Good idea to swap McIntosh amp stock tubes?

  The fact of the matter is that every version of the MC275 which I believe currently are 1 - 6 starting in 1961, has it's own specific tube compliment that may comprise a specific brand or multiple brands of tubes for that production run. To say that they designed it using a poor-quality tube from a certain manufacturer is very misleading. So happens that other companies like Luxman design certain parts of their tube circuits around JJs simply because they're the most consistent and reliable and readily available tubes at the moment. It doesn't imply that they used them in every circuit in a specific line of product currently in production because they'll design each circuit around whatever device that satisfies their design goals at the time. As an example, my LX-380 still uses the JJ 12au7 in it's gain and phase inverter sections but uses a 6L6GC that bears no resemblance to the Electro-Harmonix 6L6GC that were used in earlier production runs. I'm sure that the power output circuit as well as the phase inverter was tweaked and the bias set accordingly for the new device. I can't determine who's 6L6GC it is by looking through the ventilation grill on top because there is no silk-screening on the envelope or the base of the tube but the plate looks exactly like the TAD red base and the glass envelope also looks exactly the same. The plate also looks exactly the one in the Psvane 6L6GC but the shape of the top of the envelope does not. I won't crack the unit open to look at the structures at the top of the tubes since they're obscured by the gettering, so I'll probably never know. They may even be using a 5881 instead of the 6L6GC which is very similar and could be interchangeable except for it's lower plate dissipation

Essentially, I'm completely adverse to ignorantly mickeying around with engineers very thoughtful designs and deferring to people who are more technically savvy than myself if my equipment needs servicing. 

Just for kicks, a tube's sound doesn't change over time and it's life expectancy is not determined by months or years but decades or if it fails prematurely due to some unforeseen occurrence. Here's another one, gain circuits don't amplify without negative feedback. Class AB tube amplifiers using a pair of output tubes per channel never produce more than a handful of watts no matter the circuit topology, output transformer or tube compliment over the range of human hearing unless several sets are parallelled together producing only a few tens of watts. The disadvantage of which that they suck so much current and generate so much heat that you don't want to listen in the same room with them, not only that but the reliability issue it creates having all those pairs of tubes you have in there whose bias ifs floating all over the place with constantly changing current demands.

The MC275 based on the best information I can find is a 14 watt per channel amp on it's best day. The new Audio Research Reference 330M reviewed in December 2025 Stereophile, the manufacturer claims produces 330 watts per channel could not be verified by John Atkinson. If you look at the wide band distortion vs frequency plot JA states 12.65 volts or 20 watts to remain under his threshold 1% THD spec from 20Hz - 20Khz. The spectrum of the 50H sinewave, DC-1Khz at 20 watts bears this out as the tallest spike at 180 Hz to the right of the signal is at -60dB. Also, the IM plot where the 1Khz side band is at approximately -70dB and -63dB higher up the scale. He also says that the amp will perform best if it is used with speakers whose impedance is higher than the nominal of the transformer tap it is connected to. 

I could go on here with other choice bits of knowledge but you'll have to pick my brain if you ever pull your head out of the sand and I'll direct you to countless links of individuals who know what they're talking about. Getting late.

@faustuss 

MC275 = 2 channels @ 75Watts @ 8ohms.

I’ve re-tubed every generation of the MC275, I am quite familiar with all of them and their different tube complements. You are correct that they used to come with great tubes. But they don’t anymore, and that is a fact.

If you were responding to me, let me be clear that I didn’t say they came with poor quality tubes. I said they were fine but don’t sound as good as NOS tubes. Respectfully, I don’t think I’m the one that’s being misleading. 

Were I you friend, I’d be careful where you throw rocks when it comes to who has knowledge and who doesn’t. You blatantly quoted the wrong spec for an amp that is named literally for how many watts per channel it has, and then started hurling rocks at me while you called me a dummy. 

I swear I couldn’t make this stuff up if I tried.

BS.  Half the fun of owning a tube amp (or preamp) is trying new (equivalent) tubes.  Not always better, but often different and worth the effort.

I don’t own a Mac amp, but I’ve really enjoyed tube rolling and dialing in the sound just the way I like it.  There should be no reason you can’t do the same.  You can always add the stock tubes back in the mix.  

Tube amplifiers that reflect sound changes when tube swapping occurs may not be designed very well.

A bit of an obnoxious assertion / wording on MC's part. It's true that if a circuit design runs its tubes in an egregious way (e.g. way out of spec, or far from optimal conditions for a given tube type), then tube rolling can have much more sonic impact than normal. But I've always found that tube rolling has audible effect, even in well-engineered components and for slots that have been advertised to "not" reveal tube rolling differences (e.g. cathode followers, grounded grid outputs with SS CCS).