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In Reply to: RE: Nice! I love my MC-30s... posted by Rod M on February 14, 2022 at 18:31:09
The single biggest difference is the vacuum rect for the 30 power supply compared to the silicon voltage doubler for the 40. This alone probably accounts for the 10W power difference. The 40 also incorporates an additional feedback loop from the cathodes of the finals to cathodes of the 12AX7 drivers (g1 of finals). Schematics for the latest version of the 30 also show this additional feedback loop but I've never seen an actual 30 with it. There are many other minor differences such as a hum pot on the 30 vs none on the 40, bias supply circuit etc. Also, Mac made many changes during production runs so schematics don't always match reality.
Follow Ups:
I have a 240 for a second system, and while I adore it, there is a definite difference in it's sound vs. the 30's that I chalk up to SS vs. tube rectification. It's the latter which accounts for the incredible midrange magic on the 30 that makes vocals especially hauntingly good, especially on a speaker like Klipsch La Scala's. I find myself looking up more from whatever work I'm doing when music is playing as I hear things, phrasing or whatever that I never heard before.
Obviously, 30s are mono, 240 is stereo so PS sharing is present in 240. There's also the addl feedback loop in the 240 that most likely isn't present in the 30. Hopefully since your amps are in frequent use, critical passives have been replaced, esp but not limited to the bumblebees. Unless the passives are the same for each amp, there's another possibility for sonic differences betw them. Further confounding this is the fact that Mac's unity coupled designs are inherently high feedback with nested positive and negative loops (if stock) which may diminish differences in passive characteristics. From my perspective, it's difficult to point to any one or two physical difference betw the amps as the cause of sonic differences.
Edits: 02/15/22
Both amps have been completely restored but you're right there are other differences that would account for sonic differences.
Mine came with a hum pot or a bias pot, though I recall John saying it was for bias. In any case, I think he might have done it for the previous owner and didn't like it, so I left them for him to sort it out.I know the bias is dead nuts and it's over 30 watts after he replaced a few tubes with Amperex.
I'm pretty good with computers, but borderline dangerous on schematics.
What I wonder is how the same driver tubes do stereo on the 240 versus the sames tubes for a mono block. It seems like I have twice as many per amp. My wild ass guess is the tubes half two halves and only one is used for the 30s.
-Rod
Edits: 02/14/22
If you don't count the rectifier the tube complement /count of a 30 and 40 are the same. A 240 almost doubles the count of the 40. The difference is the input 12AX7. In a 40 (30 too), only one of the two triodes is used the other is disabled. On a 240 one of the triodes is used for the right channel and the other for the left. So the tube count for a 240 is (2X40)-1 or 11 vs 6.
Ok, I got confused and had been looking at an MC-225 which has two 12BH7s and has two 12AU7s instead of the two 12AX7s on the MC-30. Obviously, it completely different.
-Rod
The 225 dispenses with the 12AX7 cathode follower driver so it has two fewer tubes than a 240 (or a 275). The 225 is one sweet little amp. . . arguably Mac's best sounding of the vintage series. Too bad power output is limited.
Rod M, the lab I worked in as a grad student had a number of MC 60s and several VOT A-500s. I borrowed a pair for listening at home. They were particularly good for the brass in Aida.
db
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