From The Vintage Knob, one of the best vintage resources I know: The F55 is a low-THD, low-TIM, low-NFB design thanks to the high bandwidth of its output bipolar transistors (80MHz) and Pulse Power Supply (PPS).
High-fT transistors are made of a string of normal transistors connected in parallel offering lower switching distortion and better stability.
The PPS rectifies the line voltage, chops it at 20kHz, DC-AC inverts it and then rectifies and filters it again to output pure, clean, stable, DC current.
The freon-filled Heat-Pipe allows to dissipate heat more evenly over the multiple thin fins and also to place the output devices closer to both pre/power stages to avoid long cabling ; due to the immediate cooling of the system, the output devices are mounted directly on the pre-driver board !
Like most mid-end Sony amplifiers, the F55 can receive MC cartridges and even has two-step capacitance and impedance switches !
Internal cabling is done entirely with 99,99% pure copper and the preamplifier stages are separated from the main amp by a FET buffer stage.
Source selection is done by electronic switches relaying switching MOS-Fets placed in the signal circuits.
The real twist is the strange design retained for the volume control : if the push buttons were already old salad by 1979, the volume indicator itself is... not what you think it is : the F55 uses a green ribbon with a motor to pull said ribbon behind the indicator window !
The other LEDs really are real LEDs.
It sounds fantastic...very detailed and authoritative, very musical. The volume setup is as strange as it sounds, but works well. The attention to detail in every aspect of the design, and manufacturing quality gives this baby magnificent musical abilities.