Anyone made a DIY Marantz 7/7C?

Discussion in 'Tube Audio' started by Sam Cogley, Aug 21, 2009.

  1. Sam Cogley

    Sam Cogley Last of the Time Lords Subscriber

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    I've been thinking about a scratchbuild tube project - I have plenty of tubes and iron around from scrapping several organs. The Marantz 7 seems to be a very well-regarded preamp, and from the schematics I've seen, the circuit doesn't seem to be too complicated. Any suggestions? Should I look for something else to build?
     

     

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  2. jaymanaa

    jaymanaa AK Subscriber Subscriber

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    I'd like to build that circuit myself. I worked on one for an AK'er and it was the best pre I ever heard. I think that you can even buy boards on epray that are mapped out for the 7c circuit. I know that the original PT is potted too, and the tubes ride on a "floating" subchassis of sort.
     
  3. thefragger

    thefragger Certified Crazy.

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    There's a '7C Enhanced' kit that I'm eyeballing on eBay right now, I wonder how that would compare to the original... ? Seeing as how simple the original ciricuit is, how could one improve upon it?? :scratch2:

    I'm thinking of using it as a front end to a Gainclone build :)
     
    Last edited: Aug 21, 2009
  4. thisOne

    thisOne Listen...

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    I believe that the legendary 7C sound is derived from the use of those vintage "bumblebee" capacitors, named after those colored bands painted on them, and available back in the 50s. Replicating it with modern components will result in an ordinary preamp. In fact there was a reissue of the 7C and the sound was very ordinary (I ended up selling my reissue version to someone in Bangkok)

    [​IMG]
     
  5. Sam Cogley

    Sam Cogley Last of the Time Lords Subscriber

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    I wonder what all PIOs would do...

    Aren't Bumblebees still available?
     
  6. thisOne

    thisOne Listen...

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    you can get them still on ePay, but it might take a while to assemble all you need. They are apparently also used in and sought after for Les Paul guitars

    Here is a thread discussing the 7C original and reissue

    Here is a schematic for the 7C
     
    Last edited: Aug 21, 2009

     

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  7. crooner

    crooner Tube Marantzed

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    thisOne is right.

    There's something about those old Bumblebees that cannot be duplicated today. Even the selenium rectifiers contribute to the legendary sonic flavor of the preamp.

    The 1996 reissue used film caps and standard silicon rectifiers. Also the wire used was fancy silver plated type. The resulting sonic signature was very different.

    The volume control potentiometer is also key in the sound of the original unit. Modern plastic conductive types are a no-no for instance.

    The very exotic Shindo line of tube gear uses carbon pots exclusively...

    One of the highlights of the Model 7's design is indeed the floating tube bed. This feature (and pretty much the whole circuit) was copied by McIntosh in their C11/C22 preamps.

    The Model 7 is amazingly transparent for a 50 year old design. Very revealing and detailed, yet musical and never bright or harsh. Residual white noise is totally out of the way, even in the phono position.

    Building a successful true-to-the-original DIY version would be a challenge today. Consider that every Model 7 was painstakingly tested at the factory. Channel to channel balance was 1dB or less in key aspects of the circuit, for instance. Ditto for RIAA and tone control curve deviation.
     
    Last edited: Aug 22, 2009
  8. battradio

    battradio Electron trainer Subscriber

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    The Bumble Bee's are not the run of the mill bee's in a Marantz 7 acording to Sal Marantz they were poly and not paper .

    Mark
     
  9. crooner

    crooner Tube Marantzed

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    Given the longevity and performance of these Bumblebees, I would not be surprised if they are poly.
     
    Last edited: Dec 14, 2009
  10. battradio

    battradio Electron trainer Subscriber

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    A short conversation with Saul Marantz at the CES in Vegas in 1979 in the Hafler Suite .

    Mark
     
  11. GordonW

    GordonW Speakerfixer Subscriber

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    How many people have actually heard a direct comparison between the original Marantz 7 and the reissue (made by VAC)?

