811A

The 811A is a transmitting triode originally designed for RF amplifier and modulator service, with a graphite plate and thoriated-tungsten filament. In audio use it can deliver 10-15W in single-ended Class A, though it is more commonly seen in push-pull configurations where it can produce 30-50W. Its relatively high mu (~35) for a power triode makes it easy to drive but gives it a less pure triode character than lower-mu DHTs like the 300B or 2A3. The 811A is inexpensive and readily available in current production, making it an accessible entry point to directly-heated triode amplification.