Filaments


People have asked me if there is an alternative to the filament supplies I use with the batteries. Not everyone wants/needs batteries due to different issues (size, trouble, expense). Below I have listed three topologies I use to light up my directly heated tubes. For indirectly heated tubes I either use AC of a rectified and voltage regulated supply.

 

The simplest method of heating a DH tube (triode or otherwise) is to use AC, just solder on the two wires and you're off. Not all tubes like AC, well the person listening really, on the filament. Specially preamp or gain tubes give quite a bit of hum when using them on AC for obvious reasons. To get rid of this hum we need to run the tubes on a DC supply. A rectifier (diodes) and a capacitors sets up the beginning of this supply. if you already have the voltage you need and it is stable, stick with it... but you will probably need some kind of regulation to get things quiet.

The first thing that comes to mind when the term "regulator" is used is that it is used as voltage regulator. The general census is that these don't sound so nice compared to AC, so it defeats a purpose. The solution lies in using the regulator not to hammer down the voltage, but to regulate the current that is flowing. Below is a schematic of how this is accomplished.

The resistor in series with the output is the component that determines the current that will be passed. This circuit is a current source, it will try to maintain the same current even when connecting a different load. Within limits the regulator will keep the current constant. I've drawn an LM350 here, but any kind of adjustable regulator will do, provided it can handle the required current (see datasheets). The choke in the first section are to form an LC filter, they are not a must, be do some of the pre-regulating of the supply.

This same method can be used using a battery as can be seen below. This is the setup I normally use to get the correct voltage for say the 26 (in my preamps) to get to the desired working voltage for the filament.

 

Whenever possible I like to use as few components as possible. The setup below is as simple as it gets. It can be accomplished with a rectified circuit as well, but as said before most tubes will still produce hum. The capacitor over the battery might seem a bit redundant. I place a Black Gate 1000uF/25V NX here (bipolar) purely for HF response. The two resistors are used to give the right amount of drop for the tube in question.


Nice article on the lifetime of thoriated tungsten filaments: download .PDF