crooner
Tube Marantzed
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:
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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