Building an Ampex MR-70 playback preamp

If this doesn't prove I'm as crazy as a loon nothing will. I am going to try building from scratch one of the most iconic pieces of electronics ever designed. The Ampex MR-70 tape recorder is regarded as the ultimate vacuum tube based machines (only 80 were ever built) in history. It used nuvistors throughout and required 3 years to design and build. I've sent the schematic of the playback electronics to my contact in the Ukraine to see if she can work up a circuit board design. Wish me luck, seriously!



  • pitdogg2
    pitdogg2 Posts: 22,407
    edited January 16
    Lost me at "input" :#
  • SeleniumFalcon
    SeleniumFalcon Posts: 2,729
    edited January 24
    Some tangible progress. The Ukrainian engineer, Olesia Serba, has been working on converting the schematic to a file that can produce the circuit boards. The first step was for me to select the individual parts I was going to use and send her the lead spacing dimensions. Since I wanted to not use any electrolytic capacitors anywhere the parts would tend to be larger than normal. She has provided a parts outline file:


    This screen shot was reduced slightly, not to actual scale.
  • SeleniumFalcon
    SeleniumFalcon Posts: 2,729
    Yesterday I received the nuvistor sockets I found on eBay these will be mounted on an external panel.

  • heiney9
    heiney9 Posts: 24,744
    Very cool. Way beyond my capabilities, but I'll be watching this thread, so lots of pics and explanations.

    "Appreciation of audio is a completely subjective human experience. Measurements can provide a measure of insight, but are no substitute for human judgment. Why are we looking to reduce a subjective experience to objective criteria anyway? The subtleties of music and audio reproduction are for those who appreciate it. Differentiation by numbers is for those who do not".--Nelson Pass Pass Aleph 30 | EE Avant Pre | EE Mini Max Plus DAC | MIT Shotgun S3 | Pangea AC14SE MKII | Legend L600 | BlueSound Node 3 - Tubes add soul!
  • msg
    msg Posts: 7,680
    pitdogg2 wrote: »
    Lost me at "input" :#
    I like the bouncy squiggles and seesaw bits.
    I disabled signatures.
  • msg
    msg Posts: 7,680
    This is gonna be fun to watch, Ken. Looking forward to progress notes and photos, and, ultimately, performance review.
    I disabled signatures.
  • SeleniumFalcon
    SeleniumFalcon Posts: 2,729
    Olesia has sent me a schematic drawing she made from the original with connection points to external trim capacitors, variable resistors, potentiometers and the nuvistor connections.

  • SeleniumFalcon
    SeleniumFalcon Posts: 2,729
    edited January 29
    She also sent a file showing component placing that can be printed out 1:1 so I could place the actual components on the surface to make sure everything fits.


    There will be two boards for each channel. The yellow squares are the mounting holes for attaching the boards to the chassis.
  • SeleniumFalcon
    SeleniumFalcon Posts: 2,729
    Olivia has finished the two Gerber files needed to build the PCB and she provided some 3D drawings:

  • audioluvr
    audioluvr Posts: 4,803
    You, my man, are certifiable! But most geniuses are. Looking forward to following this!
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    There is about a 5% genetic difference between apes and men …but that difference is the difference between throwing your own poo when you are annoyed …and Einstein, Shakespeare and Miss January.

    by Dr. Sardonicus
  • SeleniumFalcon
    SeleniumFalcon Posts: 2,729
    From my recent drivers license:

  • maxward
    maxward Posts: 1,353
    I remember the Professor!
  • SeleniumFalcon
    SeleniumFalcon Posts: 2,729
    The company that is making the circuit boards has a way to keep track of the process:

  • stangman67
    stangman67 Posts: 1,952
    What an awesome project! I thought I was handy, you bring it to a whole nother’ level! Seems like you have some very knowledgeable people helping you out as well. Looking forward to following along
    2 Channel - 2 Channel in my home attic/bar/man cave

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  • SCompRacer
    SCompRacer Posts: 8,033
    Nice work! Let us know when your manufacturing company goes public. :)
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  • Jetmaker737
    Jetmaker737 Posts: 940
    the skillset to do something like this is rare indeed. It's a pleasure to watch. Kind of like watching a superhero movie where you're thinking to yourself "I wish I could do that"!
    SystemLuxman L-590AXII Integrated Amplifier|Sonus Faber Cremona Loudspeakers|PS Audio Directream Jr|Sansui TU-9900 Tuner|TEAC A-6100 RtR|Nakamichi RX-202 Cassette
  • pitdogg2
    pitdogg2 Posts: 22,407
    You're a rare breed Ken!
  • SeleniumFalcon
    SeleniumFalcon Posts: 2,729
    edited February 7
    One of the choices in getting a circuit board made is the thickness of the copper traces: either 1 ounce or 2 ounce. The copper thickness of 2 oz copper is 2.8 mils (0.0028"). 1 oz copper thickness is 1.37 mils (0.00137"). The idea is that the thicker copper is more suited to circuits that handle more current (speaker crossovers, for example) and less copper is needed for smaller signal handling. The 2 ounce copper is a bit more expensive and can add to the wait time, but I went with the 2 ounce choice and it didn't delay the production that I could see.
    I believe there is even thicker traces available (up to 20 ounce), but that wasn't offered in the choices available.
    Post edited by SeleniumFalcon on
  • newbie308
    newbie308 Posts: 498
    "Selenium Audio Kits" sounds like a cool business name...
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  • Some parts are starting to arrive, the circuit boards (I wish they were the same color, but the undersides are, not the component sides). Some of the necessary nuvistors have posted, 5 from Turkey (I checked with the supplier and he is okay, nowhere near the devastated area) and some from the US. The 7895 is near to impossible to find, but the 6CW4 is the identical device with, perhaps, an increased sensitivity to microphonics. I've found enough of the 7895 for the first and second positions in the design and I'll use 6CW4 for the rest. I've been lucky to find NOS of them. The 7587 came from Turkey and are unused, these are tetrodes and along with the output transformer form the balanced output stage.

  • pitdogg2
    pitdogg2 Posts: 22,407
    Ken how long do those super tiny nuvistor tubes last?
  • pitdogg2
    pitdogg2 Posts: 22,407
    I kind of figured they would outlast both you and I. Exspecially since the R2R will not be used like it would have been back in the day. :D:D
  • tonyp063
    tonyp063 Posts: 989
    Out of curiosity, who are you using for your board fabricator?
    I like the looks of those much more than the usual suspects.

    I continue to enjoy & be fascinated by your threads. Thank you.