The 5751 is a special quality double triode that was designed to resist the effects of shock and vibration. The triodes are high μ types similar to the 12AX7 but with gain reduced to 70.
Robert Coleman has replaced all the 12AX7's in his Blues Junior Amplifier with 5751's to achieve the sound her prefers for his guitar. Guitar amplifiers differ from Hi-Fi amplifiers in that the former are biased to achieve distortion as the amplifier is part of the instrument, whereas the latter are biased so as to minimise the colouration introduced by the amplifier itself.
The anode has a central depression that forms the working face and is placed close to the grid. The grid can be seen to be wound on copper supports. The composite structure that connects to the outside workd consists of three metals, the pin, the red plat and the internal material is copper. The latter was not used in earlier valve production in this context.
The two triode sections are separated by a central electrostatic screen.
The thin glass tube envelope is 21 mm in diameter, and excluding the B9A base pins is 49 mm tall.
Reference: Datasheet. Type 5751 was first introduced in 1953.