The 25Z6G contains a a pair of rectifiers. This rectifier was designed for AC/DC receivers using a 110 to 120 V mains input. The rectifiers can be used to build full wave, half wave and voltage doubler circuits. In the half wave mode both sections may be used in parallel.
Electrically the 25Z6G is identical to the 25Z6, 25Z6GT and 25Z5.
The twin rectifier sections. The heaters are in series and joined by the tape at the top.
Made in USA but the octagon containing the Type designation suggests RCA as the maker.
A closer look at the stitched carbonised anodes and multiple strand heaters.
The classic envelope is 38 mm in diameter, and excluding the IO base pins is 89 mm tall.
References: Data-sheet & 1040. Type 25Z6G was first introduced in 1936.