Russian tube 6P3S FOTON factory, output beam tetrode. Best quality, the sound is nice - authentic vintage tone. Near equivalent: 6L6, 6L6GT, 6L6GC and 5881
Rare Soviet 6S5S tube - generator, detector or low frequency amplifier. It is direct replacement for the 6C5 (metal envelope tube – so this one has another shape), 6C5GT tube and the chinese 6C5P