Details
Used old stock octal beam power tube. Various brandings, but all old stock ST-bulb (coke bottle shape) 6550 tubes were made by TungSol. Three holes on each side of the plates.
NSN/FSN = 5960005773027, 5960006200705, 5960008809933, 5960009858984, 5960010150381
HP/Agilent Number = 1923-0009