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Millett "Starving Student" hybrid amp

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  1. c12mech
    I tried a 60W bulb, it measured 17.1ohms, with the PSU and it did nothing.  Well, it went from 48V to less than 1V as soon as the load was added.  I'm sure that the PSU is my problem now.
  2. Beftus
    What's the risk if I use 50 volt output caps (C3,C5) for a 12AU7 version? In normal operation these caps have ~12.5 volts accross them. Why the high margin?
  3. wiisus
    I built the amp! 

    When I first turned it on, the left side was ridiculously feint.  After spending a while troubleshooting, I realized I had R7 (33k) and R8 (390k) mixed up.  Changed that and it worked amazing!  

    But I turned it off for a bit.  Than I came back and turned it on.  They both started up than suddenly the left one just stopped working...neither the MOSFET nor the left Tube are working now, and there's just a loud hiss in the left channel.

    Any ideas on what the problem may be?



    I switched the tubes around; It's definitely the tube that went out.
  4. ArtemF
    check the resistance of the heater. The one that is working shines too bright for me - normally they should glow.
    I suspect the heater was burnt. If that's the case, look for the reason. Otherwise you'll fry the second tube.
  5. the_equalizer

    Agreed, seeing the other one shine it looks like your build is hitting the tube heaters with much more voltage than they can take.  I'd revise all the wiring step by step to catch that bug, particularly the MOSFET and it's associated resistors; it looks as if you had wired the tube heater to the MOSFET drain, which is connected to the PSU 48 v line.
    In the meantime, as ArtemF says, do not turn the amp with the tube plugged in, or you risk blowing the second heater.

    When the amp starts up, the heater looks like a short circuit thus there's no voltage drop across it, so, for a while, the DC voltage appearing at the MOSFET source (and the cap's + terminal) is very close to the supply voltage.
  6. Beftus

    Cool, thanks! I'm gonna make another one but can only find Elna Silmic's 470uF in 50 volts... Ordering from the "big three" isn't an option, shipping cost is too high for my liking.
    I'm gonna try my luck. Besides, the PSU is regulated @ 48 volts, surely a 50 volts rated cap must be able to handle 48 volts for a while.
  7. Rangiroa
    update to my build,

    I have decided that I need to start from scratch for my build, so i will be ordering new caps and resistors.
    I was able to get faint noise from both channels, but only if i pushed around some of my wires.

    It seems that I have hooked everything together correctly for about 98-99 percent of the connections and I am missing one thing to just make it work.
    I might try and post some pics before giving up, see if someone here can spot my errors. At this point im ready to just give it up.

    I wish my mistake was more blatant because i could have found it by now. But you know how it is, after a long time of looking at something its impossible to see your wrongs.
    EDIT: forgot to mention that i ordered heatsinks and thermal protectors for mosfets off the original SS BOM. That has helped immensly, but I think there is a problem in my wima and electrolytic output area.
    I will post pics later with my thoughts. I believe the tubes are correct, and i believe the mosfets are correct.

    I am iffy in regards to the Wima output caps that connect to the neutrik jack
    (I am just thinking out loud, I will revist this idea with pics when i get off work.)
  8. Ikarios


    I wouldn't trust running even theoretical 48v across a 50v cap. Have you looked into ordering from a place like Beezar? Tomb has the caps you probably need, and I don't think it would kill him (or you) to ship out a few.
  9. wiisus
    Found out my issues.  Had the 390k and 33k mixed up in both channels.  Damn it.  Anyone happen to have a spare 12AU7? :p
  10. Rangiroa
    Here are some more pics of my build, As I said earlier the green and white wires are 99 percent connections and the blue and orange wires are ground wires.
    I have many problems and I am open to all suggestions at this point. my first problem is how to stabilize my connections. I was thinking about getting some heat shrink and a gun, would that make a large difference here?

    My amp will randomly produce a small, muted sound that sounds like music coming from my source if i press on my connections in random places. There is a 50 percent chance that i put the whole thing together and cram it into the case and it works lol.
    I am hoping that someone here can figure out something that I can't I have exhausted all options and I have stared at it for far too long. lol.
    This pic is intended to show how my output caps are connected, because since my sound was muted i assumed there is a problem with these cap connections, thoughts?
    This next picture is a pic of how my mosfet pin 2 connections are wired. The resistor in the upper part of the screen in front of the panasonic cap is connected to mosfet pin 2 by a wire. CIMG3655.jpg
    This next pic is a shot at my power area. it has the upper tube (not working) as well as the power jack and the wire running to the switch. the green terminal is used to ground wires. all wires in that connection are solidly screwed down.
    This last pic is the top of my amp. The tubes are actually on and lit up, although the heaters are very dim.its hard to see but the tops of the heaters are a light red and both tubes are lit up.
    Thoughts on why the tubes are so dim?
    Thanks for all your patience and help regarding my project. I believed i could do this before I started, and its very frustrating to be stuck and feel so close. Any help or ideas to help clean up my wire mess would be appreciated. Where can I find those connection terminals/braces that lots of people have used in their build? its the one where people usually bolt it down to their tube socket. If i knew what that was called i would definately use those for my next build.
  11. c12mech
    What you are looking for are terminal strips.  Mouser sells them P/N 534-812. 
    You said that you had the resistor going to pin two of the mosfet.  If that is the case then it is not right.  The resistor should got to pin one.  Find the data sheet for the mosfets and look carefully at the way they are laid out.  I had to look a couple of times to make sure that I had it right.  That could be why your tubes are not getting any brighter. 
    From what I see your output caps are wired correctly although not very pretty.  I cannot say much as mine is not a really pretty job either.
    One more suggestion.  Where you have multiple parts tying together, make the lead from one of the parts into a loop, then bend the other leads through the loop and solder.  It makes a much cleaner and stronger joint, IMO.
  12. Beftus

    Perhaps you have bought tubes with enclosed heaters. My Electro-Harmonix 12au7's are like that, no tube glow to be seen. A tube led will light them up nicely though.
    BTW a word of warning. Your heatsinks appear to be very small for a 12au7 version IMO. They look like 1.5" tall to me. Correct? They will get bloody hot in use, trust me.
    My 2" tall heatsinks reach 60 °C easily on top, that's 140 °F. They get uncomfortable to touch very quickly. [​IMG] For my next build I've bought 2.5" tall ones.
  13. Beftus
    I know of an amp of a respected builder with a 23-25 volt supply that has 25 volt caps in the PSU. Those caps see 25 volts continuously. Apparently it isn't a problem.
    A check on my 12au7 version shows a slowly rising voltage until it reaches ~12.5 volts and stabilizes. Not 48 volts at start up and decreasing to 12.5 volts. [​IMG]
  14. the_equalizer


    Remember all components have a tolerance rating, so using such closely matched values is a risk, even if it's done by a 'respected builder'.
    If you feel comfortable with it, use the 50 volts caps or even 25 or 18 volts caps, since according to your measurements in normal operation there are never more than 12.5 volts in those caps.
    Just understand that there could be some failure scenarios where the caps get hit by 48 volts and they'll breakdown, letting all that DC pass through to whatever 'phones are connected to the amplifier's output.
  15. Juaquin


    This is never recommended. It may work, but you're risking longevity and cap failure. I'm not sure why a builder would do that, except maybe if a certain boutique cap is only available in that rating (or that rating has better specs).
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