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ID = 57009
United States of America (USA)
Brand: Penta Laboratories; Chatsworth (CA)
Developer: Eimac (brand name) 
Tube type:  Transmitting Tetrode, air cooled   VHF 
Identical to PL-4D21 = 4D21 = 4-125A
Similar Tubes
Other characteristic (electr. data):

Base Giant 5 pin medium shell B5D Top contact with a cap.
Was used by Transmittertube (Radio)
Filament Vf 5 Volts / If 6.5 Ampere / Direct

This is just the 4D21 with the prefix of the manufacturer, PL = Penta Labs

Output watts increased from 125 to 175

Dimensions (WHD)
incl. pins / tip
x 124 x 62 mm / x 4.88 x 2.44 inch
Information source - - Manufacturers Literature   Catalog October 1963
Tube Lore (II)   p. 87

pl_4d21_so.png PL-4D21: ARRL Handbook 1972
Anonymous 10 Collector

Just Qvigstad
PL-4D21: Manufacturer's Literature
Anonymous 15 Collector

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