@retiredaudioguy very cool! I did as you said and learned that filamentary triode tubes like the two a three can be powered with either AC or DC to the filament. Here’s a little exerpt from the AI explanation. Thanks for prompting this little learning experience!
AC Filament Heating
- Standard Specifications: 2.5 Volts AC at 2.5 Amperes. [1, 2]
- How it is used: It is powered directly from a dedicated low-voltage winding on an amplifier's power transformer. [1, 2]
- The Challenge: Because the filament is also the cathode, the 60Hz (or 50Hz) mains frequency alternating through it can cause audible "hum" in the speakers. [1, 2]
- The Fix: Designers use a hum potentiometer or a center-tapped transformer winding to find an electrical balance point, canceling out most of the AC hum. Many audiophiles prefer AC because they believe it yields a more organic, open soundstage. [1, 2]
DC Filament Heating
- Standard Specifications: 2.5 Volts DC at 2.5 Amperes. [1, 2, 3]
- How it is used: The AC from the transformer is passed through a rectifier bridge and smoothing capacitors (and often a voltage regulator) to turn it into pure, flat DC before entering the tube.
- The Advantage: It completely eliminates AC-induced background hum, making it ideal for high-efficiency speakers where background noise is highly noticeable. [1, 2]
- The Tradeoff: One side of the filament will always be slightly more positive than the other, causing a tiny imbalance in electron wear across the filament over time