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Millett "Starving Student" hybrid amp

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  1. dsavitsk
    Quote:

    Originally Posted by nullstring /img/forum/go_quote.gif
    What if we just attached ground to mains Earth?



    That should be fine. The only problem is that the Cisco doesn't make it easy to do.
     
  2. the_equalizer
    Quote:

    Originally Posted by timmyGCSE /img/forum/go_quote.gif
    oh yea..thanks man, yet another excuse to do this project all over again!! haha naa just kidding great work! you're doing such a brilliant job in keeping this project alive. Maybe one for the future for me...



    Quote:

    Originally Posted by ezzieyguywuf /img/forum/go_quote.gif
    +1 at the_equalizer for keeping this thing alive! I'm still waiting for a chance to order and install a new pot. :-D



    Thanks guys ! [​IMG] I just like tinkering with this beautifully simple circuit.

    @timmyGCSE - Actually the beauty of it is that you do not have to do it all over again. Just make R2, R8 = 220 Kohm and plug a couple 17EW8 tubes into any of your 12AU7 builds

    Quote:

    Originally Posted by dsavitsk /img/forum/go_quote.gif
    It's a little less than ideal though not horrible. At a normal operating point, mu is 50, Gm is 4600 which puts rp at about 11K. With both sides in parallel, this is reduced to 5K5 which certainly works here. However, since we are running very low current through the tube, the reality is that rp is a lot higher, how high is hard to know. Also, mu of 50 is a lot of amplification, too high for many setups I'd think. But, it is a decent find and should be fun to play around with.



    Thanks for posting your thoughts Dsavitsk. Actually after doing the mod, and before plugging my headphones I measured the quiescent values, just to check everything was in order. I used my trusty analog Kyoritsu FET VOM for the measurements. Here are the numbers:

    Plate voltage: 29V
    Cathode voltage: 1V
    Current through each triode: 0.25 mA
    gain = 10

    For gain measurement I used test track no. 55 from here. I set the volume in my computer until I measured 1V AC at the grid with the volume control set on full. Then I measured AC voltage at the plate. With 1 volt going in I got 10 volts (11 volts on the left channel) coming out.

    So the gain is about the same as for a standard CMoy. Admittedly I only use my SSMH with relatively low output level sources (iPod, discman, computer headphone out) so the gain does not seem overwhelming or hard to control even with Grados or my ER-6i IEMs.

    cheers!
     
  3. timmyGCSE
    Quote:

    Originally Posted by the_equalizer /img/forum/go_quote.gif
    @timmyGCSE - Actually the beauty of it is that you do not have to do it all over again. Just make R2, R8 = 220 Kohm and plug a couple 17EW8 tubes into any of your 12AU7 builds



    very true! I have my original SSMH that could be modded and of course I have plenty of 220kohm resistors left over from my original build [​IMG]
     
  4. iPoodz
    Hey guys, I have a quick question.

    I recently noticed that two of my favorite headphones started to develop in the right channel, which was pronounced in low frequency sounds. It first happened to my AKG Sextetts, which I sent off for repair, faulting the the ancient drivers. Then, it happened to my AKG 140s... This could totally be coincidence, because the 140s are equally ancient, but I'm wondering if anyone thinks it could be caused to a faulty build, which either caused, or exacerbated the problem.

    I was advised by a trusted friend to measure the DC offset, as a high DC offset could have been the issue, but I measured 1 mV in both channels, which is very good, if I'm interpreting it correctly.

    Other possibly helpful info: I have the audio and power signals coming through copper wire, with electrical tape on the exposed parts. I never have the headphones plugged in when I turn on the amp. I listen at volumes that are 45 to (maximum) 135 degrees from 0 volume.

    Any advice/thoughts would be greatly appreciated!
     
  5. the_equalizer
    Quote:

    Originally Posted by iPoodz /img/forum/go_quote.gif
    Hey guys, I have a quick question.

    I recently noticed that two of my favorite headphones started to develop in the right channel, which was pronounced in low frequency sounds. It first happened to my AKG Sextetts, which I sent off for repair, faulting the the ancient drivers. Then, it happened to my AKG 140s... This could totally be coincidence, because the 140s are equally ancient, but I'm wondering if anyone thinks it could be caused to a faulty build, which either caused, or exacerbated the problem.

    I was advised by a trusted friend to measure the DC offset, as a high DC offset could have been the issue, but I measured 1 mV in both channels, which is very good, if I'm interpreting it correctly.

    Other possibly helpful info: I have the audio and power signals coming through copper wire, with electrical tape on the exposed parts. I never have the headphones plugged in when I turn on the amp. I listen at volumes that are 45 to (maximum) 135 degrees from 0 volume.

    Any advice/thoughts would be greatly appreciated!




