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Millett "Starving Student" hybrid amp

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  1. Wiljen
    I've used the SSMH with the HD700 as well and really like it depending on which tube you put in. There are a couple industrial tubes with slightly lower gain than the standard 12AU7 that I find give me more usable range on the volume control when using the HD700s or other low impedance cans.
  2. kickngas
    Care to share what tubes you have used?
  3. Wiljen
    RCA 5963 Black Plates are a favorite for me. Slightly lower Mu 12au7 varient that takes a bit lower plate voltage which is perfect for the SSMH.
  4. kickngas
    Thanks, but what is "Mu"? New to tubes vernacular.
  5. Wiljen
    Also IBM 6350s work well but you need to swap the grid and plate wires for each triode as the 6350 reverses those pins. IF (and that is a big IF these days) you can find a good supply of 6350s (most were either GE or Sylvania for IBM) it is worth doing the rewiring of the pins as the low mu combined with very tight tolerances on these makes for very low microphonics and really good sound with lower impedance cans.
  6. kickngas
    I found "Mu", apparently it basically is the tube amplification. I'm sure there is more to it than that, but for now, I am satisfied with the simplistic definition.
  7. Wiljen
    Mu is roughly equivalent to gain. Typically:
    12au7 = 19
    12av7 = 40
    12ay7 = 45
    12at7 = 60-70 depending on variant
    12ax7 = 100

    You can substitute 12at7s into a SSMH but when you do you usually only get about 30% of the volume pot that is usable before you reach ear-splitting levels. Putting an 12ax7 in the SSMH would probably result in it being too loud even with the volume pot set at the lowest audible level.

    On the other end, there are very few tubes that are directly replacements for the 12au7 with a lower Mu as they were a pretty low gain tube to begin with (as you can see from the chart) so you are left looking for makes and years that are known for being slightly underpowered, or industrial tubes that were dual triodes with similar specs and lower mu like the 6350 I mentioned.

    If I was new to tubes I would start out with the Sylvania 5963 and remember that with the geometry of the SSMH you don't need matched pairs. Matching will likely help with tubes lasting roughly the same length of time so you'll wear both out at once, but are not required to balance the output like they can be in some other designs. The Sylvania tubes should be something you can find for <$20 each and make a good starting point. For many, the journey will end right there as they sound good and follow the original concept of the SSMH of budget concious sound.
  8. kickngas
    Thanks for the tube lesson!!
  9. Wiljen
    its worth noting that if you have the 19J6 version of the SSMH, I am unaware of any acceptable direct substitute for the 19J6 tube. all of the tubes I was previously referencing work in the modified SSMH that uses the 12a_7 tubes by default. Count the pins on the sockets if in doubt 19J6 is a 7 pin while 12au7 is a 9.
  10. kickngas
    Yeah, I messaged member Fred_fred2004 for one of his pc boards. His is modified for 12_7's, not sure what other options are out there for modified versions. I would rather not use ptp wiring.
    Last edited: Oct 31, 2018
  11. tomb
    Just to be clear, the current kits on beezar.com are the original 19J6 tubes and use the original Dsavitsk-designed PCB.
  12. kickngas
    Tomb, clear as a bell. Is there a supply chain for the 19J6 tubes, or a suitable replacement (possibly 18FY6A?) In case one would be needed in the future?
  13. tomb
    Yes, but they're about the price of run-of-the-mill 12AU7/12AT7s these days, instead of the $2 or less they cost when Pete first designed the amp. Keep in mind that you can pay $hundreds for certain 12AU7s or 12AT7s. However, a run-of-the-mill NOS 12AU7 should cost you about $8-$10.

    A quick search on ebay for "19J6 tube" gives 6 legitimate domestic results and 2 international - ranging from $6.95 to $27.95 for a single tube. I believe most tube sellers sell them for $8 - $10 - vacuumtubes.net, vacuumtubesinc.com, tubedepot.com, esrcvacuumtubes.com, etc. You'll just have trouble if you call one of these places and ask for a dozen or more. That would be an issue.

    Again - just to be clear: I am not saying they are as common as 12AU7s or 12AT7s. Actual, new manufacturing sources exist for those tubes, not just NOS. But if you need a replacement 19J6 tube, there are readily available sources and they don't cost an arm and a leg, figuratively speaking.
    Last edited: Nov 1, 2018
    kickngas likes this.
  14. kickngas
    Got it, thank you for the information. Very helpful!
  15. Nestade
    Hi :)

    Anyone got the Eagle-Files for the 12AU7-version?
    I'd like to etch the PCB on my own.
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