Thinking of building a 300B preamp

Discussion in 'Tube Audio' started by kward, Aug 12, 2017.

  1. Palustris

    Palustris Well-Known Member

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    Another reasonably priced tube (about $10 on epay) with a low plate resistance of 230 ohms is the 7233. The plate curves look very linear, but they will burn up a lot of current. The 9 pin base is convenient.
     
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  2. thorpej

    thorpej AK Subscriber Subscriber

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    Wow, no kidding about them being cheap! Because of the shared cathode, I guess you would have to either run the two triodes in parallel (at which point you'd cut the output impedance in half, to 140 ohms, right?), or perhaps as a differential pair.
     
  3. nerdorama

    nerdorama AK member Subscriber

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    6AS7 doesn't have a shared cathode. They are independent triodes.
    [​IMG]
     
  4. BinaryMike

    BinaryMike Pelagic EE Subscriber

    There's a long-running thread over at diyAudio on what is apparently a very successful triode-strapped 6V6 design. If I needed this sort of preamp, this is the one I would try first.
     
  5. knockbill

    knockbill Addicted Member

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    Not aware how to use 6AS7 in a pre amp, but a pair of them makes a very nice sounding PP amp,,, no pre necessary!!! and they are dirt cheap! 6080s even more so than 6AS7G bottles...
     
  6. kward

    kward AK Subscriber Subscriber

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    For the price of one new production 300B tube, you could make three 6V6-based preamps. This is definitely something to consider. It would be so inexpensive it seems ludicrous not to try. If you don't like it, nothing lost (except a few hours of assembly time and a few bucks in parts--not including the power transformer). Yes, this is something to consider.

    I do have a bucket of 6AS7G's. Several Russian NOS samples (Winged C 6H13C to be exact), as well as some Sovtek new production stuff. For this project, I would use them in the power supply, but in past attempts, I've also used them as cathode followers.
     
    Last edited: Aug 13, 2017
  7. knockbill

    knockbill Addicted Member

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    Kevin,, Have you ever used them in a power amp? With your skills, it would be amazing... The one I built (NOS RCAs)makes me smile a lot!!!
    Now back to your pre amp design!
     
    Last edited: Aug 13, 2017
  8. kward

    kward AK Subscriber Subscriber

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    It's on my list! (using them for a power amp). Unfortunately my list is growing...
     
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  9. thorpej

    thorpej AK Subscriber Subscriber

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    D'oh, yes, my mistake!
     
  10. nerdorama

    nerdorama AK member Subscriber

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    About 10 years ago I saw what looked to be a cool 6AS7 amp in AudioXpress. It had a differential output stage with separate CCS's for each 6AS7 triode section since I've read that they are notorious for having unmatched sections. Then he connected the cathodes with a film capacitor. It didn't have high power but the author claimed it was a big hit with his local audio club.

    http://www.triodeguy.com/Triodeguy PDF files/6AS7 Current Balanced PP Amp.pdf
     
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  11. BinaryMike

    BinaryMike Pelagic EE Subscriber

    Since the 6AS7 keeps coming up, I will mention Pete Millett's lo-mu preamp project: http://www.pmillett.com/lowmu_preamp.htm This preamp ended up with too much hum, but I'm sure that's not a fundamental necessity.
     
  12. BinaryMike

    BinaryMike Pelagic EE Subscriber

    It occurs to me, after reviewing Millett's lo-mu preamp schematic, that you could get the desired gain from a 6AS7 by operating it as the input stage of a cascode. The CCS tubes in the lo-mu preamp could be converted to cascode upper stages with 430R anode loads, yielding 3X gain and 430R output Z if 6AS7 gm is close to the datasheet value of 7000. Another very nice way to accomplish this might be with PNP transistors in a folded cascode arrangement. The transistors are sonically transparent when used in this way.
     
  13. nerdorama

    nerdorama AK member Subscriber

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    If lower output impedance is needed, a CCS or gyrator anode lode and using the Mu output will do that. You will get the full gain of the tube as well. Or use of a FET follower of some sort will do that too. The follower kind of defeats some of the benefit of the 300b though.
     
  14. kward

    kward AK Subscriber Subscriber

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    Thank you all for your input. I've kinda narrowed down the choices to:
    1. Single stage 300B.
    2. Single stage 6V6 (as referenced in BinaryMike's post).

    The 6V6 by itself would offer more gain than I want. But unbypassing the cathode might get me right in the 2.5x to 3x ballpark (haven't done the math, just guestimating). The 6V6 strapped as a triode appears to be something kind of special, and appears to approach the linearity of the 300B.

    ===

    I've got some ideas about the power supply, regardless of path chosen.
    1. Silicon regulated DC filaments.
    2. Tube rectified B+ supply
    3. Tube B+ regulator (series pass, with error amplifier), maybe 6BM8-based.

    The "6V6 line preamp" referenced above used a Maida B+ regulator in its initial incarnation, and then later swapped over to an active shunt regulator with purportedly better sonics. Not sure I like the Maida for a preamp--seems more appropriate as screen regulator duty for a power amp or other duties similar to that. But again that's just speculation at this point.
     
  15. prelius

    prelius AK Subscriber Subscriber

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    Two suggestions, if I may... Not necessarily better, but a bit different:

    1. Use a Russian 6S19P (6С19П) tube. It has very low internal impedance (between 350 and 400 ohm, depending on the plate voltage)
    2. Use SRPP topology for a single stage pre-amp.

    Below is a link to a very good specs page: http://www.klausmobile.narod.ru/testerfiles/6s19p.htm

    I have not built a pream using this tube, but I did build a few headphone amps and flea power amps using it with very good results...

    Cheers, Paul
     
  16. nerdorama

    nerdorama AK member Subscriber

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    If you find yourself considering the 6V6, might be worth a look at the 6W6/12W6 as well. I've read good things about them triode connected. The gain is a bit lower than triode 6V6 as is plate resistance. The 6S19 mentioned above could be really good in a line stage. 12B4 has been used by many and well liked.
     
  17. BinaryMike

    BinaryMike Pelagic EE Subscriber

    I'm interested in any available information or examples of 6W6 triode operation.
     
  18. nerdorama

    nerdorama AK member Subscriber

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    I've only looked at the data but I've read a couple of positive posts on the web. I have a pretty collection of both 6W6 and 12W6. Someday I'll get them used. I'm mainly thinking of using them to drive interstage transformers in a power amp.

    There are a couple of references on this page which is where I first saw it mentioned. It was suggested as a reasonable non DHT replacement for 45's in the driver stage of an amp: http://www.nutshellhifi.com/triode2.html

    Here's another thread: http://www.diyaudio.com/forums/tubes-valves/64259-6w6-driver.html
     
  19. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    Cool stuff
    Think I'll stick around maybe I'll learn something :lurk::thumbsup:
     
  20. gadget73

    gadget73 junk junkie Subscriber

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    A triode-strapped 1619 may also be worth considering. I understand it makes a very good sub for a 45 triode. They're directly heated and 2.5 volts though, so it involves a little more power supply nonsense than a more common 6v tube.
     

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