Each tube circuit is carefully analyzed and a new transistor circuit is tried, adjusted and tested to provide maximum amplification without getting any strange oscillations or other undesirable results. The impedance match from drain to coil is done by using a choke and small value coupling capacitor. The impedance of the FET gate is high and approximates a tube grid however a small coupling capacitor is used to avoid overdriving the input. Oscillators must have special handling and parts are usually selected by trial and error until the desired results are obtained. There is more to know but basically each tube is replaced with a unique circuit. Sometimes trial and error coupled with experience is the only way to get the best results.
The strong signal overload performance is also likely degraded, an important consideration for users in certain locations (where powerful broadcast transmitters are in daily close proximity).
