Measurements were made with L1-3 tube tester.The parameters of all the tubes in the set 
correspond to the parameters specified by the factory :
Anode Current  Ia = 45 +/- 12mA , steepness S= 4,1+/-1 mA/V

The main criterion for selecting pairs and quartets is the anode current of the tubes, not the year and batch.
The tubes in pairs were manufactured in 1970 - 1979.See the photos for the measurement results for each set.

The original packaging was not preserved due to its very poor condition. Only the inner passports were preserved.

6P6S -  output beam tetrode, direct substitute for 6V6GT.
The 6V6 family came in many constructions and were very popular as the final audio stage of broadcast receivers.We have the original American rugged metal valve produced in the mid 1930s and subsequently used for military designs. Also a miniature glass tubular version the 6V6GT. The 6V6G was of classic glass envelope shape.