Akarakrit Uamkruea, blogger and audio gear maker who passionate in vacuum tube, audio & music.
Producing electronics projects since high school. Seriously upgrade and modify vintage audio equipment for “Real Sound” concept.
He is a co-founder of Soundaries, all audio solution provider.
Experienced and lively explore the listening rooms & audio shops in Japan as his most favorite to do.

Sunday, January 17, 2016

Marantz Model 7 After Modify with "Real Sound" Recommendation

I'm excited to get great result after using Arizona capacitor and other high quality components

that Mr Shirokazu Yazaki from SPEC Corp. have produced and recommended for "Real Sound".

After finishing modify McIntosh 275 amplifier with capacitors adventure.

http://mellowgroovy.blogspot.com/2016/01/mcintosh-mc275-modify-part2.html

I started continue modifying Marantz Model 7 to use them together.

This is a link for inside Model 7 before do anything.

http://mellowgroovy.blogspot.com/2015/12/marantz7beforemodification.html

The result can be predictable for upgrade sound quality of Marantz Model 7 to higher level.

And I want to share what did I do with this world famous tube preamplifier.


Components for do this "upgrade".


Top view.


Bottom view.


Rectifier section, both high voltage and filament supply.

Original electrolytic capacitors are in working condition and still produce great tone.


I use my favorite IRC GS3 metal glaze resistor to replace carbon composite type 

that too high tolerance for this section.


Change old selenium to silicon bridge rectifier for tube's filament supply.


Using STTH6110TV2 ultrafast recovery diode to replace selenium in high voltage rectify too.


Replace cement coated type with better wirewound resistors from Ohmite.


Use 680k carbon composite resistor to replace 500k potentiometer + 180k resistor

in level adjust before last tube stage.

This potentiometer almost had dirty contact, need to replace if somebody still want to use it.


47k resistor RA series from Tepro did it best in cartridge loading position before phonostage.


And all coupling using paper & polyester dielectric in oil-filled capacitors from Arizona capacitor.

Almost use C50309 "Blue Cactus" except 0.1uF that use C50313 "Green Cactus" 

for tuning some kind of timbre. 



Tube's cathode bypass capacitor using axial Mallory capacitors.


Coupling capacitors under the board.



Another coupling capacitors that stay in selector position.


Sing-along with McIntosh 275, produce mellow tone and "Real Sound" like expected.


Cheer !!!
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18 comments:

  1. NestorJune 29, 2016 at 2:55 AM

    Hi!
    Congratulations for your superb job on these Marantz Model 7 restorations!
    I am currently in the process of upgrading a Model 7 in the 14000 serial number range.
    I have ordered the same AZ Caps as you. I have also obtained the same high voltage B+ rectifier and a assortment of IRC GS3 and Ohmite Brown Devil resistors for the power supply.
    However, I see you are using a standard silicon bridge for the heaters. Was this the best option to your ears? My preamp had the selenium replaced long time ago by the SEN SLA600LCD silicon bridge rectifier. Is this a good sounding rectifier? How about going back to Selenium? I have a vintage 1958 International Rectifier selenium bridge which is an exact replacement. I am thinking of using that one.
    Cheers,
    Nestor

    ReplyDelete
  2. Akarakrit UamkrueaJune 29, 2016 at 9:25 PM

    Hi Nestor,

    Thanks for your comment.

    For heater rectification, I think the best option is using 4 schottky diodes bridge together.

    But silicon bridge module still present thicker sound in middle range for my opinion.

    However selenium should be replace cause from its danger if it short and smoke happen.


    Please don't forget to tell me about result after you finish modification.

    Cheers,
    Nattawut

    ReplyDelete
  3. NestorJune 30, 2016 at 1:47 AM

    HI Nattawut,
    Thank you for your quick reply. What kind of Schottky's would you you recommend? Is a snubber capacitor needed?

    The SEN SLA600LCD silicon bridge currently in the preamp is very hard to find information online. Only that auction on yahoo Japan. I should try listening to it first to see if I like it...

