb, double the capacitance for each doubling of filament current. You may also need to decrease or increase the series resistor value (and possibly wattage) depending on the load. 1/4W 1N4001 o--1.5ohm--+-->|--+----------+-------o + | | + | 6.3VAC | --- 470uf --- 0.1uf* | --- 16V --- | | | o-----------------+----------+ 12.6VDC | | + | | --- 470uf --- 0.1uf* | --- 16v --- | | | ---|<--+----------+-------o - 1N4001 *Optional 0.1uf ceramic capacitors There are several advantages and disadvantages to using a voltage doubler. One advantage is reduced hum in the heater circuit because you are feeding the tube heater with DC instead of AC. However, the circuit draws most of it's power at the tops of the waveform and that causes current surges in the filament supply. That can generate pulsating magnetic fields in the power supply circuits which can potentially get into other parts of the circuits. In a radio this is not likely to be much of a problem, but in an amplifier, especially one with high gain circuits, it may cause trouble. Part of the purpose of the series resistor is to minimize the surge current as much as possible. If you adapt this circuit for filaments that require higher currents, remember that the surge currents through the diodes will be high because they have to supply all of the power at the peaks of the AC waveform. So although the average current will be equal to the output current, the peak current will be much much higher. The size of the capacitors and the series resistor will determine the peak current. Always use a rectifier diode capable of handling the high currents. Since this is a doubler circuit, it requires the heater to be floating. If the circuit requires the heater to be attached to ground or other circuits, the voltage doubler will not work properly unless the 6.3V power source is floating. Also remember that over time electrolytic capacitors wear out. How much depends on the quality of the capacitor. As the capacitor wears out, the output voltage will slowly sag. Shorting out the series resistor will boost the voltage, but it will also wear out the capacitors faster. When selecting the capacitors, make sure they are intended for power supply filter operation (high ripple) such as the Nichicon HE series. You can potentially use ceramic capacitors instead of electrolytic ones. However there are some limitations to be aware of. The first is that the current technology limits the availability to around 47uF at 16V. There is a 100uF at 6V available, but it shouldn't be used for this work for several reasons, the most important is that it would not be able to properly handle the voltages in the circuit, but also that particular capacitor typically uses Y5U dilectric material which is not very stable. For more information on the pros and cons of using ceramic capacitors see the http://www.fourwater.com/files/mlcc-caps.txt file. Tantalum capacitors are another option, and can work ok in this application. However, keep in mind that when a tantalum capacitor fails, it tends to short out and catch fire. So while a tantalum will normally last longer than an electrolytic, their failure mode is a bit more nasty. Also always make sure that the 6.3 volt power supply you are getting the power from can handle the added load from the circuit. The current required from the 6.3V supply will be twice the current used by the 12.6V heater. And of course the usual disclaimer; Remember that tube circuits contain lethal voltages and you should not work on them if you don't know what you are doing.