Nice choice of amps, with minor work that unit will perform against just about anything. Unit is a pentode operated in ultralinear, with a HUGH power supply (in the neighborhood of 550 joules). The first series of 79's were not noted as an 'A' but are called 'A'. The 'A' version was a (not exactly) D76A front end with this power supply, the 'B' was designed with the D90 front end and the 'C' is designed around the D115 (and D115 MKII on the 79C-MKII). There is only around 25-30 79'C's, not sure about the others. Very well built and can deliver top performance with very little maintenance. Keep one thing in mind...ARC amps work best with top quality tubes. I'm currently working a large one over after someone ran 'China' type tubes..resistors and circuit took a hit when the tube(s) let go. Get Sevtlana 6550 or Kt88 or for the best protection look at ARC's tube..costly but most reliable. I'm not saying others are inferior just appropriate quality when it comes to these tubes..Ram, others as well will work. Also, the regulator tube helps to have high transconductance. Down side to this amp is the power out of 75-80 watts per side, can be small or huge if coming from the single ended camp. Holman, e-mail me and we can get in touch, I will explain more about this superb amp. KMc.