MC30 Recap: Round 2

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Back in 2011 I had my MC30's restored by a local tech - this was soon before I started doing my own electronics work. At the time money was tight and I wanted him to just make the amps safe to operate by the least expensive means possible, so the 'Bumblebees' came out and an assortment of generic caps was installed (I also supplied a PS restoration kit that I bought from Yves Beauvais that included authenticap filter cans). Since then I've had zero problems and I always thought the amps had good fidelity.

But they were never the spectacular-sounding MC30's that people seemed to rave about and lately I've been desiring more... More detail, more depth, and more realism.

Fast forward to a few days ago- I reached out to our good friend Jim McShane after reading many glowing references and he put together a new recap kit for the amps, including new resistors for critical positions, and a pair of T-S ECC803S and a Genalex B749 tube for my Scott preamp. I'll be reusing the Authenticap filter can since it's identical in height to the original C-D multicap.

Jim just informed me the parts have been shipped. I'll try not to harbor unrealistic sonic expectations but I'm optimistically looking forward to experiencing my music after the recap.

 

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First one on the bench, ready to go under the iron.

 

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One down, one to go. Note these late MC30's have a slightly different input/preamp section than what is commonly found... I intend to convert them to the 15329-and-up revision since they share the same transformers, as to prevent headaches for the next person to work on these. For now just some component upgrades will suffice.

 

[dshoaf]

Lookin' good! They look well perserved, cosmetically.

When you're done with the rework, make a copy of the schematic and take voltage readings where it is called for on the schematics. Use a pencil to record your measurements aside each one called for. Use a separate schematic for each amp.

Then, in a few weeks of general use, go back in to each amp and check the voltages again. The original voltages will tell you what's changed. I'll bet some of the readings will have changed and will be related to the old carbon comp resistors. Measure them again and see if they've changes value. I've seen it bad enough that all you had to do was heat up the resistor body with an iron and you could see the resistance change!

I've restored a couple of sets of MC30s and all had that characteristic that the old resistors would start drifting after the heat-up/cool down cycles went on for a while.

Cheers,

David

 

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I made the voltage measurements described in the manual and marked them down and dated...the readings are elevated from using a high impedance DMM, but are all within 5-7% and consistent. I'll measure again in a few weeks as you recommend.

I've listened for about 20 hours on the reworked amp and have A/B'd it with the "original" one using a resistor loaded selector switch and my speakers in parallel mono. So far the most notable improvement is an increase in top-end detail. The acoustic space within the recording is more pronounced, giving the illusion of music eminating from behind the speakers instead of in front or out of them.

As I understand the soviet PIO's will take a while to settle in. I was thinking about building a linestage to burn in the next batch of caps before they go into the other amp. Are there any other simpler ways to do this?

 

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I measured the first amp again this morning and I'm getting very close to the same readings, so I guess that's a good thing. I get .4v on the cathodes of the output tubes instead of .8 but everything appears to be operating the way it should...could it be electrical differences between the 1614 and 6L6?

Here's the second amp recapped and converted to the 15329+ version that I did over the past few days. I'm burning it in with a combination of music and sweeps @1-2W through a 1 ohm resistor for about 24 hours before reconnecting it to my system. I'll do a listening comparison between the two revisions later on (stay tuned!).



The first mac has about 50 hours of play on it now and the caps are settling in nicely. I did a final listening comparison between the mylar-capped amp and the upgrade capped one. It's hard to describe the improvements, but the amp with the updated caps has more realism...Instruments sound convincing, and not like it's coming out of a loudspeaker. Drum solos have more attack and sound more dynamic. I also detect more background detail, like jazz musicians humming, or the faint clicks of Miles' trumpet valve action.

I'm already impressed and the caps still have a long way to go...

 

[dshoaf]

Tep, you’re getting there!

Cheers,

David

 

[x3workshop]

I've rebuilt 2 pairs of MC30's with Jim's cap selections, but I also replaced ALL the carbon comps with metal films. The amps sound spectacular. Give those K40Y's about 100 hours to really show their stuff. Enjoy!

 

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The amps have both passed the 100 hour mark since recap and the sonic characteristics between the two versions have become apparent. I've listened to the two revisions together in stereo as well as individually, switching between the two with the speakers in parallel. In my system, the 15329-and-up version is more exciting, vivid and natural sounding than the original 7Exxx circuit, which is a little more sterile (think "digital-ish") by comparison. Sorry, I have a hard time describing this kind of stuff quantitatively...

I converted the other amp to the 15329 version last night so they match and sound the same. (Ahhh . . . A coherent soundstage at last!...)