The 4-125A or 4D21 is a 125 watt dissipation tetrode. The usable frequency for maximum ratings is 120 MHz. Eimac's development started in late 1943 and lasted through 51 experimental stages ending in about mid 1946. Early experimental and production tubes had finned anodes. See: X130. The second example is an Eimac that has straight sides and is slightly shorter. The third example is an Eimac but has no base insulation and straight sides similar to the RCA example. The fourth example was made by Lewis and Kaufman (LK-4D21), the fifth by Penta Labs (PL-4D21), the sixth by GE and the seventh by Sylvania. The eighth example is an RCA of the usual construction and the ninth is a newer RCA with no base insulation, straight sides and only a perforated skirt. The 6155 is the number used by RCA and others for a smaller version of this tube. Amperex also made a similiar small tube the AX4-125A. The 4F21 is the same tube in Japan.
Length = 5.69" Diameter = 2.72"
Max voltage = 3000
Max current = 225 ma
Fil voltage = 5
Fil current = 6.5 amps