MC2301 Power Tubes & Driver Information (plus a MC275 V bonus)

[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

[Still-One]

So much to think about with tubes.

All us SS guys worry about is finding this.

[f1 fan]

Bob...great info. I've always enjoyed reading what the guys at McIntosh have to say be it tubes, ss, design and engineering of the product. Great read.

Dave

[kb7215]

My MC2301 tubes are marked as follows: Front-to-Back (Right Side) as V1-, V2-,V3-, V4-, V5-. The other side, Front-to-Back (Left Side): V5+, V4+, V3+, V2+, V1+. IOW, my front four tubes are V1-, V2-, V5+, V4+. This configuration layout is the same for the other MC2301 in my pair.

Does this change your explanation in any way?

[PHC1]

Good to know Bob. Thanks for sharing this with us.

[vintage_tube]

Originally Posted by kb7215

My MC2301 tubes are marked as follows: Front-to-Back (Right Side) as V1-, V2-,V3-, V4-, V5-. The other side, Front-to-Back (Left Side): V5+, V4+, V3+, V2+, V1+. IOW, my front four tubes are V1-, V2-, V5+, V4+. This configuration layout is the same for the other MC2301 in my pair.

Does this change your explanation in any way?

Thanks for bringing that to my attention (I corrected my post) & welcome to AA. Congrats on owning what we already know -- one of the best tube amps ever produced. I know what happpened -- while placing call to Ron-C, the door bell rang, took care of that, Ron answered, went back to the amps that had tube covers on and attempted to answer his question on what side were the +/-'s on and without taking the tube covers off -- thought I read +/- thru the cage and made a mental note. Now, we know I can't read jack****.

Bob

[f1 fan]

kb7215...Welcome To AA and you own some great tube amps.

Dave

[Formerly YB-2]

Great info. I was just getting ready to order a quad of 12AT7s for my MC275 and now know that I want a pair of triode matched for V3/V6. Thanks.

[vintage_tube]

Originally Posted by Formerly YB-2

Great info. I was just getting ready to order a quad of 12AT7s for my MC275 and now know that I want a pair of triode matched for V3/V6. Thanks.

One of the many reasons for this web site and more than glad to help (and save you unnecessary spending on premium tubes when it's not absolutely essential).


Bob

[Formerly YB-2]

Yep........thanks, again. I've used NOS JAN Philips 12AT7s in the MC275 for several years and find they provide excellent sonics, so that is what I am ordering. I am quite pleased with my newest MC275MkV and the sonics it provides with the McIntosh branded tubes. Considerably better sounding than the tubes they were supplying with the McIntosh name back in '07-'08.