Directly Heated Filaments
    Many of the previous heater tricks will not work with directly heated tubes like the 2A3 and 300B, as these do not have heaters: their cathode and filament are one. Consequently, connecting the filament to the wall voltage, for example, is the same as connecting the cathode to the wall voltage.
    Like a heater element, however, the filament can be heated by DC or AC voltage. If AC voltage is used, the following technique can be used. The usual configuration for an AC powered directly heated tube is to use a center-tapped transformer with the outside leads connecting the filament and the center-tap connecting to ground  either directly or via a cathode bias resistor. In the latter case, the cathode resistor is usually bypassed by a large valued electrolytic capacitor.

     Bypassing this capacitor with a smaller valued high quality film or oil capacitor is both common and recommended. In spite of this effort to "speed" up the electrolytic capacitor, the signal must still travel through the filament transformer's secondary winding. The assumption being that this winding represents zero impedance to all frequencies. (I would like to see a test of various filament transformers to see what contribution they make to a directly heated tube's frequency response. If you have ever thought that you might get away with using a power transformer as an output transformer, you should test its frequency response first before going any further. The results will probably prove amazingly disappointing. )

     But what if instead of using just one small film bypass capacitor, we use two small bypass capacitors and connect them to the output side leads of the filament transformer? Now the transformer's secondary is bypassed as well. The highest frequencies will take the path provided