    A 1 mV (milivolt) DC offset is completely inocuous. The "electrical tape on the exposed parts" sounds a bit scary, but I doubt that caused the problem.

    You mention you don't have your 'phones plugged in at power on... but what about power off ? At power on the output caps charge through your cans' coils if they're plugged in; at power off the output caps would discharge through them, also causing a loud thump.

    cheers!
     
  6. iPoodz
    Oh! Is electrical tape bad? Can you recommend an alternative? I thought that's what electrical tape was for.

    I try to have my headphones out of the socket when I power off, but I'll be honest, and say that I have left them in on occasion.

    Thanks for the thoughts, equalizer!
     
  7. Juaquin
    Electrical tape works but for long-term peace of mind, I'd use heatshrink.
     
  8. m11a1
    Tomb,

    Do you remember how I had that volume imbalance problem? Well, I finally had time to work on my SSMH today. The first thing I did was checked all of the resistors and then I removed the 2 LEDs along with LED resistor. I also changed out new MOSFETs that I just brought from Mouser (3rd pair). Anyways, I figured I'll just try it out see if anything change and what do you know....the imbalance is gone! It's completely balanced and it sounds great. So I'm not really sure what fixed it....regardless it works well now.

    However, I have a question for you. As I was checking the resistors, I noticed that 4 of the resistors had very strange measurements. R2/R8 and R4/R10, according to the schematics were supposed to be 220K ohms, but I got 110-120 ohms when I measured them. Despite this, the individual resistors are labeled "2203", which is correct for 220k ohm. Is this a problem that I should be concern about...even though it sounds great at the moment.

    Thanks a lot!
     
  9. Juaquin
    Did you measure them in the circuit? If they're connected to the rest of the amp then current can flow through those other connections, making the overall resistance between those two points lower.
     
  10. m11a1
    Quote:

    Originally Posted by Juaquin /img/forum/go_quote.gif
    Did you measure them in the circuit? If they're connected to the rest of the amp then current can flow through those other connections, making the overall resistance between those two points lower.



    Yes, I read them right off the circuit because they are all soldered in...that's really interesting because the rest of the resistors give me the right reading according to their values...

    ...despite that, you're still right.

    Well either way, I suppose I don't have to worry about it then. My SSMH is done!

    Thanks!
     
  11. fogmachine
    okay, i've read all the pages of the thread, well done to every one on their builds some great looking amps there.

    got one question to anyone that might be able to help, the 12sr7/gt build where do you connect pins 4 & 5, the diode plate no. 1 & 2 ?

    would like to give them a go but my valve knowledge is minimal so dont want to start pluggin them in anywhere.

    cheers in advance

    fog.
     
  12. the_equalizer
    Quote:

    Originally Posted by fogmachine /img/forum/go_quote.gif
    okay, i've read all the pages of the thread,

    </snip>




    Man, I admire your patience and will power !

    Quote:

    Originally Posted by fogmachine /img/forum/go_quote.gif
    </snip>
    got one question to anyone that might be able to help, the 12sr7/gt build where do you connect pins 4 & 5, the diode plate no. 1 & 2 ?

    would like to give them a go but my valve knowledge is minimal so dont want to start pluggin them in anywhere.

    cheers in advance

    fog.




    Pins 4 and 5 of the 12SR7/GT should be grounded. If you have a non-GT variety (with a metal casing) pin 1 should be grounded too.

    cheers!
     
  13. timmyGCSE
    Quote:

    Originally Posted by the_equalizer /img/forum/go_quote.gif
    Man, I admire your patience and will power !



    woah me too..342 pages?! good on you fogmachine [​IMG]

    my gf came to mine this weekend (she lives by birmingham [​IMG] ) and all too often she berates me for my headphone purchases lol but I think she was quietly impressed with my SSMH and she's ended up stealing my RX700's lol. It was interesting training her to make sure the headphones are not plugged in before turning it on or off though lol
     
  14. fogmachine
    Quote:

    Originally Posted by timmyGCSE /img/forum/go_quote.gif
    woah me too..342 pages?! good on you fogmachine [​IMG]



    it just made sense, if your planning on building something like this and there is that much help, suggestions, issues etc all ready documented, make the most of them or at least use the search function and stop asking the same questions over and over again! tbh i'm surprised some of you are still replying to some of the questions that have been asked soo many times already.

    oh, and i didnt do it in one sitting, about 100 pages at a time [​IMG]

    right back to the breadboards, got a birdsnest to unpick, looking forward to see how much the 12sr7gt glow, both 12au7 and 12ax7s dont glow nearly as much at the 19j6's, but of couse its not all about how it looks [​IMG]

    cheers for the help, my initial guess was to ground them, but is always good to check. thank you head-fi'ers
     
  15. fogmachine
    wicked, works just great
    [​IMG]
     
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