Purposely Running
Low-Heater Voltages
    We have covered over-voltages and their danger, but what of under-voltages: are they also dangerous? Many old textooks claim that any voltage lower or higher than the specified heater voltage will shorten the tube's life. A few sources offer a different view: namely, the higher the heater voltage, the shorter the tube life and the lower the voltage, the longer the tube life. Who is right? My guess is the second group, as I have experimented a little with lower heater voltages and I have found the tubes glow less brightly at turnon, which must be for the good. Besides it seems odd to me that a heater element would suffer from being under heated. I know of one pro-audio company that makes hybrid tube gear that uses a 5 volt power supply to feed all its 6.3 volt tubes. I have been told that the tubes work fine and last forever in these units. The following diagram from Richard Shea's "Amplifier Handbook" shows transconductance over time with three different heater voltages.

by the capacitors instead of the transformer. Do not worry about the capacitors shorting out the transformer except at the highest frequencies, as their reactance is too low at 50 or 60 Hz.

Voltage regulators for the 2A3
     Directly heated filaments benefit from DC voltage, just as indirectly heated cathodes do. Unfortunately, no fixed voltage regulators are available with a 2.5 volt output. But 1.25 volt fixed regulators are available; they are called adjustable voltage regulators. Most adjustable voltage regulators have a 1.25 volt base output voltage and use a voltage divider on the output to scale this voltage up to any greater voltage up to the regulator's limit.
   What good is a 1.25 volt regulator when the 2A3 needs 2.5 volts on its filament? A center-tapped 2.5 volt regulator power supply can be made out of two 1.25 volt regulators.

     The 2A3 requires 2.5 amps of current, which precludes the use of the LM-317 and LM-337 regulators. The LM-350 and the LM-333 have 3 amp current limits, however. 

5 Volt Regulators for the 300B
     300B users are lucky, as there is the greatest number of options when it comes to 5 volt regulators. A good choice is the LM2943CT, which is a low dropout 3A voltage regulator. With as little as 1 volt to spare this regulator will maintain regulation. Dropout voltage is the specification most beginners ignore, unfortunately. A voltage regulator needs to have some overhead to function correctly and not giving it what it needs is a big mistake.

    The first thing I noticed was that the 6.3 volts line fell off depressingly quickly: after just 10 hours or so the transconductance begins to slowly fall off at an amazingly linear rate. The 5 volt line, on the other hand, remains pretty much flat over the whole 5,000 hours of use. Why not try a 5 volt heater power supply and see if the extended life and greater consistancy in Gm, albeit lowr Gm, is not worth the effort. If you have had any experience with this approach, please drops an e-mail.
                                  //JRB