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Millett "Starving Student" hybrid amp

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  1. hellomoto
    Hi Beftus,
     
    I think you just found out what my problem was... I used a Bergquist pad, from Beezar and thought the metal tab were insulated from the casing...
     
    I used the kit from Beezar, and mounted it as shown here (http://www.diyforums.org/SSMH/SSMHheatsink.php), and I used the kit wo shoulder washer... I now understand the problem... Okay, first DIY project, I learn ^^
     
    Thanks for your help Beftus, I should now be able to make it work! I guess my mofsets are dead now? Is there any way to control them? Are there other parts that could have burnt at the same time, and again a way to control them?

     
    Quote:

    I just used the metal case as ground for my RCA, should I insolate them? I have the propers plastic rings, so it no big deal: what's the best solution?
     
    About the power input, if I understand right, I just have to solder a second wire to the ground pin, and solder it to the case, using a file to remove a bit of black paint, and solder it there?
     
    Thank you so much for your help, I really appreciate it!
     
    Antoine
     
  2. Beftus
    Using a metal case as a ground conductor is to be avoided at all times. The ground can take an unpredictable path which can result in hum. The best method is to attach all grounds to a single point (star ground). From the star ground run a single wire to the metal casing (yes, remove some paint first). This way the metal casing acts as a Faraday cage and keeps out noise and interference.
     
    Take a look at this bit. It's from the lower left corner of Pete Millett's original Starving Student. What you see is the single connection from ground to the metal casing. 
     
      gnd.jpg
     
    As for the mosfets, I would mount them properly and then fire up the amp again. If the heatsinks get warm first and then too hot to touch for long, they're OK and you turned your Sparkling Student into a Starving Student [​IMG]. If not replace them with new ones. IRF510's are cheap. I don't think other parts got killed-in-action. Resistors and film caps are likely still OK, if the electrolytics appear normal they should be fine. Tubes are probably fine too.
     
    If all goes well during power-up, check DC-offset @ left and right channel first! Measure between out-gnd & out-left and out-gnd & out-right. A reading of several millivolts is OK. Then plugin a cheap set of headphones to check sound. Good luck Antoine!
     
  3. Ikarios
    hellomoto,
     
    If you go back and reread a couple of my posts from my own troubleshooting you'll see that I had the exact same issue you did. Your mosfets should be fine, as long as the only problem was a short from the drain tab to ground, and not a miswire of the mosfet (which would be much more likely to kill it).
     
    as for ground - it's very difficult to isolate the RCAs from ground, even with the isolation washers - the metal body of the RCA is quite likely to touch the inside of the hole anyway. This is evidently not an issue, because if you go and read TomB's tutorials on assembly, you'll notice he omitted the inside isolation washers, which means that at least two components are touching the back panel - the DC jack and the RCA jacks.
     
    Personally, I'm using a star ground, where I have wires from every ground connection on the entire schematic join together at one point in one big soldered blob. I don't think I have any hum issues - maybe a little higher noise floor than i'd like, but not hum-like...
     
  4. hellomoto
    Thank you very much Beftus and Aflac (your MSSH looks really amazing!!!),I hope everything will be fixed soon, i'll order insulated washers asap, i'm pretty confident, maybe shoudln't i ^^!
     
    Thanks again!
     
    Bye
     
    Antoine
     
  5. Ikarios


    Quote:

    Don't worry about the insulation washers - TomB just uses them for cosmetic reasons (when you tighten the nut on the RCA, it's quite likely to scratch the finish of the back plate. the isolation washer helps stop that, but a regular washer will work just fine too. it also depends on your type of RCA jack. As far as I know it shouldn't make a difference if your RCA grounds touch the back plate.
     
  6. hellomoto
    Hi,
     
    I was talking about insulation washers for the mofsets lol!
     
    Oh, a last question, should I also solder a second wire from the ground pin of the power jack to the case, or the one going from the pin to the blanck PCB board is enough?
     
    Thanks
     
    Antoine
     
  7. Ikarios


    Quote:
    Oops! yes, definitely get mosfet insulators. In addition, make sure to deburr the mounting hole you're using - this is probably what caused my problem. You can use mica insulators - which Radioshack carries - and use thermal grease on both sides of the insulator, or if you're ordering online, use a sil-pad or a Bergquist pad.
     
    I would connect your DC power ground to your common ground (in my case, a star ground), just to not cause any complications, unless you plan on connecting your star ground to the back panel - in which case it wouldn't make much of a difference. I'm not too sure of how your case/grounds are set up so it's a little hard to advise here.
     
     
  8. livewire
    Dang! I've been listening to my SS for a week now,
    and all I can say is WOW! Best $50 bucks I've ever spent.
    This little tuber has opened up a whole new dimension of sound for me.
    Listening to Lady Gaga "Bad Romance".
    Not exactly audiophile music, I dig her wild climatic ranting and
    the synthesizer's spacial separation and clarity is out of this world on this little amp.
     
    Just a n00b bitten by the bug. Had to go out and buy a DAC and some better cans.
    Maybe a Little Dot MKIII is on the way? Poor wallet....
     
  9. Secretninja
    Hi all, I've been lurking for a while and had this amp in mind.  Read through all 364 pages over the past few weeks, while waiting for some ebay bids to go through.  Finally won a set of 4 12au7 tubes off ebay today for $15 dollars shipped, so I guess it is time to start sourcing parts. I had 1 question, though.
     
    I plan to use one of the stainless steel pans that you see in kitchens that go into hotboxes, over sternos, etc., (my boss said I could have one for free) and had originally planned to just ground everything to it.  After reading that using a separate ground, then grounding it to the case would provide better shielding, I was wondering if a 3-5 inch section of copper pipe would work for this purpose?
     
  10. hellomoto
    Hello,
     
    I received two insulators washers from Beezar, I mounted them, as showed on DIYForums, bith a Bergquist pad, but there's still conductivity between the mofset and the case... What is the best way to insolate the mofset completely then?
     
    Thanks for your help!
     
    Antoine
     
  11. tomb


    Quote:

    Is it possible that you torqued down too hard on the Bergquist pad and the MOSFET tab cut through to the case?  Also, is the hole de-burred?  You may need to show us a pic, if you can.
     
     
  12. hellomoto
    Hello,
     
    I think the Mofsets are isolated from the case. What confused me is that there are conductivity between the mofsets and the case, but this conductivity is still present when the mofset isn't mounted on the case. Is that normal? My build is with a blank cooper board to use as ground, it isn't linked to the case, I didn't suceed in soldering a wire on the case, but the RCA are both linked to the case and to the board, that may emplain this conductivity?
     
    I tried to make it work again, I plugged it in, but nothing happened, no spark, but the tubes were not lighting. Maybe some components get damaged during the first try. Is there a way to verify if a condensator, a tube and a mofset is working, using a DMM?
     
    Thanks for your help guys!
     
  13. Ikarios


    Quote:
    I would check for a short elsewhere. Follow the drain line to the MOSFET and see if you have any other accidental connections. A good macro picture would do us good here, as we may be able to help you spot a problem.
     
     
  14. tomb


    Quote:

    Agreed.  If the conductivity between the MOSFETs and case still exist when the MOSFET is not even mounted, then the short to ground lies elsewhere in the circuit.
     
     
  15. the_equalizer

     
    Quote:

    It would do fine, but it might be challenging to solder to it with a regular soldering iron. A 3 - 5 inch section of 'thick' copper wire would also work well as a ground point (that would be called a 'bus' ground) and would be easier to solder to.
     
    So, are you planning to build two amps or just keep the extra 2 tubes as spares?
     
    cheers!
     
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