    Thanks,
    Nestor

    ReplyDelete
  4. NestorJune 30, 2016 at 1:59 AM

    I forgot to mention what your opinion is on stepped attenuators. The original carbon volume control is still in the preamp. But I have the opportunity to custom manufacture a silver contact MIL-SPEC 32 step attenuator, 500K, log taper with 1.2 dB steps. Precision metal film resistors. It would be manufactured by the successor of the vintage DAVEN company here in the US.
    Would this sound better (more resolution) than the old carbon potentiometer?

    Thanks again!
    Nestor

    ReplyDelete
  5. Akarakrit UamkrueaJune 30, 2016 at 9:17 AM

    Hi Nestor,

    MBR360 (60V/3A) from ON Semiconductor would be great for this job.

    If you want to use snubber cap, should try Mica type capacitor.

    But snubber cap not necessary too much.


    Your attenuator will worth in more resolution but if you still listen to the music from vinyl format, I think high quality carbon potentiometer make the great result in tonal balance and natural sound.

    Cheers!
    Nattawut

    ReplyDelete
  6. UnknownJune 30, 2016 at 11:36 AM

    Hi Nattawut,
    Thanks for your recommendation! I went ahead and ordered several of these Schottky diodes. I also purchased a nice black phenolic turret board to hard wire the rectifiers on. It should preserve the original Marantz vintage look! Can't wait to start working on my own version of "real sound".
    As for the potentiometer, I think I'll stick to the carbon pot for now.
    Again thanks,
    Nestor

    ReplyDelete
    Replies
    1. Akarakrit UamkrueaJune 30, 2016 at 5:09 PM

      Hi Nestor,

      Hope you enjoy with your own version.

      Cheers!
      Nattawut

      Delete
  7. UnknownJuly 2, 2016 at 10:31 AM

    Hi Nattawut!
    I have a question about the proper B+ HV voltage. With the STTH6110TV2 rectifier I am getting around 330V right before the first filter cap. Spec according to original Model 7 schematic is 328V. However, often the selenium rectifier has softened due to age and voltages of 300V or less are common. What voltage are you getting on your Marantz 7? Have you felt a need to reduce it using a series resistor?
    Thanks again!
    Nestor

    ReplyDelete
    Replies
    1. Akarakrit UamkrueaJuly 3, 2016 at 8:16 PM

      Hi Nestor,

      Plate voltage that not different more than 10V does not effect with tube's life.

      Sound also. ;)


      Cheers!
      Nattawut

      Delete
    2. UnknownJuly 4, 2016 at 12:38 PM

      Hi Nattawut, thank you for your reply. That's what I thought then something happened. I have an all original "Green chassis" Marantz 7 from 1958. It is my reference for sound quality. It still has the original selenium rectifiers. I measured the output before filtering and it was 283V DC at 117V line voltage. The schematic calls for 328V!

      The funny thing is that it sounds fantastic that way! Very smooth, non fatiguing, but detailed and three dimensional at the same time.

      Out of curiosity I disconnected the HV selenium and installed the STTH6110TV2 rectifier I obtained to modify my other Model 7. I put the preamp back in my system and listened.

      Something was not right. It sounded detailed but more fatiguing. I was not able to get "lost" in the music. Something was lost.

      Perhaps the STTH6110TV2 needs more hours to "burn in". Or I need to add a series resistor to bring down the DC voltage closer to the selenium...

      I also noticed the original schematic says the output after the rectifier should be 328V. This is with selenium. However, with a silicon rectifier I get 330V on both my Marantz 7's! This is not correct since silicon has a lower voltage drop than selenium.

      I am guessing the 328V is incorrect. And the voltage is 300V or less. Maybe even 283V like my green chassis reference Model 7!

      Very interesting indeed!

      Regards,

      Nestor

      Delete
  8. Akarakrit UamkrueaJuly 4, 2016 at 7:04 PM

    Hi Nestor,

    I've forgot to told you that I use series resistor after HV diode to drop voltages until get closer to 280V, 265V, 245V as in Model 7 schematic.

