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Millett "Starving Student" hybrid amp

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  1. the_equalizer


    Quote:
     
    Yeah, they're one of the good tube vendors IMO. They also have the 12SR7GT listed for $4.00 US each.
     
    cheers!
     
     
  2. wiisus
     
     
    My amp is 95% finished!  I'm going go through, and redo some of the grounding eventually and maybe give it a new case.  Using the box the parts from mouser came in.  But for now, it's good.  
     
    I kept hearing this weird buzz every once in a while, and it was driving me insane.  I couldn't for the life of me figure it out.  Turns out, it would only happen when my cellphone was near [​IMG].
     
    Overall, fun build, and it sounds awesome.
     
     
    DSCN4576.jpg
     
     
    DSCN4578.jpg
     
    Thanks everyone for all the help and info!
     
  3. Zink


    Quote:

    Thanks a lot. I went back and read what the the_equalizer had to say about his 17EW8 version and it seems that I missed that he liked it better than the 12AU7 the first time I read the thread. I ordered three 17EW8 from vacuumtubes.net and I also ordered two "PCP9" which are supposed to be 9 pin chassis mount sockets. Are these the right nine pins for mounting on the top of a case PTP wired or is there a mini variant that I need? Will there be pins in the socket or do I need to buy those as well? I got the rest from Mouser and some at my local shop so now I think I have everything but the power supply ordered.
     
  4. the_equalizer

     
    Quote:

    Well, It's not that the 17EW8 version is superior to the 12AU7 version, just slightly different :)  I'm sure you'll enjoy any of the two, but since you asked if those 12AU7's were the cheapest tubes one could use I felt compelled to answer.
     
    As to the socket, I'm not sure what you mean by 'Will there be pins in the socket'.
     
    cheers !
     
  5. c12mech
    Zink,
     
    Those sockets are exactly what you need.  They are the same that I used.  They mount to the top of the enclosure and are pretty easy to work with.  If I had to do it again I would have put a little more thought into what order I soldered things together.  I gets kind of tight around the socket as more connections start to be added.  Altogether it was a pretty straight forward build.
     
    As for the amp, it is starting to sound better each time I turn it on now.  I usually listen at a low volume and am hearing things that I never knew was in some of the recordings I listen to.  This is the best bang for the buck amp out there, IMHO.  
     
    For my next project I'm going to use another of Pete Millett's amps as the base for a tube ipod dock similar to the Bose sound dock.  I know it's not for headphones but I need something for my toolbox at work. 
     
  6. Juaquin


    Quote:

    My cellphone can do this. When we moved apartments and I had room to put the amp on my desk, I suddenly noticed more interference. Turns out my wifi router (2.4GHz and 5GHz channels) was causing interference, and only in the tubes (if I blocked the tubes with my hand, all was well). I need to figure out something to shield them. Maybe just some foil wrapped on the outside of a couple 35mm film capsules fitted over the tubes - that would certainly be starving student!
     
  7. Beftus
    Have you tried adding grid stopper resistors to eliminate interference?
     
    You could also make one with those metal tubes: the 12SR7 mod. Perhaps the metal enclosure can be tied to ground?
     
     
  8. the_equalizer


    Quote:
     
    Indeed, the metal case is connected to a pin which can (and should) be grounded.
     

     
     
  9. Juaquin
    The SSMH kit that I have isn't easily modified into a 12SR7 or similar (given the socket difference), but it's not really a big deal since I don't use the amp on my desk much. As soon as I have time I'll try modding it a bit.
     
  10. Ikarios
    I just got the 5814a tubes I ordered today, along with some other stuff like sockets, RCA jacks and heatshrink. A couple thoughts and questions...
     
    they're a lot smaller than i thought. for some reason I thought these things would be like at least film canister size. This probably means the whole Hammond case will also be a lot smaller than I thought as well.
     
    there are NINE pins! I thought this thing was supposed to be an 8-pin tube... why are there only eight pins as indicated on the 12AU7 variation schematic? D: do I just ignore pin 9?
     
    also, I'm having a heck of a time getting my tubes to insert into the sockets I ordered. I've tried wiggling the pins around but one socket refuses to have a tube put into it. is this what I get for ordering cheap sockets, or is there a way around it?
     
    I had the bright idea of using a variable resistor/pot (rheostat?) for RLED, but I don't know if it's viable or not. First, what does the resistance rating of a pot refer to, the highest resistance point? Second, I wouldn't even know how to wire up a single pot as RLED... would I run a single supply voltage line, split it to two, connect to the inputs, then wire the LEDs to the outputs? wouldn't that count as resistors running in parallel and require some not-straightforward calculations? I'm looking to vary the RLED current between 5 and 12mA, ideally running all the way down to zero.
     
