Ultimate Marantz Model 7 Restoration Thread

The snubber is aiming to localise and damp any transformer winding transient energy at diode turn-offs. Quite a few diyer's tune their snubber to the transformer, to minimise the noise glitch - there are a few threads on diyaudio regarding the Quasimodo test circuit that assists tuning. It looks like your snubber values were just an educated guess. Even noise glitches in the heater supply can escape in to the amps audio circuitry, such as via wiring or through valve heaters in to cathode circuitry.

The heater supply is only 600mA nominal, but would be more heavily loaded at turn-on, so even a couple of parallel UF4007's for each 'diode' would be a good choice. The concern with using big bruiser or schottky diodes is that their off-state capacitance is much higher than 1-2A pn diodes, so glitches find it easier to pass through.

The schematic in that other thread indicated the B+ loading is about 11-12mA total, so only 6mA per side, but splitting the rails for better channel seperation would screw up the aesthetics of the filter cans.
 
I hope you all had a great Labor Day weekend!
Today I went ahead and bypassed the output level controls on the preamp. I replaced the 500K pots and 180K resistors with 680K vintage Allen Bradley resistors on each channel.
The cathode follower output level pots have limited usefulness when driving a Marantz power amp since they remain set at their max positions for the best S/N ratio.
So eliminating them made sense. Should yield an improvement in sound quality.

I tried to arrange the wires as neatly as possible. Dressed the wire bundle in a black mesh to match my other cables....

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I hope you all had a great Labor Day weekend!
Today I went ahead and bypassed the output level controls on the preamp. I replaced the 500K pots and 180K resistors with 680K vintage Allen Bradley resistors on each channel.
The cathode follower output level pots have limited usefulness when driving a Marantz power amp since they remain set at their max positions for the best S/N ratio.
So eliminating them made sense. Should yield an improvement in sound quality.

I tried to arrange the wires as neatly as possible. Dressed the wire bundle in a black mesh to match my other cables....

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Do you cover plastic tubing with braided shield ?what is that inside plastic tubing? And what to use on rca connectors ?
 
For the cables going to the volume control I simply covered them in braid with shrink end tubing.
For the cables coming from the mode switch I used Belden shielded cables with the shield connected to ground at the balance control (one end only). I dressed the cable with the same mesh for uniformity.

The cables going to the output jacks are the same, just covered with the Mesh. I did bypass the output level controls with 680K resistors for each channel. The RCA connectors are all original. I cleaned them as much as I could. I don't like the newer gold plated or audiophile types. I wanted the preamp to look completely stock on the outside...
 
I have been a fan of the Model 7 preamplifier for almost thirty years now. It was one of the first american vacuum tube Hi-Fi products that appreciated dramatically at the beginning of the 1980's as demand from the far east, specially Japan, often outstripped the supply.

The Model 7, introduced in December 1958, was considered an instant classic with features and design widely copied not only by domestic competitors but also the fledgling Japanese audio industry as well. The McIntosh C11 and C22 were virtually identical copies in circuit and component/chassis layout. The later Audio Research SP-3, a sonic benchmark of the 1970's, also borrowed heavily from it.

Restoring one of these have always been a challenge. Somehow the old parts seem to influence the overall sonic character of the preamp. However, there are other mysteries that I will be uncovering in this thread. A near archaeological research was undertaken to try to learn as much as possible from this design and what affects its sound and overall performance.

Looking at literally hundreds of inner pictures of Model 7's (accumulated from over ten years worth of eBay auctions and my own examples), show all kinds of repairs. From simple recaps using more modern film caps (Orange Drops seemed to be the choice by the 1970s and 80s) to some wild circuit modifications. The original Sprague "bumblebee" or Black Beauty oil impregnated paper caps are often said to become leaky with humidity or cracks in their molded plastic bodies.

In the examples I have owned over the years I have lucked out, as the Bumblebees have performed well with no need for replacement. However, the question of long term longevity always lingered in my mind: after more than 50 years of use, they could be failing at any time now...

Then we have the always controversial question about selenium rectifiers. These were being replaced with silicon as early as the late 1960's as seen in some examples. The rectifiers, like the rest of the parts on these preamps are very conservatively operated. As such they tend to "soften" as they age rather than fail catastrophically. I know of no documented incident in which a selenium has shorted or caused toxic fumes to be released in a Marantz Model 7.

