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Line stage schematic suggestions sought:

Discussion in 'Tube Audio' started by winters860, Dec 8, 2018.

  1. winters860

    winters860 AK Subscriber Subscriber

    Messages:
    4,192
    Location:
    Dirtbag, Ohio
    I know these threads get done to death, and become a grab bag of suggestions only tangentially related to the subject matter ("Buy a used Adcom GTP-500II instead!" "Psvane makes the best new-production tubes and I'll CUT anybody that says different!") but that's okay, right?
    :whip:

    I'll be building a stereo tube preamplifier soon. This will be my first scratch build, to be done concurrently with a SEP EL84 stereo amp.

    Objectives:
    1) Be able to drive a wide variety of amplifiers
    2) Be relatively simple and inexpensive to build
    3) Be compatible with 280V-0-280V power transformer secondaries (I have a nice NOS transformer)
    4) 9-pin miniature tubes from the 12A*7 family strongly preferred (I have lots)

    Suggestions, anyone?
     
    Last edited: Dec 8, 2018

     

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  2. winters860

    winters860 AK Subscriber Subscriber

    Messages:
    4,192
    Location:
    Dirtbag, Ohio
    Looking hard at a cathode follower design, like the one from the RCA Receiving Tube Manual or one of the ones from Shine7.

    12AU7-Cathode-Follower-Schematic.png 12ax7_schematics.gif

    If all the power supply calls for is a single B+ voltage, I'm comfortable at this point designing my own.
     
  3. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    37,033
    Location:
    Southern NJ
    Not much to go wrong with the RCA one.

    Similar, but with no coupling cap and no cathode bypass cap

    https://p10hifi.net/tlinespeakers/lineStage-12AU7.gif

    You get local feedback at the voltage amp stage with the unbypassed cathode resistor and no chance of phase shift or anything of that nature by ditching the coupling cap. its pretty much what the Bottlehead Foreplay circuit is, and those sound quite nice.
     
    sfox52 and winters860 like this.
  4. BillWojo

    BillWojo AK Subscriber Subscriber

    Messages:
    2,787
    Location:
    Burlington, NJ
    Check out Vacuum Tube Audio, the Aikida preamps. They sell board kits for both 9 pin and octal tubes. I use one that was built from a complete kit with chassis and all and love it, found it on CL. Lots of great info out there on them. You can even find the schematics and do point to point.

    BillWojo
     
    winters860 and BuzzK like this.
  5. santa

    santa AK Subscriber Subscriber

    Messages:
    614
    Location:
    Oakley, Ca
    Look up glassware audio
    They sell a number of kits suited to your needs here
    I would go with the CCDA pre kit and one of the power supply kits like the ps1 which will give you nice clean power without trial and error

    The manuals are excellent and will teach you a lot as you assemble the parts
     
  6. PakProtector

    PakProtector AK member

    Messages:
    3,715
    Location:
    Ann Arbor, MI
    I would offer up a grounded cathode 12B4. Simple grid to volume pot, and output coupled to the plate with a 2-5 uF capacitor. If you wish to cheap out on filtering B+, use a CCS instead of a resistor for the plate loads. You can get WAAAAAY more carried away too, and I present you with the following evidence:
    http://www.emotiveaudio.com/epifania_info.htm
    cheers,
    Douglas
     
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  7. golana

    golana analog Subscriber

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  8. PakProtector

    PakProtector AK member

    Messages:
    3,715
    Location:
    Ann Arbor, MI
    bjlefebvre and golana like this.
  9. GordonW

    GordonW Speakerfixer Subscriber

    Messages:
    17,901
    Location:
    Marietta/Moultrie GA USA
    My pet preamp is similar topology to the RCA, but with the volume pot between the gain stage and cathode follower, and with lower-impedance plate and cathode resistors. I usually use something like a 47K 2w plate resistor and an 820 ohm cathode resistor on the gain stage, and a 47K cathode resistor on the cathode follower. I put .22uf caps before and after the volume pot (which is 100K ohms), to couple from the plate of the gain stage and to the grid of the cathode follower.

    Regards,,
    Gordon.
     
    winters860 likes this.
  10. BuzzK

    BuzzK AK Member Subscriber

    Messages:
    1,797
    I agree with BillWojo. The Aikido is hard to beat. Doesn't seem to color the sound, i.e. is faithful to the music.
     
    Lavane and winters860 like this.
  11. trainbuftony

    trainbuftony Electron Herder Subscriber

    I built a "forewatt" as one of my first DIY projects in the vacuum tube realm. I still occasionally use it, and I'm planning to someday add a phonostage to it. You mentioned having some premium iron which you would like to use. Well, when I built my forewatt, I saved a few bucks by designing my own power supply, using what I had laying around, and it turned out great. It sounds fantastic, and uses some of the cheapest tubes out there the 12AU7. The main thing keeping me from using it is the way I designed the chassis, with the tubes coming out the top,I can't stack anything on top of it and it's inconvenient to change the volume.
     
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  12. JohnW

    JohnW Member

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    winters860 likes this.
  13. GordonW

    GordonW Speakerfixer Subscriber

    Messages:
    17,901
    Location:
    Marietta/Moultrie GA USA
    I went back and looked at my notes- and here's the last iteration of the circuit I used.

    stupid-simple-buffered.jpg

    I usually use a three-stage power supply for this (RCRCRC)- with something like 100uf caps in all three C positions, for a stereo unit. B+ can be anything from 250-300V. I think that higher B+ sounds a little better, IME.

    Regards,
    Gordon.
     
    Last edited: Dec 9, 2018 at 11:01 AM
    winters860 likes this.
  14. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    37,033
    Location:
    Southern NJ
    I find that simple is better for preamps. My PAS has a 12AU7 stage that Gordon built for me, and I like it just fine. Its basically invisible, which I think should be the goal.
     
  15. winters860

    winters860 AK Subscriber Subscriber

    Messages:
    4,192
    Location:
    Dirtbag, Ohio
    Thanks all! Many intriguing variations on the cathode follower pre here. You've given me a lot to chew on.

    Further thought:

    Thinking seriously about adding a phono section. Does anybody do this any more?

    Should I just keep the phono tubes on all the time, like in vintage designs, or do they switch off the B+ and heaters when only the line stage is in use? If it's preferable to power down the phobo tubes when not in use, is there a more elegant solution than a toggle switch for phono B+ and heaters and a seperate toggle switch for inputs? Would I be asking for noise putting power and signal on the same switch? Am I inviting trouble putting B+ and heaters on the same switch?
     

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