Having fun with 1920's triodes..
Late 1920's the first indirectly heated triodes were developed. Primarily to replace directly heated triodes, hence the 4-prong base and side/top-pins for filaments.
![Cardon AC 373 triode](http://www.dhtrob.com/_blog/userfiles/image/100_5550.JPG)
The Cardon AC 373..
![Kellogg 401 triode](http://www.dhtrob.com/_blog/userfiles/image/100_5554.JPG)
And the Kellogg 401..
Running both 3 Volts at 1 Amp for the filament and a B+ of 90 Volts at 4mA.They make for a very nice sound!