    Arizona capacitor and STTH6110TV2 need "burn in" time as you said, you will hear different after 100 hours.

    Cheers!
    Nattawut

    ReplyDelete
  9. UnknownJuly 5, 2016 at 8:55 AM

    Hi Nattawut!
    Thanks again for your insight. After the series resistor (and before the first filter cap), how many volts are you getting? The schematic shows 328V here. I think this is incorrect. Because I can easily get 330V with no series resistor installed! It should be much less than that, methinks.

    ReplyDelete
  10. Akarakrit UamkrueaJuly 5, 2016 at 11:12 AM

    Hi Nestor,

    How about 3 HV points that you measure?


    Cheers!
    Nattawut

    ReplyDelete
    Replies
    1. UnknownJuly 5, 2016 at 12:56 PM

      Hi Nattawut,
      Those are derived from the initial voltage output of the rectifier. So for instance if I get 283V right at the rectifier output (before filtering and voltage dividing resistors), the voltage can only get lower for the derived 3 HV points. How much lower, I would have to measure, but I think it is important to start with the minimum input voltage that will produce the required HV points...

      Regards,
      Nestor

      Delete
  11. UnknownAugust 15, 2016 at 3:20 AM

    Hi Nattawut,
    Please feel free check out my thread on Audiokarma with my own Model 7 restoration. Thanks so much for your own restoration which inspired mine.
    Check it out!
    http://audiokarma.org/forums/index.php?threads/ultimate-marantz-model-7-restoration-thread.727823/

    Regards,
    Nestor

    ReplyDelete
  12. Akarakrit UamkrueaAugust 15, 2016 at 9:58 AM

    Hi Nestor,

    Nice work!!!

    Hope you enjoy with Model 7 and others in future.


    Cheers!
    Nattawut

    ReplyDelete
    Replies
    1. NestorApril 22, 2019 at 10:59 AM

      Nattawut,
      I hope you are doing well. I was wondering why did you use a 680K ohm resistor to ground when bypassing the output level controls? Most DIY schematics simply connect C91 to the input leg of the pot and disconnect R53 (180K). Wouldn't this reduce the gain of the preamp? Thanks again! Nestor

      Delete
    2. Akarakrit UamkrueaMay 5, 2019 at 6:55 AM

      Hi Nestor,

      Seem you enjoy with this.

      This is what Shirokazu Yazaki had mention.

      "The maximum gain of the tone amp is 24.5 dB, and just 12 dB turns the level down from the max output gain.

      So I changed the R54 A&B 500K Ohm potentiometer to a 500K Ohm Caddock thick metal film resistor for each channel. It’s my opinion that thick film resistors are basically well-balanced and pliable sounding, and they could be promising for audio use in the future.

      For the original Model 7’s tone amp the maximum output gain is specified at about 22 dB, which is 2.5 dB lower than Model 7K. The difference is due to the type of channel balance volume control on the front panel.

      The type MN volume control was adopted for the Model 7K, and the type AC volume was used in the original Model 7. The benefit of the MN volume control is that there is no loss at the center position of the balance volume, but the AC volume control at the center loses around 2.5 dB, so the MN volume control would be a desirable factor for the sound quality of Model 7K compared to the original Model 7.

      Anyways, changing the R54 A&B output level potentiometer for an original Model 7 has to be adjusted for that difference, so for example, going from 300K Ohm pot to a fixed resistor with same resistance, and so an original R53 A&B 180K Ohm fixed resistor should be changed to 380K Ohm (180 K ohm + 500 K ohm – 300 K ohm ) fixed resistor.

      Changing to these fixed resistors could realize around 15 dB gain for the original Model 7’s tone amp.

      The changing to fixed resistors, as I mentioned, would be very preferable both for being fully free from noise, for long term reliability compared to using a potentiometer, the accuracy of channel balance, and furthermore an improvement to sound quality."

      Cheers!


      Delete

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