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Millett "Starving Student" hybrid amp

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  1. the_equalizer
    Quote:

    Originally Posted by timmyGCSE /img/forum/go_quote.gif
    would this tube be 'suitable'
    http://www.r-type.org/pdfs/6bq7a.pdf
    6BQ7A, Tube 6BQ7A; Röhre 6BQ7A (6BQ7A)

    the pinout may be different though [​IMG]




    As I posted here it could be made to work but it'd require mammoth size heatsinks for the MOSFETs as the 6BQ5 heater is rated at 6.3 volts (you'd be dropping ~42 volts on the MOSFETs) @ 400mA (more than twice the current for the 19J6, 12A_7, 17EW8).

    cheers!
     
  2. timmyGCSE
    ah ok well nothing a bit of active cooling wouldn't sort out [​IMG]
    lol just joking, I just read they are part of the same family as the 17EW8 and they seem be easier to get hold of than the 17EW8's in the UK. But ok thanks for the info.

    Any thoughts on the buzzing? cheers [​IMG]
     
  3. tomb
    Quote:

    Originally Posted by the_equalizer /img/forum/go_quote.gif
    As I posted here it could be made to work but it'd require mammoth size heatsinks for the MOSFETs as the 6BQ5 heater is rated at 6.3 volts (you'd be dropping ~42 volts on the MOSFETs) @ 400mA (more than twice the current for the 19J6, 12A_7, 17EW8).

    cheers!




    Yep. If a 6.3V heater would work, we'd have gone with 6J6's, but it's an insurmountable problem with the present design, IMHO (Dsavitsk's too).[​IMG]
     
  4. dsavitsk
    Quote:

    Originally Posted by blippster /img/forum/go_quote.gif
    Someone please correct me if I'm wrong, but I don't think the tubes would be too affected by running them on 12V heaters, since effectively that's just dissipating less power in the heaters. The extra 7V drop over the MOSFETs is balanced out by a drop in the bias current from 150mA to ~105mA, so the amp still runs much cooler, and I'm not a fan of huge bias currents anyway. Perhaps that may (or not) compromise sonically, but (imo) 105mA is still plenty, especially with comparatively high-Z Sennheisers.



    Well, from a tube life perspective, running heaters too low is probably not ideal, but this circuit runs tubes so gently that it surely does not matter here.

    There is some controversy regarding starving the heaters -- with directly heated tubes (2A3, 300B, etc) it seems to lower the distortion slightly. Some people think this is a good thing to do, others not so much. Other people claim this is the case with indirectly heated tubes, too, though I've not seen any research to back that up.

    It probably does not matter here because the low voltage and current across the tube likely lead to a high 2nd harmonic component in this amp. However, too low of a heater voltage and the tube won't work properly -- probably can't drive the fet's gate capacitance which could lead to treble roll off.

    Quote:

    Originally Posted by blippster /img/forum/go_quote.gif
    Just measured with the DMM. I get a reading of 13.7V across the heater pins with the 17EW8, and ~13V for the 12AU7. Looks like they were being run a bit hard.



    The difference in current across the FET would account for the extra .7V on the lower current tube.


    Quote:

    Originally Posted by the_equalizer /img/forum/go_quote.gif
    But what seems interesting to me from your post is that it's the MOSFET gate bias (the gate to source voltage) what determines the MOSFET source voltage



    Yes,

    Quote:

    Originally Posted by the_equalizer /img/forum/go_quote.gif
    You see, what I'd thought would happen was that the MOSFET source would sit at ~18 volts (the 17EW8 tube heater voltage) and, with R2, R8 = 390 K the MOSFET gate would sit at ~17 volts; so the MOSFET wouldn't turn on.



    No

    The Mosfet's source will be ~4.5V below the gate. There is a relationship between Vgs and Ids (current from drain to source). This is what allows a mosfet to be used as a CCS -- set that voltage and the fet will only allow the corresponding amount of current to flow across it.

    Quote:

    Originally Posted by the_equalizer /img/forum/go_quote.gif
    So now I see that tube heaters do not behave like constant voltage sources, but rather like constant current sources! [​IMG]



    No, I think something is wrong with your measurements -- heaters are resistors. Like all resistors, their resistance changes with heat -- happens that they change a lot more than a standard resistor. But, the current is simply determined by V=IR.


