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Millett "Starving Student" hybrid amp

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  1. Beftus
    Weird. Mine doesn't cycle at all. I have a 48VDC 380mA Cisco ADP-18PB.
     
  2. the_equalizer


    Quote:

    It's noticeable if you have some kind of power indicator (a LED or pilot lamp). Then you see the indicator pulse a few times before the tubes come up.
     
    cheers!
     
  3. Ikarios
    Considering some of the problems involving the PSU (the power cycling, the capacitance between the PSU ground and true ground = possibility of killing non-protected DAC), are there any possible replacements? Would any 48VDC PSU with a higher current work? Perhaps a modified TREAD? another easy-to-find wall wart PSU?
     
  4. Beftus
    Quote:
    Both my tube LEDs do not blink at power up. The voltage doesn't drop at power up either, it remains steady at 48.3 volts. Perhaps the size of C1 and C6 makes a difference? I used 470uF for those caps instead of the specified 680uF in the BOM.
     
  5. c12mech
    I will work on it some tonight when I get home.  I'll post what I find out later on.  I was thinking the same thing about the size of C1 and C6 since I did use the 680uf caps there.  How big of a difference does it make to use a smaller cap there? 
     
  6. Rangiroa
    Ok everyone,
     
    Update to my project as it was a few days ago since i fried my mosfets (actually i think the mosfets are fine, they are not burnt) But i used new mosfets anyways to be safe.
     
    My first build of this project has spanned over 3 weeks, including casework which i tried to make look as professional as i could with the tools that i had, including making some mistakes.

    I shouldnt have used a 21 dollar case for my first case...but i digress :)
     
    I used the original millet BOM and got the heatsinks and protective pads for the mosfets, rewired all that and boom the tubes light up correct, and it gets as hot as people says, and theres no sparks when i turn it on (ALL GOOD THINGS :D )
     
    Anyways, now i dont have any sound with a source plugged in. I suspect its either in my potentiometer or my neutrik wiring. I wasnt sure which side of the potentiometer to wire both grounds so maybe i have it backwards, my grounds are on the right side as you look at the pot from the connection side. On my neutrik jack, do i wire the l, r , and ground on the right or left side of the jack? i couldnt figure that out either.
     
     
  7. the_equalizer

     
    Quote:
    Quote:

    Bigger caps definitely demand more current to chage up.  I just checked my 17EW8 build and I too used 470 uF eletrolytics for C1, C6.  
     
    Could you try a couple of 470 uF electrolytics and let me know if that solved the problem? If that solves it I guess I'll update the 17EW8 schematic setting C1,C6 = 470 uF just to be on the safe side.
     
    cheers!
     
  8. c12mech
    I played with it some tonight and am pretty sure the PSU is the problem.  I pulled both tubes out and turned it on just to see what would happen.  I kept a meter hooked up and let it sit for a few minutes.  The voltage started coming up very slowly.  It finally got to around 22V and I turned it off and just put one tube in.  Back to ~5.5V.  Pulled the tube back out and again a very slow voltage gain.  Since I don't have another 48V PSU I plugged in a 12V one just to see if there was a big drop.  I got 12V every where I should have.  The big caps charged to 12V almost instantly since I left the tubes out.  I may try to order another PSU with a higher amp rating and see what happens.
     
    If it did not defeat the whole SS theme I would build a 48V power supply in a separate enclosure.  As with most projects the PS would cost more than the amp.
     
  9. ArtemF
    wait a second - you mean that the voltage was not equal to 48V when BOTH tubes were unplugged?
    And at the same time the unloaded PSU shows 48V? I'm a bit confused. Can you measure the resistance
    of the unpowered circuit without tubes? It might be that the PSU has gone crazy so it can't power any reasonable load.
    What is the power rating of this PSU? 0.15A? Can you take a separate 350Ohm load (40W light bulb?) and test the
    PSU without the circuit?
     
     
  10. the_equalizer

     
    Quote:

    Certainly, any 48 VDC with 350 mA or more of current capacity will work. I've used a 30 V HP printer supply to build a version using 12AE6 tubes.
     
    The simple amplifier circuit has very low rejection of power supply noise and ripple, so you'd have to be carfeul about that point. Voltage is not that critical and I'm pretty sure that anything between, say, 45 and 53 volts will work OK.
     
     
    cheers!
     
  11. Rangiroa
    can anyone explain to me which side of the potentiometer the ground wires are connected to?
     
    Also, The Neutrik Hphone Jack, which side to connect the wires, I wired mine as the original Millett was pictured.
     
    For the potentiometer, I am unsure.

    If i am not receiving sound, which tube pins should i measure resistances etc to?
     
  12. nikongod
     
    Quote:

    I usually just measure it. When the pot is turned to "minimum volume" the resistance from the center pin to the one that is typically grounded should be roughly 0ohms. the resistance from the center pin to the one where signal comes IN should be roughly the nominal value of the pot +/- a few % depending on your luck (czek the datasheet if you really care). 
     
  13. c12mech

     
    Quote:

    The answer to your first two questions is yes.  As soon as any load is applied to the PSU it drops its voltage.  It is a 380ma rated PSU.  Everywhere I measure I get ~.3ohm except across the resistors where I get indicated values.  I will try the light bulb tonight when I get home.
     
    I think as you said the PSU has gone crazy.   
     
     
  14. ArtemF
    I meant the resistance of the whole circuit when you unplug the power jack and put the ohmmeter leads to "+" and "-" of your circuit.
     
  15. c12mech
    I'll try that tonight as well.  I'll do it with the tubes in and with them out.  I don't really think that is a problem since it worked fine for about a week than decided to quit.  As I said earlier, the PSU does not even cycle when turned on now.  It just drops voltage and sits.
     
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