[vintage_tube]

I found this I wrote back in 2010:

Had a couple emails w/Ron-C and a very short conversation w/Ron Evans (MC2301 designer/engineer @ McIntosh) regarding the power tubes for the MC2301 and thought I'd share what I know and what I learned so owners and prospective owners are a bit smarter & a wee-bit more knowledgeable when it comes to the power tubes in a MC2301 (and the 12AT7’s).

**First off, as we have heard here and elsewhere, matching of KT88's is not a big deal for this amp. "Don't get hung up on testing/matching power tubes as this will not increase their lifespan".

**What's critical for amplifier performance is based on two designs of the unity coupled output:
a. How well matched the idle current is on the right and left power tube banks.
b. The maximum cathode and plate current capabilities.

**Degradation of a power tube(s) will display itself with clipping as power is increased (there is a way in which to troubleshoot to a degree w/o a tester --keep reading).

**If you notice the 8 power tube slots are labeled V1-/V1+, V2-/V2+, V4-/V4+ and V5-/V5+. Each - (right bank) and + (left bank) pair works together push/pull, cathode and plate since they are unity coupled.

** Now, this is the most significant tidbit I learned -- those 8 power tubes are further grouped into a quad -- the front four (V1-/V5+, V2-/V4+) being separate from the rear four (V4-/V2+, V5-/V1+) as a design feature of the balanced design. What's cool about this is, each quad (per se) can be removed and the amp powered up and operated (carefully) -- this allows one to help isolate at least down to a quad which tube may be susceptible to giving the owner fits.

**Again, stock power tubes should be "looked at" after around 3000 hours and expect them to last between 5K & 10K hours.

** The most critical tube in the MC2301 are the two 12AT7A’s (slots V3-/V3+) which "work hard as they drive the KT88s". These are preferred to be matched and in optimum operating condition.

**Unlike all other McIntosh tube amp designs, the MC2301 is balanced with separate hot and ground connections for 2, 4, and 8 ohms. All other McIntosh tube amps use one common ground for all three impedances.