[vintage_tube]

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.

**Having stated the above, if you are capable of testing tubes and know the value of each, it's optimum to attempt to balance each quad with similar rated tubes; rather than randomly slotting them down one side and up the other. The way I understand it, one quad could have a better set than the other and not affect performance characteristics of the amp.

**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.


Bonus Tidbit: for those MC275V owners, the most significant tube in that unit is the 12AT7's which ocuppy slots V3 and V6. Those that ocuppy slots V4 and V7 are not as critical and work as a pair. Emphasis should be placed on matching tubes slotted in V3/V6.

Bob