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Follow-up on MC-30 Rebuild

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Posted on May 15, 2016 at 07:20:46
Lee of Omaha
Dealer

Posts: 1800
Location: Omaha NE
Joined: September 8, 2006
 I'd heard that K-40Y capacitors need some break-in time. The amps sounded great immediately after the rebuild, so I didn't "get" the break-in time thing.

Now I do. We're about 40-50 hours in, and holy cow. The thing that's most objective is that on a great recording (SACD titled "Kenny Burrell and John Coltrane [most highly recommended; great music, excellent original recording, great remaster]) instruments playing in unison are exactly that--individual instruments playing in unison, rather than the mashup most amps present.

I'd sure like to know the physics behind break-in of a capacitor.

 

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RE: Follow-up on MC-30 Rebuild, posted on May 15, 2016 at 09:02:05
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 "I'd sure like to know the physics behind break-in of a capacitor."

Dielectric absorbtion is a factor.


Eli D.

 

How?, posted on May 15, 2016 at 10:33:14
Lee of Omaha
Dealer

Posts: 1800
Location: Omaha NE
Joined: September 8, 2006
 I know the ill effects of dielectric absorption. But what changes as the capacitor is used?

 

RE: How?, posted on May 15, 2016 at 13:12:05
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 A certain amount of energy builds up in the dielectric, over time. When things reach a "steady state", you notice it.

BTW, a certain amount of observed "run in" is associated with the listener getting used to what is being heard. I certainly will not guess where Physics leaves off and Psycho-acoustics begins.


Eli D.

 

RE: How?, posted on May 15, 2016 at 13:27:34
Triode_Kingdom
Audiophile

Posts: 10044
Location: Central Texas
Joined: September 24, 2006
 I think psycho-acoustics is responsible for about 95% of the opinions around here. The other 5% are just psychos. :)


 

RE: How?, posted on May 15, 2016 at 13:41:59
Lee of Omaha
Dealer

Posts: 1800
Location: Omaha NE
Joined: September 8, 2006
 I'm wary of hearing what you want, too. I don't think that's what going on here. I'm hearing detail and especially separation of instruments (not spatially, but in an indescribable way that is live-like, not recording-like. The only variable is the amps' burn-in.

I think there is some physics going on. "Magic" bothers me; I like to know the "why" of things. I was always a pain in the ass to my teachers because of that.

 

RE: How?, posted on May 15, 2016 at 14:38:33
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 Lee
Dielectric absorption in coupling caps can't really affect sound in a bad way. Coupling caps are supposed to block DC while passing the AC signal so who cares if the DC gets absorbed into the dielectric.All the better I would assume.
Dielectric absorption simply does not affect the way a capacitor passes the signal. Dielectric loss becomes a problem when significant high frequency signal voltage appears across the capacitor, but is rarely even measurable as a loss at audio frequencies that we would listen to.

"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

Are you sure about that?...., posted on May 15, 2016 at 16:40:51
Steve O
Audiophile

Posts: 12362
Location: SE MI
Joined: September 6, 2001
 ...if one considers the theoretical electrical model of DA, "smearing" of transients might be expected. It's not clear how audible this would be and I've never seen any definitive listening based studies addressing the issue. Since the DA model is relatively simple, I'm curious what a sim might show us, esp using a non-repetitive driving signal.

FWIW, the two types of caps with the greatest DA I've encountered are electrolytics of any nature and to a lesser extent, K40 PIOs.

 

Best post of the month! nt, posted on May 15, 2016 at 19:02:08
Paul Joppa
Industry Professional

Posts: 7295
Location: Seattle, WA
Joined: April 23, 2001
 nt

 

RE: How?, posted on May 15, 2016 at 19:12:41
Paul Joppa
Industry Professional

Posts: 7295
Location: Seattle, WA
Joined: April 23, 2001
 Dielectric absorption causes hysteresis, similar to magnetic hysteresis in iron cores of transformers. In both cases measurable distortion occurs. Mylar has high hysteresis, teflon is low. Ordinary iron has high hysteresis, nickel is low.

In my experience, both caps and iron components have a breakin effect, which can be confirmed by comparing two identical circuits, only one of which has been broken in without any listening (drive a resistor instead of a speaker). I have done this experiment, as have many others here I'm sure.

Fortunately for me :^) I am more an engineer that a scientist, so the observed fact is more powerful than the theoretical prediction. I would love to find a plausible reason for the breakin effect but I have not yet found one.

 

RE: Are you sure about that?...., posted on May 16, 2016 at 21:33:41
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 I'm curious what a sim might show us, esp using a non-repetitive driving signal.

That is what would be hard to measure in overall application because even tho paper in oil caps have a higher DA,some like Steve Bench have cited the fact they have very stable characteristics with freq change.
This goes back to one of my original points in that we typically do most testing with a single frequency at a time and simple logic tells us that we don't listen to just one frequency at a time.
Now lets say we drive a capacitor with a 20 to 20k sweep and monitor it on a PC thru an 8903 or just a nice audio analyzer in general,I wonder if we could spot any obvious conclusions like non linenerties or excessive distortion at different frequencies of one cap vs another? I would love to be able to measure real world conditions as used in an audio amp or preamp..

I put caps on my Sencore capacitor analyzer and I can measure the most important aspects of the cap from an engineering standpoint but in audio use it can be tricky and sometimes things that are perceived as negative in the engineering world,can be a good thing in audio applications.
I would love to make some conclusive measurements as it pertains to our hobby..Help me set something up.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

Follow-up on Follow-up, posted on May 17, 2016 at 17:47:29
Lee of Omaha
Dealer

Posts: 1800
Location: Omaha NE
Joined: September 8, 2006
 After reading the long and illuminating discussion on the possible merits of replacing the 12AU7 with a 6CG7, we tried it.

Result, if you own McIntosh tubed amp(s): Do it. Now. You won't look back.

We used NOS American tubes (don't remember what brand) that tested NOS.

Expectation: I either didn't expect to hear a difference or thought that maybe after extended listening I'd be able to hear some small difference.

Test method: Single blind testing. One amp was modified and hooked to one speaker (I was not told which one). The preamp was set to mono. The balance control was panned from one channel to the other.

The difference was immediately apparent. I thought the amps were amazing before. With the 6CG7 they're substantially amazinger (that's a word, now). The upper mids and highs especially are alive, more detailed.

We then interchanged which amp was connected to which speaker, to make sure one speaker wasn't substantially different from the other. Nope, it's the amps. We then immediately modified the other amp.

The mod is the easiest one in the world that involves solder. The pinout of the two tubes is the same except for the heater. The 12AU7's heater pins are 4,5 and 9, with 9 being the center tap. The 6CG7's heater pins are 4 and 5, with pin 9 unused. So just remove the bridge from pins 4 and 5 and move the wire on pin 9 to the now-empty pin. That's it.

Your ears will immediately tell you that you did the right thing. And restoration to stock is equally easy (but you won't be doing that).

Thanks to all who discussed the relative merits of doing the tube sub, and especially Jim McShane for providing original guidance on what parts to use in the rebuilds. We used most of his advice and bought some parts from him.

 

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