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Millett "Starving Student" hybrid amp

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  1. amc

    So after gathering dust on my bench for the last 5 months while there was a ban on melting lead in the house due to pregnancy, my 2nd daughter Ava has been born.  I pulled out this frustrating project and had a go at it with renewed enthusiasm and it is now working fine and sounds great.  To close the loop on my troubleshooting and resolution here is what I did.  
    I followed the advise and painfully dis-assembled the heatsinks yet again to go over the bottom of the board with a fine tooth comb.  I did find solder joint I was not happy with and for good measure reflowed a lot of the ground connections with a larger tip than the last time - this sped up heating the groundplane significantly! I re-did the LEDs with thinner wire and re-attached the heatsinks and still nothing.  Cr*p!  
    So I started probing again and found the problem - so simple!  I did not have continuity between the switch wires regardless of the position of the power switch.  Checking closer I determined I did not have a good connection between one of the fast on connectors and the wire connecting the switch to the board.  So I removed the switch and jumpered the switch connection and the LEDs powered up, the tubes started to glow and the amp worked.
    Moral of the story - take small steps and eliminate the easy things before you just to conslusions.  I could have saved myself a tonne of pain!  Thanks to Tomb and the_equalizer for helping me out.
  2. the_equalizer
    Well.. what can I say... congratulations, first and foremost, for your new-born child!  and secondly, congratulations on finding that little bug. I hope you enjoy your amp !
  3. Beftus
    I know I'm late to the party, but please bear with me. I want to make one too. I gather that 19j6's are in short supply ATM. There's the 12AU7 mod though, thanks for that! I have a few questions about this 12AU7 mod.
    The yellow marked parts are optional but what purpose do the following parts serve?
    1. R14/R15
    2. R16/R17
    3. C7/C8
    I see the interstage caps C2/C4 are changed from 0.1uF to 0.22uF. Why the change? Or will 0.1uF do too (just like in Peter Millett's original BOM)?
  4. the_equalizer


    Hi Beftus,
       R14, R15, R16, R17 are "input resistors", they help with two things:
    a) By attenuating the input signal a bit, they help pad down the gain of the amp, which can be a bit excesive under some circumstances (sensitive IEMs + full line level input signal, for example).
    b) Help compensate for the volume potentiometer channel mismatch at lower volumes.
    C7 and C8 are the "cathode bypass" capacitors. These help increase the gain of the amplifier by bypassing the AC audio signal around the cathode resistors R5 and R11
    Finally, a 0.1 uF coupling cap will do; the 0.22 uF caps help extend the low frequency -3dB point of the coupling stage down a bit more, thus increasing the low frequency response of the amp. 
    I hope I've answered your questions.
  5. Beftus

    Many thanks!
    I thought that increasing the output caps (C3,C5) from the original 150uF to 470uF already enhanced low freq. response. [​IMG] What's the -3dB point with the 0.1uF cap?
    I tried to find 19J6's locally this afternoon. No luck however. The 12AU7 (or ECC82 as it's known in Europe) is available new. Asking price for Sovtek 12AU7's is 10 euros a piece. Are Sovtek's any good? 
  6. the_equalizer

    You are correct, the output cap interacts with your 'phones impedance to form a high pass filter too.  You want both the interstage coupling and the output coupling to have reasonable -3 db points.
    You can calculate the -3 dB point with the formula

    where R is in Ohms and C in Farads ( http://en.wikipedia.org/wiki/High-pass_filter ).
    The formula gives a -3dB point of 7.95 Hz with the 0.1 uF cap and  3.97 Hz with the 0.2uF cap (R = 200,000 ohms, the value of R4).
    I used JJ new production 12AU7's for my build.I haven't used Sovtek 12AU7, though I have Sovtek 12AX7s in my integrated tube amp and they sound very nice.  I also tested a set of new production 'Mullard'  12AX7s in the "Starving Student" and I thought they sounded stellar, but they costed $17.00 USD each :S
  7. Beftus
    Awesome equaliser, many thanks! All the information I needed to help me decide to make one for myself. [​IMG]
    I think I can live with a corner frequency of ~8Hz. I have some 0.1uF MKP1837's I want to use for the interstage caps. Now it's time to go sourcing all the parts my humble parts box doesn't contain.
    Did I say thank you? [​IMG]
  8. the_equalizer

    You're welcome, I'm glad I can help.  Good luck with your build and let us know how it went.
  9. Rangiroa
    Hey guys,

    I have just commenced building the starving student, I am very excited but I have already made tons of mistakes :) Its not my first electronics project but it is the first project in about 10 years, so obviously I need to brush the cobwebs out of that part of my brain.