    Also, I had a question regarding ground for this amp - some people have mentioned using the chassis as ground, but then some people mentioned not using the chassis as a "conductor". does this mean to connect ground to the chassis, but not directly (i.e. run all grounds to, say, a copper plate, then connect the plate to the case)? What about, say, for the pot or RCA jacks, where the ground tab will probably touch the metal case?
     
    I have a lot more questions but since my last question post got ignored for the most part, I'll post them slowly this time... :D
     
  11. Juaquin
    For grounding - basically what you want to avoid is actually passing current through the case. Therefore, you want circuit ground to touch the chassis at really only one point (if it connected at two points, there's a possibility some current would flow through the case, which may or may not be a good conductor and hence add noise). So you can either use star ground (run ALL grounds to a single post connected to the chassis) or use a large ground plane (like a copper plate) and connect that plane to the chassis at one point. For the jacks and the pot, you'll probably be fine just letting them touch the case (if they need to and they're not insulated). You'd still run a ground wire to the jacks anyways - electricity takes the path of least resistance, and since the case likely has a higher resistance than the wire you wouldn't be running much current through the case even in a worst-case scenario.
     
    A pot is rated for it's highest resistance (so a 1k pot goes from about 0 ohm to 1000 ohms). To use a pot for RLED, pick a pot that is equal to or higher than the value you would have used for RLED. Hook one of the outer pins (doesn't matter which, except which way you want to turn the pot) to one side of where RLED would go and the wiper (middle pin) to the other side. Keep in mind that RLED on the SSMH needs to be rated for at least 2 watts because of the 48V supply, from what I remember. I'm not sure most pots meet that rating.
     
    So if you had a pot rated for 2W+, and you were shooting for around 10ma through the LED on a 48V supply, you would do 48V/10ma=4800ohm. So I'd go with 10k pot so you could lower the brightness further (48V/10000=5ma). This is all assuming you can find a pot with a suitable power rating.
     
    As far as inserting tubes into sockets - it can be hard sometimes. You can use some force as long as you're careful not to break the tube. You could also stick a screwdriver in there and loosen the socket a bit, but overdoing it could lead to a bad connection.
     
  12. Beftus


    Quote:
     

    Indeed, nine pins, pin 9 isn't used. I even removed pin 9 from the tube socket when I was modifying the socket to accept a tube LED.
     
    DSC_0574.jpg
     
    The LED needs a pot with a high watt rating, as outlined before. I plan to install a on/off/on switch giving me the option to run the LEDs at a low current, a high current or turned off.  
     
  13. Juaquin


    Quote:

    Yes, this might be the better idea. Set up a switch to switch between open circuit, one resistor, and a higher resistor. That way you can use any standard 2W+ resistors and still have control over brightness.
     
  14. Ikarios


    Quote:

    I think I found a suitable pot for RLED - http://www.mouser.com/ProductDetail/Bourns/3852A-162-503AL/?qs=sGAEpiMZZMtxdMMi52izyhVRLOm6MJTqb9DViJ%252b2ghU%3d
    compared to buying two 2W resistors (about $2 each) and a DPDT switch (~$5-8) it's not a bad deal. 50K ohms for the ability to use more of the pot, and if it goes down to ~0 ohms it won't matter, right?
     
    Since it only has three lugs, do I connect one end of each LED to the wiper pin and the connection from power to the far side pin so it starts at 50k, then goes down? I think I've been confused about the whole 2-LED thing, do most people use one RLED for both LEDs? or two resistors and two connections from power for each LED?
     
    EDIT: I did some more thinking on the matter and I think instead of only using a 50k pot like this, what about using a 10k pot with a ~5k resistor right before the LED, so the LED current will swing from ~3.2mA to ~12mA? This way, there's no possibility of blowing out the LED if someone accidentally turns the pot all the way up. The main disadvantage of this method is that there's no way to turn the LED completely OFF... but I don't know if an LED fed by 3.2mA will be very visible when placed underneath a tube anyway.
     
    As an aside... how do I know if my tubes' heaters are enclosed? One of my main concerns about this whole LED thing is that some people have very bright orange tubes, whereas others have zero tube glow. I plan on using the sea green LEDs from Beezar, which will probably not look good with the standard orange-yellow tube glow...
     
  15. Juaquin


    Quote:

    This is a good idea. 3.2ma might be visible but not much, depending on the LED. You can connect both LEDs to one resistor/pot - that's how the SSMH kit did it.
     
    I'm not sure which tubes are enclosed and which aren't, or if you can tell by looking at them. I suppose the easiest way would be simply to hook up the heater and find out. You don't have to build the rest of the circuit just for that.
     
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