In de datasheets van General Electric vond ik nog :
"The 1T4 is a remote-cutoff pentode for use in battery operated equipment. The remote-cutoff characteristics makes it suitable for use in stages to which AVC is applied."

"The 1U4 is a sharp-cutoff pentode intended for use in battery operated receivers."

Dat zou volgens Wikipedia betekenen dat de twee buizen elk een eigen specifieke toepassing hebben en dat er dus geen sprake is dat een 1U4 een verbeterde 1T4 is, zie: https://en.wikipedia.org/wiki/Pentode

Variable transconductance ("variable-mu", "remote-cutoff" or "super-control" tubes in general are those with a non-uniform grid wire spacing to allow them to handle a wide range of input signal levels without excessive cross-modulation distortion, and so useful in radio frequency stages where automatic gain control is applied to the pentode.

Sharp-cutoff ("high slope" or ordinary) pentodes have the more ordinary uniform spacing of grid wires, and so mutual conductance decreases in an essentially uniform manner with increasing negative bias, and has a more abrupt cutoff. These pentodes are more suitable for audio amplifiers.

Willem