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marantz 7c preamp

71.251.20.191
Posted on November 27, 2009 at 18:47:16
rws62


 
i would like to change the selenium rectifier to a diode . which side of the selenium is pos. what diode? 1n4007 400v ? and which tube sockets are the phono stage thanks rob

 

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RE: marantz 7c preamp, posted on November 27, 2009 at 19:56:27
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 V1,V2,and V3 are phono tubes for left and right channel as they are shared.
The side of the selenium that is positive,hooks to the series of 1k resistors and the negative side of the diode,goes to the transformer wire..In other words,the new diode's banded end,goes to the 1k resistor string.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: marantz 7c preamp, posted on November 28, 2009 at 06:46:09
rws62


 
thank you!......rob Ps. what brand 12ax7's nos or reissues would you use in this preamp

 

Restoration question; same subject, posted on November 28, 2009 at 09:36:44
caffeinator
Audiophile

Posts: 1729
Location: Pacific Northwest
Joined: August 22, 2003
 I just happen to be restoring a 7C and have been eyeing that aged selenium rectifier myself. Question is, from a collector's point of view, I'd like to hear opinions on the options - one being leave it alone if it's working (not to the point I can test yet), another being try to replace it with the same thing (can those be sourced any more?), and then the diode replacement choices; replace and discard the old rectifier, or replace but leave the disconnected rectifer in place.

Sorry for such a long question, but I would appreciate the input.

Happy Holidays to everyone here - this forum is a great resource and a unique community as well.

Cheers!

 

RE: Restoration question; same subject, posted on November 28, 2009 at 09:40:11
Allen Wright
Manufacturer

Posts: 5652
Location: Schaffhausen
Joined: January 31, 2002
 A silicon diode will work far better than a selenium diode, hence it may well have higher output voltage.

Then you will need top look at the electro caps it powers, maybe they are not up to the task.

But the 7c is such a classic, it deserves a full rejuvination - all new caps & Rs for starters, and I would change that diode in a heartbeat.

Regards, Allen

 

RE: Restoration question; same subject, posted on November 28, 2009 at 10:15:12
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 It goes further than AW said. Selenium rectifiers are ticking toxic time bombs. It's a question of when, not if, they will fail. Given the value of Saul Marantz's work to collectors, disconnect the Se rectifier and leave it in place. That way, the unit can be returned to "original" condition.

Opinions vary, but my take on classic tube units is that they exist solely to play music, not occupy space on some shelf. So, I say no holds barred in conducting a restoration. Saul knew what good sound was. Follow in his footsteps and make the "beast" sing SWEETLY.


Eli D.

 

RE: marantz 7c preamp, posted on November 28, 2009 at 10:27:24
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 Look at the schematic. V1 and V2 are critical phono section tubes. Use the Sovtek 12AX7LPS there, for its clean and quiet nature. Use the "reissue" Mullard ECC83 out of Saratov, Russia, for its tone, every place else.


Eli D.

 

A 1N4007 is a TRASH diode, posted on November 28, 2009 at 12:41:17
grhughes
 It causes too much garbage spike hash in the noise floor. Use a hexfred. Ray

 

RE: A 1N4007 is a TRASH diode, posted on November 28, 2009 at 13:35:56
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 Ray,

I agree that the 1N4007 is too noisy for B+ rectification duty. However, a correctly snubbed UF4007 is less noisy than a FRED, while costing less.

"Zero" switching noise will be present if a 600 PIV Schottky diode is used.

Might it be worthwhile to install a bridge of 2X snubbed UF4007s and 2X Schottkys? B+ ripple would get a kick in the pants and "standing" DC would be removed from the B+ winding.


Eli D.

 

I snub all my PS diodes with a 470k ohm resistor and a .01 sbc716P, nt, posted on November 28, 2009 at 13:55:07
grhughes
 nt

 

RE: marantz 7c preamp, posted on November 29, 2009 at 06:31:59
rws62


 
replaced selenium with diode, cleaned pots , new mullards. sounds very nice. At start up voltage spike is 370vdc, settles down to 330vdc .origial can cap 350vdc. any suggestions on electrolytics? Brand,mounting and installation tips? .Thanks ...Rob

 

RE: Restoration question; same subject, posted on November 30, 2009 at 17:33:18
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 Like Eli and Allen said,the selenium has to go as do the coupling caps. You can redo a vintage piece with new caps and diodes without hurting its value one iota,as long as you don't cut up the chassis or drill holes in it. In other words,always do mods that you can reverse back to original.
Now,the biggest reason you want to replace coupling caps is,when the bumblebee caps get leaky,you do things like put DC voltage on volume pots and balance controls many times and this will destroy them.. Trying to find a replacement many times is next to impossible.

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

 

RE: A 1N4007 is a TRASH diode, posted on September 17, 2020 at 18:48:34
dbxdx5
Audiophile

Posts: 1
Joined: June 8, 2014
 Ancient thread, I know. . .

Eli: I'd like to replace the original selenium rectifier with a UF4007, but I could use some advice on how to properly snub it, i.e. what values you would recommend. This is a Model 7.

 

RE: Restoration question; same subject, posted on September 20, 2020 at 10:43:13
Lee of Omaha
Dealer

Posts: 1800
Location: Omaha NE
Joined: September 8, 2006
 100% totally agree. Replace the nasty selenium (unreliable, emits toxic gas as it fails) with a UF4007 or a 1N4007.

You'll have to increase the dropping resistor as the new rectifier will only drop 0.7 - 1.0 volts as opposed to tens of volts for the selenium. That gives you the opportunity to add another stage of filtering, if you desire. The extra stage will not be original, but it will result in better regulation and less ripple.

If there's room, we leave seleniums and original electrolytic capacitors physically installed but electrically disconnected to retain the original look.

 

RE: Restoration question; same subject, posted on September 20, 2020 at 10:59:36
rivervalley817
 damn, I miss that guy ... what a keen intellect

 

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