Vacuum Tube - 4-125A / 4D21, Tetrode, Power Amplifier
New-Old-Stock (NOS) tube.
The 4D21 / 4-125A is a four-electrode tube designed for use as a power amplifier and oscillator. The anode is capable of dissipating 125 watts, and cooling is accomplished by radiation. The cathode is a thoriated-tungsten filament. Maximum ratings apply up to 120 megacycles.