Hi all ,
I'm planing to build a Marantz 8 B CLONE , for my own use .
Does anyone knows the impedance ( PLATE TO PLATE ) of
an original Marantz 8 B output transformer ???
And the TURNS RATIO of the screen grids taps ( 10% , 33% ,
43% or what ... ??? )
I already have the original Service Manual , and this is the
only data that I do not have yet .
Thanks in advance for any input .
Regards for all ,
Carlos
I'm planing to build a Marantz 8 B CLONE , for my own use .
Does anyone knows the impedance ( PLATE TO PLATE ) of
an original Marantz 8 B output transformer ???
And the TURNS RATIO of the screen grids taps ( 10% , 33% ,
43% or what ... ??? )
I already have the original Service Manual , and this is the
only data that I do not have yet .
Thanks in advance for any input .
Regards for all ,
Carlos
Thanks rdf ,
I already had seen , this article , BUT S. Kobayashi , has
adopted the primary impedance as 4,400 ohms plate to plate ,
and I do not know if this is the correct value ( neither him ) , like the original Marantz 8 B .
What I need to know , is the exactly value of an ORIGINAL
Marantz 8 B , and the turns ratio of screen grids taps .
I can not believe that anybody on this forum , has not measured
this value , yet .
Please guys , my question is still "on the air" , does anyone
know the answer ???
Thanks in advance ,
Carlos
I already had seen , this article , BUT S. Kobayashi , has
adopted the primary impedance as 4,400 ohms plate to plate ,
and I do not know if this is the correct value ( neither him ) , like the original Marantz 8 B .
What I need to know , is the exactly value of an ORIGINAL
Marantz 8 B , and the turns ratio of screen grids taps .
I can not believe that anybody on this forum , has not measured
this value , yet .
Please guys , my question is still "on the air" , does anyone
know the answer ???
Thanks in advance ,
Carlos
Carlos,
John Atwood had measured a number of commercial output transformers many years ago. I can no longer find his results on the web, but I kept the data. With an 8-ohm load on the 8-ohm tap of the Marantz 8B’s OPT, Atwood measured a primary impedance of 6.2K plate-to-plate. He measured the ultralinear taps at a 44% ratio. By the way, with a 4-ohm load on the 4-ohm output tap, he measured 7.3K ohm on the primary, showing that the turns ratio of the secondary windings was not quite perfectly set at 1.414:1, and/or that DCR was having an impact. I would use the 6.2K primary impedance as a target.
John Atwood had measured a number of commercial output transformers many years ago. I can no longer find his results on the web, but I kept the data. With an 8-ohm load on the 8-ohm tap of the Marantz 8B’s OPT, Atwood measured a primary impedance of 6.2K plate-to-plate. He measured the ultralinear taps at a 44% ratio. By the way, with a 4-ohm load on the 4-ohm output tap, he measured 7.3K ohm on the primary, showing that the turns ratio of the secondary windings was not quite perfectly set at 1.414:1, and/or that DCR was having an impact. I would use the 6.2K primary impedance as a target.
Transformer Tests
Here is the page with my transformer test results. Read the attached letter to see some of the limitations and to avoid misunderstandings. If I were to do the tests today, the results would be more sophisticated, but I believe the 1990 results are still valid, if you realize that they are really based on a comparison in a P-P 6550 amp.
Page: http://www.one-electron.com/Trans_Tests.html
- John Atwood
Here is the page with my transformer test results. Read the attached letter to see some of the limitations and to avoid misunderstandings. If I were to do the tests today, the results would be more sophisticated, but I believe the 1990 results are still valid, if you realize that they are really based on a comparison in a P-P 6550 amp.
Page: http://www.one-electron.com/Trans_Tests.html
- John Atwood
Thanks John. I had perused your site this morning intending to post a link to these tests, but I must have overlooked where you had placed them (but there they are, under "Tech Library").
BTW, do you attribute the slight difference in primary impedance between the 4-ohm loading and 8-ohm loading to the secondary turns ratio not being 1.414:1, or to a secondary DCR-related difference?
BTW, do you attribute the slight difference in primary impedance between the 4-ohm loading and 8-ohm loading to the secondary turns ratio not being 1.414:1, or to a secondary DCR-related difference?
Hello Brian ,
THANK YOU very much for your reply , and for your
comments too . Great !!!
Talking about it , I think that my guess is 6600 ohms
plate to plate , with 43% screen grids taps , because
this value was “more or less” a standard at that time ,
for EL34’s push-pull output . But the target of 6200
ohms is good , too .
I am not completely sure about that , as I said , only my
guess . About the difference I think you are right , the
DC resistance , may have some influence . Every time
I had measured , any kind of output transformer I
NEVER could read the same primary impedance value
with 4 , with 8 or with 16 ohms secondary load .
Hello John ,
CONGRATULATIONS for the excellent job . It is really
amazing and complete .
This kind of job does not become older , it will be always
up-to-date , for sure .
Best Regards ,
Carlos
THANK YOU very much for your reply , and for your
comments too . Great !!!
Talking about it , I think that my guess is 6600 ohms
plate to plate , with 43% screen grids taps , because
this value was “more or less” a standard at that time ,
for EL34’s push-pull output . But the target of 6200
ohms is good , too .
I am not completely sure about that , as I said , only my
guess . About the difference I think you are right , the
DC resistance , may have some influence . Every time
I had measured , any kind of output transformer I
NEVER could read the same primary impedance value
with 4 , with 8 or with 16 ohms secondary load .
Hello John ,
CONGRATULATIONS for the excellent job . It is really
amazing and complete .
