[old_tv_nut]

For linear amplifiers, transistors generally want lower impedance than tubes (less voltage, more current: smaller resistors, lower b+, bigger capacitors, smaller inductors.) Plus, a single transistor is unlikely to have the same characteristics as a single tube. Some combination of a FET with a bipolar transistor might be designed to come sort of close. However, the differences would be especially critical for circuits that depend on non-linear tube characteristics, like IF amplifiers with AGC or the color AFC control tube in your tube color TV.