This amplifier just got fully serviced by an authorized AR service . I replaced the tubes to a new factory matched set of KT 120's and we also replaced the LED illumination bulbs to a new matched set !!!!!!!!
The Reference 75 ($9000) is similar to the Ref 110 and Ref 150 in that all three use input circuits using direct-coupled JFETs and a 6H30 tube. However, the Ref 75 has only four Tungsol KT120 output tubes (in ARC's ultra-linear "partially cathode coupled" topology), compared with eight in the Ref 150, and eight 6550Cs in the Ref 110. Warren Gehl told me that, compared with the 6550C, the KT120—apparently the first output tube specifically designed for audio applications—is more robust and powerful, with greater dynamics, lower distortion, better tonal balance, a larger soundstage, and a greater sense of presence. The Ref 75 shares the same oversize power supply (520 joules of energy storage) as the Ref 110, but has improved interstage coupling capacitors, and more damping material on its capacitors and heatsinks.
I asked Gehl what makes the Ref 75 special.
The circuit design is straightforward, and careful attention was paid to ensure that signal paths would be as short as possible. Removing the cover and looking inside confirmed the simplicity and elegance of the circuit layout. In fact, with only one pair of output tubes per channel, there's no need for a cooling fan. Weighing only 47 lbs, the Ref 75 is the lightest tubed stereo power amp I've had in my system.
Read more at https://www.stereophile.com/content/audio-research-reference-75-power-amplifier#Y6UwzR4CetM5MJ47.99