This kind of job does not become older , it will be always
up-to-date , for sure .
Best Regards ,
Carlos
Carlos,
As you probably know, the primary impedance need not be very precise. Most speakers will vary wildly from either 4 or 8 ohms across the band, and the plates will see a stepped-up version of this varying impedance (both in magnitude and in phase). I would think that a good wideband OPT with a 6600-ohm primary and 43% taps will work fine. The only catch is that the Marantz 8 OPT used separate windings to return feedback to the first stage cathode, rather than taking feedback directly off the speaker windings. John’s table doesn’t address those windings. Note that there are several passive components used to compensate the response for good closed-loop stability. All of this would be different with a different OPT. You would have to redesign the feedback circuit compensation like Kobayashi did in the AudioXpress article. A faithful clone will not be easy. Good luck!
As you probably know, the primary impedance need not be very precise. Most speakers will vary wildly from either 4 or 8 ohms across the band, and the plates will see a stepped-up version of this varying impedance (both in magnitude and in phase). I would think that a good wideband OPT with a 6600-ohm primary and 43% taps will work fine. The only catch is that the Marantz 8 OPT used separate windings to return feedback to the first stage cathode, rather than taking feedback directly off the speaker windings. John’s table doesn’t address those windings. Note that there are several passive components used to compensate the response for good closed-loop stability. All of this would be different with a different OPT. You would have to redesign the feedback circuit compensation like Kobayashi did in the AudioXpress article. A faithful clone will not be easy. Good luck!
Secondary Impedances
I've been traveling, so couldn't answer your question right away. The inexact nature of secondary impedances is most likely due to the quantum effects of keeping a constant number of turns per layer. I've been getting into transformer design, and have found that breaking a layer in mid-winding, to get the exact impedance, is asking for trouble - mainly high leakage inductance and lousy HF response. From dissecting commercial transformers, it is common for the secondary impeances to be only approximate (although consistent). I suspect the "43%" ultra-linear tap came out the same way - if the primary was made up of 7 layers, picking the tap off of the end of the 3rd layer gives almost exactly 43%.
- John
I've been traveling, so couldn't answer your question right away. The inexact nature of secondary impedances is most likely due to the quantum effects of keeping a constant number of turns per layer. I've been getting into transformer design, and have found that breaking a layer in mid-winding, to get the exact impedance, is asking for trouble - mainly high leakage inductance and lousy HF response. From dissecting commercial transformers, it is common for the secondary impeances to be only approximate (although consistent). I suspect the "43%" ultra-linear tap came out the same way - if the primary was made up of 7 layers, picking the tap off of the end of the 3rd layer gives almost exactly 43%.
- John
Hello John ,
Thank you for the explanation . The “ mystery was solved ”.
Hello Brian ,
Thank you for the advices . I already had seen this problem
and I am thinking a lot about it . I think , the best solution is
the middle way between Marantz design and what Kobayashi
did .
I am planing to do an additional 8 ohms winding , only for NFB,
and adjust the NFB by trial and error until I can get the best
oscillogram for a 10 KHz square wave . The output transfor-
mers will be custom made by an excellent local manufacturer,
so it is not a problem .
Best Regards ,
Carlos
Thank you for the explanation . The “ mystery was solved ”.
Hello Brian ,
Thank you for the advices . I already had seen this problem
and I am thinking a lot about it . I think , the best solution is
the middle way between Marantz design and what Kobayashi
did .
I am planing to do an additional 8 ohms winding , only for NFB,
and adjust the NFB by trial and error until I can get the best
oscillogram for a 10 KHz square wave . The output transfor-
mers will be custom made by an excellent local manufacturer,
so it is not a problem .
Best Regards ,
Carlos
Hey Carlos,
I'm curious. How did your 8b clone come out? I have it on good authority that the tertiary windings on the 8b for the feedback were 4 ohms. You mentioned 8 ohms and i was wondering how it turned out. I have a pair of Acrosuond TO 300 transformers that I'm planning to use in my 8b clone. And I wondered what all you had to do to create the feedback circuit?
Kevin
I'm curious. How did your 8b clone come out? I have it on good authority that the tertiary windings on the 8b for the feedback were 4 ohms. You mentioned 8 ohms and i was wondering how it turned out. I have a pair of Acrosuond TO 300 transformers that I'm planning to use in my 8b clone. And I wondered what all you had to do to create the feedback circuit?
Kevin
I had a pair of TO300's and found their 6600 ohm rating incompatible with EL34's. Plus, Doc Hoyer told me they were crap. Kevin at VAC told me they had transformers built by the same company that built them for Marantz originally. He also told me that company was NOT an audio transformer company but wouldn't tell me their name. (He built the Marantz 8B reissues a few years back.) Kevin told me to use the Dynaco's. As a side note Doc Hoyer gave me all the necessary specs for the model 9 output transformers. I had a pair built plus power transformers. It's about time I built those amps!
mr2racer,
I am a bit surprised that you found the TO300s 6.6k primary impedance “incompatible with EL34”, given that Mullard in the EL34 data sheet listed several operating points with Ra-a all the way up to 7k. Apparently Marantz used a primary impedance of 7.3k (on the 4 ohm tap) in the 8B. Could you elaborate on what you found incompatible?
I am a bit surprised that you found the TO300s 6.6k primary impedance “incompatible with EL34”, given that Mullard in the EL34 data sheet listed several operating points with Ra-a all the way up to 7k. Apparently Marantz used a primary impedance of 7.3k (on the 4 ohm tap) in the 8B. Could you elaborate on what you found incompatible?