Rectifier tube, beautiful ST-Shape "coke bottle", NOS / NIB Novosibirsk, built 1960-1970s.
Can replace the 5Y3GT/WGT in many amps like the Fener CHAMP (5F1) or DELUXE (5E3) and provides a little extra power resulting in a little extra headroom. Also replaces GZ31, 6106.