    I ask, because I have. And in fact, the VAC version was even more consistent than the original... even more stable imaging, for example. That's because Kevin Hayes went through every detail of the original... same transformers, same type caps (as mentioned, the original 7 had a different variation of the "bumblebee" cap... just because they're bumblebee striped does NOT mean they're paper, in all cases), but with improved connectors and wiring.

    The reissue 7 was one of the quietest, most linear preamps I've ever heard. Now, some of Kevin Hayes' own designs ARE better (the Phi being the primary example), but it's not like night-and-day better...

    BTW: I'm planning to build a stripped-down 7 design... no tone controls, no filters... just the gain stages and buffer stages. I'm going to experiment a little bit... for example, trying 12AT7s for the output buffers instead of 12AX7s. Lower output impedance and more voltage drive capacity... can't hurt to try...

    Regards,
    Gordon.
     

     

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  12. LinuxGuru

    LinuxGuru Well-Known Member

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    What is the output impedance of original 7C ?
     
  13. GordonW

    GordonW Speakerfixer Subscriber

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    I can't remember the specs... but I calculated it from using a 12AX7 in the same configuration (cathode follower with the same resistance values) using TubeCAD. I then did the same calculation in TubeCAD, using the 12AT7. Output impedance was about half what it was for the 12AX7, IIRC. That's one more tape deck or amp I can drive simultaneously, with the same drive voltage...

    Regards,
    Gordon.
     
  14. crooner

    crooner Tube Marantzed

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    Lucky you!
    How was he like?
    I've heard he was a true Gentleman. Great guy.
    What else did he say?
     
  15. LinuxGuru

    LinuxGuru Well-Known Member

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    Thanks, Gordon! I do not have TubeCAD, unfortunately. May be you can recall approximate value (with 12AX7)?

    So, in order to lower output impedance you suggest to replace last 12AX7 with 12AT7 (or may may be 12AU7)?
     
  16. LinuxGuru

    LinuxGuru Well-Known Member

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    Most likely it is influence of tubes, which determine sonic signature. Influence of caps to the sound is greatly exaggerated.
     

     

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  17. Sam Cogley

    Sam Cogley Last of the Time Lords Subscriber

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    One thing I have in droves is vintage 12AX7s...
     
  18. BrokenHill

    BrokenHill Well-Known Member

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    I recently saw one of those selenium rectifiers from an Marantz 7C preamp
    on ebay for 94.95 Buy It Now price, I was wondering why the asking
    price was so high.
    I have heard from other People that those old Bumblebees have excellent
    sound characteristics in their particular amplifiers, and they tried replacing
    them with newer replacements only to be very disappointed in the sound
    quality and ended up putting the originals back in place.
     
  19. crooner

    crooner Tube Marantzed

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    A lot of these kits are actually versions of the Walt Jung/Chuck Hollander "St. Pooge & The Driaagon" 7C redesign of the early 1980s.

    This mod consisted mostly of power supply upgrades. Mainly relying on heavy solid-state voltage regulation. Wholesale parts replacement of caps, resistors and pots were also suggested.

    The sonic results, although praised at the time, are more akin to the Audio Research SP-6 of the day. More "modern" sounding than the classic Marantz 7C sound that is so coveted today.
     
    Last edited: Aug 22, 2009
  20. thefragger

    thefragger Certified Crazy.

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    Thanks crooner for the clarification-- it lead me on a search where I found these two schematics;

    [​IMG] [​IMG]

    The first one is supposedly the Marantz 7c schematic while the second one is supposedly a simplified, 'cut-down' version. While the former seems daunting to replicate, the latter seems to me within reach of a novice!

    The values in the cut down version seems to correspond to the ones in the full circuit; aside from the bumblebee caps, selenium rectification, and mediocre wiring, they should sound similar if those were both built using the same grade of parts?

    You guys are giving me ideas, now...

    :scratch2:
     
    Last edited: Aug 22, 2009

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