The preamplifier uses two selenium rectifiers. The B+ rectifier is a high voltage low current device in a half wave configuration. A NOS example will output around 300V DC before filtering. Typical rectifiers will soften to around 280V, and this will not affect operation much, if at all. This is because the circuit was designed to produce up to 15V output with very low distortion. The actual power amp drive requirements are just a few volts, typically less than 2V, so a 20 volt drop in B+ is pretty much inconsequential.

The schematic for the Model 7 shows the output from the rectifier to be 328V DC. This was a rather optimistic figure since a stock preamp with a fresh B+ selenium rectifier will barely make it over 300V DC.

The other Selenium rectifier supplying DC voltage to the heaters is a 25V 600 ma bridge. The raw output from the rectifier is 25V, which is then reduced by filtering/voltage divider resistors to around 18V.

I will be performing the restoration on a Model 7 with serial number in the early 19000 range, manufactured in late 1963, early 1964. The preamp is stock with the exception of the heater selenium rectifier which was replaced with a silicon bridge sometime in its life.

For baseline measurements I will be using my reference Model 7 which is a very early unit with serial number in the 10200 range. This preamp has very low noise and distortion and the sound quality from it is nothing short of astonishing. Very lifelike specially with female vocals and woodwinds. Instrument placement is uncanny in 3D fashion. The preamp is completely stock and fitted with ribbed plate diamond bottom Telefunken 12AX7's. All voltages seem to be in spec, except for HV B+ selenium output which hovers around 283V with a line voltage of 117V, which is the design center. Like I mentioned before, this does not seem to affect the preamp's performance in any way.

I will be focusing on trying to, at least, replicate the sonics of my reference preamp by means of modern replacement capacitors, rectifiers and assorted parts. I will also be trying a very high-end stepped attenuator which is probably a first in this type of preamp.

I hope this thread serves a "knowledge base" for this type of restoration where we can learn from years of attempts by others to modify this preamp and add new things that we discover along the way.

Attached are pictures of my reference unit and the restoration candidate preamp in "as found" condition:
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Hello Crooner and all Marantz 7c lovers-

I bought my 7c from the original owners estate, and had serviced the "Dream System" several times for the owner. He had told me that they had never been serviced prior to my work on them.
In the 7c (S/N 7-203xx, the HV selenium is there and is still working. However, the filament rectifier is not a selenium, but is a small black plastic square, that I believe to be a full wave silicone bridge rectifier.

As far as the HV rectifier, I do believe that the selenium can have a sound of its own, and should be left in place if still working.
My question is for the filament rectifier position. I really wonder if there is a sonic difference between selenium and silicone in this position?

I have checked both HV and filament voltages, and they both look spot on. The preamp, used mostly for my vinyl listening, continues to delight.

What do you think about the change to silicone for the filament supply?

Thanks, Bruce on the coast of South Carolina
 
Hi Bruce,

You should definitely try the STTH6110TV2 in place of the selenium. Add a 1K series resistor, if my memory serves me right, to bring the voltage to around 300V DC before filtering.
I used to be a big proponent of Selenium in the HV, but not anymore.

I agree the heater rectifier is not as critical to the sound, but Schottky's are so cheap nowadays that it doesn't hurt to try. I used conventional silicon Schottkys' but the new CREE's would be cool to try here.

I think most of the differences in sound folks hear from changing from selenium to silicon are the result of not properly compensating for proper output voltage. The jacked up voltage probably translates into a 'bright" sound.
 
I'm going to revive this old thread because I have some info that others may not be aware of that could be of use if one happens to be restoring a Marantz 7, or perhaps building a clone...

