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Millett "Starving Student" hybrid amp

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  1. Secretninja
    Thanks for the input.  How thick of copper wire should I use?  I am guessing about 8-12 ga would be sufficient?
    The plan is to build an amp right now in the cheapo pan right now, then an attractive wooden enclosed one over thanksgiving or christmas break when I have access to my dad's workshop.  I plan on trying to emulate the look of an old radio, and might end up trying to incorporate a dac.  But, I should probably focus on getting this amp to work first[​IMG]  I have a few cmoys under my belt, and have done a bit of other soldering work, but nothing ptp.  I'll keep you all updated as I go.
  2. livewire
    I believe that 20 to 24 ga solid is recommended for this build.
    EDIT:  Above I was referring to the point to point connection wire.
               If you were referring to the ground bus wire, yes 8 to 12 ga will do fine.
               I used a buss wire 2 inches long, 12 ga for mine. It ended up being kind of short
               to accept all of the ground connections. 3 or 4 inches should do it.
               My 30 watt soldering iron really struggled soldering to this buss wire.
               I recommend something around 75 watts for flowing the solder to these connections.
  3. Ikarios
    Most people use 20-22ga wire. I personally used 22ga solid core, which was a pain to work with in a small enclosure with lots of extra wire (the Beezar Hammond, but with a bunch of extra stuff like my gain switch, DAC, input selector). I would stick with 20-22ga stranded.
  4. the_equalizer
    As both livewire and Aflac have commented on wire gauge to use for the ground bus and general wiring, I'll only add that while doing an 'air-wiring' build is feasible (as demonstrated by many builds in this thread) using terminal strips really helps ease P2P wiring and provides support to circuit elements.
    Good luck with your builds and let us know how they go !
  5. hellomoto
    Here are a few pics I took. Such a mess, I'll try to remove some wire that is useless when I have enough time, next week i should be able to work on it.
  6. livewire
    How does your amp work? Thanks for posting your pics.
    Dont feel bad, the inside of mine doesnt look much better.[​IMG]
  7. hellomoto
    No, I still haven't find what was wrong, and I'm quite busy these weeks, so I still didn't find time to sit and really debug, I should have enought time today!

  8. nullstring
    Hey guys,
    My SSMH got damaged when I moved into my into my new apartment.
    Only one tube lights up.
    I did some debugging and came up with the following observations.
    1. If you short pin 1 of each mosfet together, both tubes light up
    2. Therefore, mosfets and tubes are fine.
    3. If you bypass C2, the tube lights up like a christmas tree. I reflowed C2, didn't help.
    Is is possible that the move damaged C2? Does that make any sense at all?
    You'd think a cap would be the last thing to be damaged..
    I don't exactly understand the circuit but.. is it possible that the voltage isn't high enough to charge the cap? but, when you short out the cap, it no longer needs to be charged, so a low voltage current activates the transistor.
    Any insight would be helpful..
  9. nikongod
    Is pin 1 the drain or source on the mosfet you used?
    If its the drain look for where the power supply wire got screwed up.
    If its the source it could be any number of things [​IMG] Do you have good voltage at the gate of the mosfet? 
  10. the_equalizer
    EDIT: After reading nikongod's post I needed to clarify that I'm assuming a regular IRF510 / IRF610 MOSFET in which pin 1 is the gate, pin 2 the drain and pin 3 the source; as mentioned in this post from Dec 09.
    Since you're talking about C2, and refering to this schematic, I assume it's V1 which doesn't light up.
    By your description of your tests it seems to me that pin 1 of MOSFET Q1 is not at it's intended voltage level, which should be ~17 volts for a 12A_7 build and ~24 V for a 19J6 build. Since bypassing C2 makes the MOSFET turn on (which in turn lights up the tube), we can assume R3 is OK. Thus I can only think of R2 and R4 as the culprits, most likely R2.
    Check the values of R2 and R4 (off circuit, of course; you can unsolder one end of each to check their values). Check their solder points. Check that the 'upper' (refering to the schematic) end of R2 does indeed sit at ~48V as the schematics point out. Check that the 'lower' end of R4 (again referring to the schematic) does indeed sit at ground ( 0 volts). If those previous conditions are true then the joint of R2, R4 and R3 must be at ~17 V. If this is true and yet the MOSFET does not turn on, then most likely the MOSFET is faulty.
    BTW, about C2, it's purpose is precisely to isolate the MOSFETs pin 1 from the DC voltage and current present at the tube plate. Thus if it had somehow failed and became a "completely open circuit" that wouldn't affect the DC biasing of the MOSFET; you'd see the amp light and warm up correctly but no sound would come out of the Q1 channel, since the audio signal would not be able to cross C2 to get from the tube to the MOSFET.
    On the other hand, if C2 had failed in other way and became a short circuit, then you'd see the tube light up 'as a Christmas tree'  [​IMG]  as you yourself saw in your experiments.  That happens because with a shorted C2 the MOSFET gate sits at the DC voltage of the tube plate, ~27 V  if I remember correctly. Thus it's source pin is then forced to be at around 22 volts, which is the voltage that feeds the tube heater: 10 volts higher than it's intended voltage, making it light up brightly.
    Since from your description we know that none of these last two things happen, we can pretty safely assume that C2 is OK.
    I hope this helps you in understanding the circuit, but more importantly, to get your amp back up and running. [​IMG]
  11. nullstring


    Pin 1 there.
    Forgive me, I would look up which one it is, but I'm at work currently.
    I really do need to learn the terminology for transistors.
  12. the_equalizer


    Yeah, that's the MOSFET gate pin, the one connected to R3/R9
  13. nullstring
    Thanks the_equalizer.
    I think I understand..
    I'll do a few more experiments when I get back home
    I recall that R4 must be working, but I suppose I am not entirely sure about R2.
  14. Secretninja
    Ok, after a somewhat crappy week of quizzes and overly involved homework (god I hate tax laws), I have ordered all the parts.  I'm into this thing $55 dollars all told, so hopefully I can get it working properly.
    I had a couple silly questions while waiting for the parts to arrive.  In the schematic here http://www.diyforums.org/SSMH/variants/SSMH-12AU7.gif , I was somewhat confused.  I know that dots where lines meet indicate a connection, and I know that the jumps added by dsavistsk for the optional parts indicate no connection, but I was wondering about the points where lines cross but no dot appears.  I am assuming that this is just a different way of indicating a no connection cross in the schematic, but just wanted to make sure.  Also, pin 9 (12au7 build) is missing from the schematic.  Is it simply jumped to ground, along with pin 5?
  15. the_equalizer

    a) Lines crossing with no dot indeed indicate a no connection cross.
    b) Pin 9 is not in the schematic because it must NOT be connected to anything. That pin is the heater center tap, used when wiring the 12A_7 heater as two independent 6.3 V parallel heaters. In the SSMH the heaters are used in series to look like a single 12.6V heater. Grounding pin 9 will short out one of the heaters and dump ~12 volts into the other one.... not good [​IMG]
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