    I ordered the Hammond case that comes in the BOM. It arrived yesterday and I started drilling out the front and rear panels for mounting. I plan to use 12au7 tubes and I have all the resistors and tube sockets for the change to the schematics. After I had drilled out the front panel I realized that the case was not going to work for me, obviously because the bottom and top do not open, so i will not be able to solder p2p in the box.

    I would just use some junk box lying around but I want this to look nice so the wife approves of it :) Anyone have the part number or link to mouser//digikey where i can get a nice looking box that opens from the top? Or maybe a brand or line of jameco or hammond that they have found works well for the p2p?
    I tried searching for this topic with no luck, so i apologize if this was discussed somewhere within this 350 page behemoth.
  10. tomb

    I'm not sure what's going on here, but Hammond refers to the removable plate on the 1455-series cases as the belly plate.  However, all you have to do is turn the case over to make it a top plate.[​IMG][​IMG]  In either situation, the plate is removable.  Which Hammond do you specifically have?  Is it possible that you didn't realize the plate is removable or am I completely off-base and you've purchased something else entirely?

  11. amc
    So,  My kit starving student is happily working away and I am enjoying it quite a bit.  I have one issue though.  I seem to get significant ringing or pinging in my cans if I touch the amp while listening.  Happens if I touch the heat sinks, the tubes, the volume knob, touch my source (Ipod), or even the side table the amp sits on.  I can get it to be really pronounced if I start tapping, even by tapping on my Ipod.  
    So are my symptoms what is referred to as Microphonics or is it something else?
    Can it be minimized? (isolation of the amp / some sort of dampening of the tubes / some tweak to the amp)
    The problem is that it uses the increasingly difficult to source 19J6 tubes - so I am hoping it is not a problem with the tubes.  Ideas on how to troubleshoot would be greatly appreciated.
  12. Rangiroa
    Hammond 1455N1201 - Black

    What is going on is that I don't believe I can wire a SSMH amp p2p if I cannot access most of the connections, it would very hard to wire most of it jamming my fat hands into the little case to make connections, etc.

    I was hoping to find a box where the top is removable but it appears this case is solid aluminum except for the endcaps. Maybe I am missing something though, will have to take a look later on this evening.
    If all else fails ill go find a cheapo box to start wiring things and go from there.
    Any tips on how to drill holes big enough for the power switch or the tube sockets? My drill only goes up to 3/8in. Im thinking about going to the hardware store for some kind of adaptor for larger drill bits.
  13. tomb

    You are definitely missing something.  The top plate slides completely out.  When the endplates are off, you have complete unfettered acces to the entire channel that makes up the body.  Alternately, you could build most of it on the top plate itself with maximum access.
    Go look in the SSMH build thread at the pics (link in my sig!) and you'll understand right away.[​IMG]

  14. tomb

    Yes, it's tube microphonics.  It's possible some tube dampers will alleviate some of the symptoms down to a tolerable amount, but it won't remove them completely.
    If they're Beezar tubes, contact me through a PM.  There may be something I can do to replace your tubes.[​IMG]  One thing that will help - try to determine which tube is microphonic.  It's possible that both are, but that would really be an unusual bit of bad luck.  If it's only one, we've got a better chance of getting it replaced.

  15. Rangiroa
    Doh you are right, Tomb.
    After i looked at your thread I definately need to look closer at my case, it felt really solid at first glance.

    Thanks for the help, when I get my build close to finish i will post pics, of course :)
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