Those Arizona capacitors are expensive. I don't know if there's another source, but Parts Connexion sells them for $25 - $35 a piece depending on the value. So, as an alternative, I'd like to propose the use of Russian K75-10 capacitors. Like the Arizonas, these are metal-case, vacuum glass-encapsulated oil-filled film capacitors using a paper-and-mylar hybrid dielectric. Because of this, they might be better for this application than the K40-Y paper-in-oil capacitors that many people use. They can be found for usually a couple of bucks on certain auction sites or those that cater to tube audio hobbyists. I use them in my speaker crossovers, and I really, really like the sound of the speakers, if that tells you anything. There are some others that might be suitable: Miflex is a Polish company that makes two series of paper-and-polypropylene in oil capacitors, with your choice of either aluminum or copper conductors. They're a bit pricier than the Russian caps but still very reasonable, all things considered. Obbligato also makes capacitors that have a mellow, hifi sound for not too much money. Particularly, if you need higher values, they have a series of polypropylene-in-oil caps that sound good. My phono preamp uses a 22uf cap in the signal path, and I had good results replacing the no-name electrolytics with these Obbligatos. Finally, there are some Mundorf caps that are oil-impregnated with film dielectrics that may be a good fit in this circuit. I should leave a disclaimer, I haven't had the opportunity to hear all of these in a Marantz 7, but if the Arizona Blue Cactus caps come so highly recommended, then these alternatives may yield similar results based on their similar construction.

I found this thread recently because I realized that, with a bunch of old ECC83s laying around (including one of the highly-desired Telefunkens!), I might be able to build a good Marantz 7 clone. I want to thank Crooner for documenting his work here. I hopefully will be starting work on my 7 clone soon and would be happy to post pictures if people are interested. (Though that might be moving into thread-jacking territory.) In any case, hopefully someone will find this capacitor info useful.

Oh, and by the way, two tweaks that I found in researching the process of building a Marantz 7 clone: 1) that output capacitor value can be changed to work better with your power amp. The capacitor and the amp's input impedance forms an RC filter that can cause low frequency rolloff and phase anomalies. If you know the input impedance of your amp, you can change this cap to a more suitable value. There's a good calculator for this, with a great explanation, on V-Cap's website. You may find you need more capacitance to get all the lower frequencies if you have an amp with a low input impedance, or you might find you can use less capacitor here. Worth a shot. 2) I think you may have to alter some resistor values, but you may reach sonically desirable results using a 12AU7(ECC82) instead of a 12AX7(ECC83) in the cathode follower position. The 12AU7 has a higher transconductance than the 12AX7, which translates to a lower output impedance, by about some 40% or so. Some people have reported good results with this, and since I have an old Sylvania 12AU7 on hand, I will use this in my clone.

Have fun, be careful with that B+ voltage, and for goodness's sake, make sure you're holding the cold end of the soldering iron! ;)
 
This is one of the most inspirational threads on Audiokarma. It was the straw which broke the camels back in getting me to pitch my lousy DIY PAS and build something better. I took inspiration from several aspects of the Marantz design. I can't afford a 7 right now, but what I have sounds great in the mean time. You probably don't realize the ripple effect caused by posting this fine work here, but its real. When you do great work you inspire others to do their best, and thats what you have done here. Cheers, and congratulations on a beautiful preamp build.
 
This is one of the most inspirational threads on Audiokarma. It was the straw which broke the camels back in getting me to pitch my lousy DIY PAS and build something better. I took inspiration from several aspects of the Marantz design. I can't afford a 7 right now, but what I have sounds great in the mean time. You probably don't realize the ripple effect caused by posting this fine work here, but its real. When you do great work you inspire others to do their best, and thats what you have done here. Cheers, and congratulations on a beautiful preamp build.
You can build a clone of the core of the 7 easily enough.
 
My "Cactus" upgraded Marantz 7 is performing beautifully 3 years after I embarked in this adventure. Now with the Arizona Cactus caps fully broken in, it's heads and shoulders superior to my completely stock 1958 unit. I don't see ever going back to a unit with original caps. The volume control was also a huge improvement. definitely one of the most important factors in the resulting sound quality of this preamp. With the varying levels on most recordings, having remote control is the icing on the cake!

One thing it didn't work out was the small switching power supply for the motorized volume control. That had to be changed to a small wall wart plugged in the back for the best S/N ratio.

I am currently trying to replicate the sound of this preamp on an early Marantz 7T. That's even a bigger challenge. But so far, it holds promise!

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I've read it's difficult or even impossible to tell the difference between a properly restored 7c a d 7t. I just don't buy that. From what I've been able to find they were supposed to be pretty similar. I also read stories about people trading in the 7c for the 7t only to return to the store where bought to get the 7c's back and to find the model 7c's were already sold. I would be very curious to hear what you think once the 7t is restored.
 
