McIntosh MC-2301 Tube Replacements

All McIntosh tube power amplifiers use the Unity Coupled output circuit (a great description can be found in Bruce Rozenblit’s book “Tube Audio Design”).  This inventive and effective circuit is able to run the output tubes conservatively, giving those tubes longer live so I bet the original Mac output tubes in your amp may be fine. However, the tradeoff (there’s always a tradeoff) is that the driver tubes are run hard.  These are likely the 12AT7s you mentioned.  I would consider replacing those first. Good news is that even though NOS 12AT7s are expensive (guitar amp aficionados gobble them up) an (almost) direct replacement tube is the 12AZ7, the only difference being the 12AZ7 pulls 400ma of filament current while the 12AT7 demands slightly less at 300ma filament current.  NOS 12AZ7s are dirt cheap and common as they were mostly used in televisions, though McIntosh has used them as driver tubes in the past, such as the MC275 Gordon Gow amp, a pair of which I owned for over 20 years. During that time I only needed to replace the KT88s once, but the NOS 12AZ7 drivers needed replacement three times.

I don’t have a Mac, but own several tube amps and gobs of NOS and some new production tubes. My Dennis Had Insipre will run KT-88, 6550, EL34 and 6L6.  The amp came with Gold Lion KT-88s. They are  excellent tube IMO.  Clean, transparent, linear with good balance and a touch warm. Rock solid construction, made in the old Reflector factory in Saratov. 

You should also consider Shuguang Nature series KT-88-T. They take a while to burn in, but offer a touch more bass and detail. NOS Svetlana 6550 are an amazing tube, bass, midrange drive, and seductive. Less transparent but make music come alive, as in sitting in the front row.

I don’t own any new production 12AT7s, all NOS. But I do have a Gold Lion 12AX7 Russian gold pin, that is V1 signal tube in another amp, great tube. Just like the KT-88s, clean, transparent, well balanced. Be sure to get Russian version, they also offer a Chinese version. It’s a bit flat and uninspired. If you want to chase NOS, go with Telefunken or Amperex.

fyi,  I am running a pair of McIntosh mc275 in monoblock mode with balanced cables, so, no V1 used.. .  I replaced all the tubes with Russian Reissue Genalex Gold Lions (gold pin) in 2014.  After 11 years,  I recently tested all my tubes and the small tubes were pretty worn out, but, the KT88s still tested very high.  I ended up just changing the small tubes - 12ax7 and 12at7.  I took my time and ended up buying NOS vintage tubes.  The sound definitely improved, but, my old Gold Lions were testing low, so, they were worn out, and I cannot compare the sound when the Gold Lions were new.  My impression is that if you decide to tube roll, go vintage NOS for the small tubes, and stick with the Russian Reissue Gold Lions for the KT88s.