    Quote:

    Originally Posted by timmyGCSE /img/forum/go_quote.gif
    ah ok well nothing a bit of active cooling wouldn't sort out [​IMG]



    Quote:

    Originally Posted by tomb /img/forum/go_quote.gif
    Yep. If a 6.3V heater would work, we'd have gone with 6J6's, but it's an insurmountable problem with the present design, IMHO (Dsavitsk's too).[​IMG]



    You can certainly run 500mA through one of these fets, and the more current generally the better. But, I do think that IRF610's perform very well for headphone use at ~100-120mA and any more is unnecessary. And, you will have not only the increased current to deal with, but a much larger voltage drop -- 40V at 350mA or so is not trivial -- ~15W per side. So, not only would you need a really beefed up PS, but you'd need a ton of heatsinking. It could be done, but you could build a better amp for less effort other ways.
     
    blippster likes this.
  5. the_equalizer
    Quote:

    Originally Posted by dsavitsk /img/forum/go_quote.gif
    </snip>
    No, I think something is wrong with your measurements -- heaters are resistors. Like all resistors, their resistance changes with heat -- happens that they change a lot more than a standard resistor. But, the current is simply determined by V=IR.




    Well, I didn't mean they behave like a ring of two BJTs active constant current source; but that 'change a lot more than a standard resistor' part makes them behave more like a CCS than a standard resistor.


    Quote:

    Originally Posted by dsavitsk /img/forum/go_quote.gif
    </snip>
    And, you will have not only the increased current to deal with, but a much larger voltage drop -- 40V at 350mA or so is not trivial -- ~15W per side. So, not only would you need a really beefed up PS, but you'd need a ton of heatsinking. It could be done, but you could build a better amp for less effort other ways.




    Agreed
     
  6. timmyGCSE
    well I've bought two RCA 17EW8 tubes this morning, the ebay seller says they are matched in emissions and construction so that should keep me happy [​IMG]
     
  7. the_equalizer
    Quote:

    Originally Posted by timmyGCSE /img/forum/go_quote.gif
    well I've bought two RCA 17EW8 tubes this morning, the ebay seller says they are matched in emissions and construction so that should keep me happy [​IMG]



    Nice! Try them with the 220Kohm bias resistors too.

    cheers!
     
  8. the_equalizer
    Oh well, after 5 months I got my Beezar SSMH back. The friend I sold it to put it up for sale and as I told him: 'if that amp is not with a close friend or relative, it must be with me', so I re-bought it.

    I had forgotten what a cool, nifty, fun little amp this is. I had to open it to spray some cleaner into the pot and while taking it apart I again enjoyed so much the clever case/pc board design. Thanks again to Dsavitsk and TomB for such a nice amp! I don't think I'll let it go again!

    [​IMG]
     
  9. Iniamyen
    Are PCB's still available or did I miss out? I want to build one of these for a friend. Can't seem to find them on Beezar. I am probably just oblivious.[​IMG]

    Edit: Haha, just read the website. That answers my original question.

    Are there any plans to do another round of PCBs in the future?
     
  10. Juaquin
    No, because it's next to impossible to find the 19J6 tubes. You can build one of the modified versions with different tubes using point to point construction rather easily - it's a pretty simple amp so a PCB isn't required for anything other than convenience.
     
  11. Sganzerla
    Some pages ago I wrote about my Millet not working well, now I discovered what the problem was and it is working, but channel imbalance continues. If I invert the tubes the imbalance follows it.
    Should I use more to see if this disappears (they have 12 hours of use) or better begin to look for a new pair? [​IMG]
     
  12. the_equalizer
    Quote:

    Originally Posted by Sganzerla /img/forum/go_quote.gif
    Some pages ago I wrote about my Millet not working well, now I discovered what the problem was and it is working, but channel imbalance continues. If I invert the tubes the imbalance follows it.
    Should I use more to see if this disappears (they have 12 hours of use) or better begin to look for a new pair? [​IMG]




    I don't think tubes age much in the SSMH since they're not driven hard (high plate voltage and current). Still, you can naturally do the experiment, that is, "burn them in" and hear if it makes a difference [​IMG]

    cheers!
     
  13. timmyGCSE
    got the 17EW8 tubes, yes..the midrange is noticibly richer and better. I like
     
  14. the_equalizer
    ^^  Great!  Did you try them with the 220K or the 390K biasing resistors?
     
    cheers!
     
  15. timmyGCSE
    390K. Though I have channel imbalance - this is not tube specific, it was happening with my old ones as well. I plugged my Denons quickly into my mobile and them seemed ok though I was using just one low quality mp3 so I will have to try my mptrois player when I get a chance and root behind my PC and plug my Denons straight into the soundcard see if that makes a difference
     
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