I've read it's difficult or even impossible to tell the difference between a properly restored 7c a d 7t. I just don't buy that. From what I've been able to find they were supposed to be pretty similar. I also read stories about people trading in the 7c for the 7t only to return to the store where bought to get the 7c's back and to find the model 7c's were already sold. I would be very curious to hear what you think once the 7t is restored.

Yes I've heard that story too. Given my experience of owning or listening to close to ten Model 7T's over the years, there is a lot of variability in the sound of them. I have heard some that sound so dull as if a blanket was thrown over the music. Others were detailed but mercilessly bright. And then there are ones that sound very close in timbre to the tubed 7. The latter tend to be the earlier serial numbers, for some reason: as if Marantz's collective auditory memory was still fresh from years of manufacturing Model 7's.

One theory of why the vast differences in sound between examples of the 7T, is the poor manufacturing consistency of certain parts. The transistors are early planar silicon types by Fairchild. It is possible the yield was pretty poor when they were made, with extensive screening and selection by Marantz before being put into the 7T. I know some examples have a lot of white noise hiss and others are as quiet as the Model 7. Some folks have reported manufacturing issues with the deposited carbon resistors as well. They appeared fragile and easy to become stressed when bent and soldered to the preamps. All of these factors may contribute to the often inferior sound attributed to the Model 7T as compared to the tried and true tubed unit.
 
Yes I've heard that story too. Given my experience of owning or listening to close to ten Model 7T's over the years, there is a lot of variability in the sound of them. I have heard some that sound so dull as if a blanket was thrown over the music. Others were detailed but mercilessly bright. And then there are ones that sound very close in timbre to the tubed 7. The latter tend to be the earlier serial numbers, for some reason: as if Marantz's collective auditory memory was still fresh from years of manufacturing Model 7's.

One theory of why the vast differences in sound between examples of the 7T, is the poor manufacturing consistency of certain parts. The transistors are early planar silicon types by Fairchild. It is possible the yield was pretty poor when they were made, with extensive screening and selection by Marantz before being put into the 7T. I know some examples have a lot of white noise hiss and others are as quiet as the Model 7. Some folks have reported manufacturing issues with the deposited carbon resistors as well. They appeared fragile and easy to become stressed when bent and soldered to the preamps. All of these factors may contribute to the often inferior sound attributed to the Model 7T as compared to the tried and true tubed unit.
I've also read Matantz didn't want to switch to transistors until they became better. Maybe he should have waited a bit longer.
 
I've also read Matantz didn't want to switch to transistors until they became better. Maybe he should have waited a bit longer.

True. I think some strong marketing forces were also in place. By this time, Marantz was owned by Superscope and I have a feeling Joe Tushinsky was putting pressure on the company as well.
 
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I've also read Matantz didn't want to switch to transistors until they became better. Maybe he should have waited a bit longer.
Same with Mac, another holdout waiting on silicon consistency. Their first gen ss has good sonic rep, but similar stories about disappointed returns for the hertage thermionic emission electron electrostatic linear gate technology. Some report good sound with the early generation Fisher ss receivers. I have one 500T in the pile that needs TLC so couldn't say myself. Same for a lone Mac MC50.
 
True. I think some strong marketing forces were also in place. By this time, Marantz was owned by Superscope and I have a feeling Joe Tushinsky was putting pressure on the company as well.
Marantz was pushing the edge in other areas to the detriment of the bottom line, the model 9 major monoblock amp, the 10/10B tuner, and the SLT-12 turntable, none of which returned on the heavy r&d investments. The best selling cash cow models 7 and 8/8B weren't sufficient to feed the r&d beast and keep the lights on, and it was otherwise, for many reasons, the twilight of tube tech industry wide.
Marantz et al afaik didn't have yet a cutting edge speaker system in development, but Saul found one such in a kindred spirit after the sale, a young hobbyist named Jon Dahlquist with some important revolutionary concept approaches that changed the industry much as did Edgar Villchur and AR a decade previously.
 
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Heck, the Model 10 a d 20 tuners almost sunk the boat for Marantz. I have a model 16 power amp and matching 33 preamp. The early Marantz SS amps like the 15 and 16 monoblocks ran on the ragged edge. When the 250 amp came along it was just a more stable model 240. Even so those old SS monoblocks sound really good once restored as do the 240.
 
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