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Millett "Starving Student" hybrid amp

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  1. Beftus


    Cool, thanks! It means I understand the math behind the conversion. Never too old to learn something new. :)
  2. Beftus
    Could the 18AQ8 be used to make a Starving Student? Has an 18 volt heater @ 150mA current draw. The increased heater could be nice to lower the heat dissipated by the heatsinks when compared to a 12A.7 version.
    Found a site selling them for €4 a piece.
  3. the_equalizer
    Just looked that tube up in google, a double triode it seems. I'm pretty sure it'll work... but there's only one way to know for sure   :)
  4. Ikarios


    Also ArtemF created a 6J6 version using a couple resistors in place of the heater -
  5. nullstring


    or... given away [​IMG]
  6. Beftus


    You are generating a hot of heat and need a power supply capable of delivering much more current to make it work though. http://www.head-fi.org/forum/thread/319231/millett-starving-student-hybrid-amp/2145#post_5384865 
  7. Ikarios
    okay, I'm too excited - I can't keep it in anymore. here's a quick teaser shot [​IMG] hopefully I should be finished with the rest of the connections later this afternoon and can take real pics tonight.
  8. the_equalizer

    Wow !   Looks really nice and professional. I'm looking forward to seeing those pics tonight !
  9. Ikarios
    Ah, crap - was all excited to get everything all set up and working, and when I plugged in the power and flipped the switch - nothing. I disconnected everything and poked around with the multimeter. In my haste to case the thing up, I shorted my mosfets to ground. I guess I didn't seat my shoulder washer correctly (I mounted my mosfets the same way TomB did in his PCB build), and the metal tab on the mosfet (drain) must be touching the screw, which is in turn touching the case/ground. Are my mosfets fried, or can I just fix up the short and have everything work magically?
    EDIT: did some more exploring regarding my mosfets.
    I switched around the nut and the screw head (i.e. screwing UP from the inside) and covered the screw threads with teflon tape, ensuring that it would not cause a short to ground, and that the shoulder washer was in place. upon flipping the power switch, a huge spark emits from somewhere on the back panel. needless to say I didn't want to find out exactly where this spark was coming from.
    I unmounted the mosfets and let them hang in midair, and turned on the amp for a few seconds - it appears to work fine, the LEDs light up, the mosfets get hot, and no spark. this ensures that I did not knock another part out of place and cause another short.
    So this means that there is a problem with the way I've mounted the mosfets. However, I'm not sure what part of the setup is causing a short. I'm going to leave this alone tonight and hope that I can find 4-40 plastic screws at work tomorrow. in the meantime, does anyone have any ideas?
    Thanks for your help!
  10. nullstring
    You should've used your multimeter to see whether or not your mosfets were shorted to ground in the first place..
    You don't need to turn it on to figure that out >_>
  11. the_equalizer

    Did you use a thermal Bergquist pad between the MOSFET tab and the case ?
  12. jamesbobo007
    Aflac, I am thinking like Equalizer, that your not using an insulator. The back of the MOSFET can not be grounded ! and if your not using a thermal pad its grounding out thru the heatsink.
    Here is some info that might help ?
    If you dont have these, you'll need to get them, Radio Shack has them if your in a hurry.
    Also, if you can make sure the heatsink is not grounded, you can go ahead and test the amp.
  13. tomb
    Just an FYI, but if the shoulder washer is used properly, it completely insulates the screw from the MOSFET's metal tab.  It doesn't matter whether the screw touches the heat sink or not (it will).
    We discussed this in a PM, but it bears repeating - there are warnings in the SSMH build thread and on the SSMH website about torquing down too much on the Bergquist pad.  The MOSFET tab can slice right through the Bergquist pad if tightened too much.  You just need to make certain that the assembly is snug and that the lock washer has been compressed.  The Bergquist pad itself will turn to almost a flowable liquid under heat and provides all the thermal contact that's necessary for optimum heat sink conduction with the MOSFET.  Note that if your MOSFET is torqued itself in how it's soldered into the circuit, that could cause the edges of the metal tab to cut into the Bergquist pad, regardless of how gingerly you tighten the assembly.
    Despite all the verbiage above, this shouldn't be a difficult task.  Only one or two builders ever had trouble with cutting into the Bergquist pad over scores of builds.  If the pads are already torn up, there's nothing wrong with simply slipping a mica insulating pad right in there and torquing down on it. [​IMG]
  14. Ikarios
    Thanks for the help. I didn't think I had cut through the bergquist pads in the first place, I didn't tighten them any more than I thought it would take to secure the mosfet in place. There must've been something wrong with my mounting methods (both the normal shoulder washer assembly and the teflon-wrapped screw method), because I replaced the whole thing with a plastic screw coming up from the bottom and a metal nut from the top, and I don't see a short anymore.
    How hot do the mosfets get during normal operation? maybe 60-70C? I need to ensure that these plastic screws don't melt or something.
    Also, are my mosfets okay even after shorting them to ground and turning the amp on? I did get two extras in case this happened, it would just be easier to replace everything at once if the current ones are shot.
  15. tomb
    MOSFETs are pretty tough and they like to run hot.70-80 deg.C. would not be a concern, IMHO - at least from the perspective of the MOSFETs.  I doubt that shorting one out would've caused much damage either, unless it got too hot.
    EDIT: I'm still a bit concerned with your use of the shoulder washer.  As I said, used properly, it completely insulates the metal tab from any part of the screw. So, it shouldn't matter if the screw is metal and it touches anything else.
    As far as plastic screws, I would definitely be concerned with those at the temps we're talking about.  Most plastic screws are nylon, I believe, and at the very least will experience long-term creep (or even in the short term).  So, it'll be an accident waiting to happen perhaps.  That's one reason I go to an extra effort and expense to get the genuine Aavid glass-filled polysulfide shoulder washers.  Most TO-220 mounting kits have a very cheap nylon shoulder washer that won't stand up very well to heat - kind of cross-purposed, but I guess they try to keep those kits pretty cheap.
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