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Millett "Starving Student" hybrid amp

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  1. tomb
    Quote:

    Originally Posted by ezzieyguywuf /img/forum/go_quote.gif
    Sweet, it turned out a lot cheaper at the site you linked than at thetubestore.com My total is now under $50!!!! Including tubes! Alls I got left is to think about a chassis. Woot. I think I'll use a breadboard as my common ground, instead of a copper plate. does that defeat the purpose or will it suffice?



    The copper plate would be better, but I guess it depends on your choice of case. If the case you end up using has plenty of metal in it to form a good ground sink, then a breadboard will be fine. This isn't a statement of precise knowledge, but I just have a feeling after building a few of the PCB versions that the SSMH needs a hefty ground to be happy. Those that have hum/noise issues in P2P versions seem to go light on the grounding material. It doesn't matter how many wires you run to ground - if the ground you're running to is insubstantial. A couple of the early PCB protos seemed to have had this issue, especially if they were left uncased.

    Just as example, in the MAX/MiniMAX, a 2 oz. ground plane was worth an additional 5-10 uVAC in ripple reduction (in the PS) over a PCB with a 1 oz. ground plane - with everything else equal. I think the SSMH is more sensitive to good grounding than that, even.
     
  2. ezzieyguywuf
    Ah. How about running them all to a breadboard (for neatness) and then connecting the breadboard to a nice hunk of copper: or the case (if it ends up being metal?)?
     
  3. the_equalizer
    Quote:

    Originally Posted by ezzieyguywuf /img/forum/go_quote.gif
    Alright I thought I could do it but still I'm epic failing at this. Anywhere I look for tubes (12AU7) they seem to be ~$16. I thought these could be had for cheaper? Or maybe its an alternative that can be had for cheaper? Also, i'm trying to find a tube vendor that will also sell me the sockets I need, as I don't want to have to go to another party, and have to pay shipping a third time, for two small components.



    You could also try e-bay. I just did a quick search for 12AU7 and found a 'Buy it Now' offer for four 12AU7's at $15.00 USD. Maybe not premium vintage 'golden sounding' Amperex or Telefunken, but 12AU7's certainly [​IMG].

    cheers!
     
  4. ezzieyguywuf
    Quote:

    Originally Posted by the_equalizer /img/forum/go_quote.gif
    You could also try e-bay. I just did a quick search for 12AU7 and found a 'Buy it Now' offer for four 12AU7's at $15.00 USD. Maybe not premium vintage 'golden sounding' Amperex or Telefunken, but 12AU7's certainly [​IMG].

    cheers!




    Ah I hadn't even thought about using ebay. Will do thanks!
     
  5. the_equalizer
    Quote:

    Originally Posted by tomb /img/forum/go_quote.gif
    Looks great! I'll try to post it today!



    Thanks Tom! I worked hard at making it look like the Beezar one.

    Now to the 12SR7 and 12AE6 schematics...

    cheers!
     
  6. nullstring
    How exactly do you calculate the resistors necessary for different tubes?

    I was bored and was looking through a few lists of tubes, and I couldn't figure out why a 14n7 wouldn't work well.

    After looking through what logistic and the_equalizer decided to use for their resistors, I am thinking that this tube may only need the single resistor change to 390k.
    This is because, 19J6, 12au7, and 14n7 are all twin triodes, and the 14n7 runs at the same voltage as the 12au7... but this is just inductive logic.

    anyway, the tubes are cheaper than 12au7's as far as I can tell... and from they seem to sound quite nice from what I am reading.

    would anyone mind confirming my theory? I really don't have any idea what I'm talking about =p
    would a 14n7 work well?

    How wonder how hard it would be to make an adapter from 14n7 to 19j6.
    I'd need a reverse socket thing... to plug into the 7pin tube socket... the rest should be easy.
     
  7. the_equalizer
    Quote:

    Originally Posted by nullstring /img/forum/go_quote.gif
    How exactly do you calculate the resistors necessary for different tubes?

    I was bored and was looking through a few lists of tubes, and I couldn't figure out why a 14n7 wouldn't work well.

    After looking through what logistic and the_equalizer decided to use for their resistors, I am thinking that this tube may only need the single resistor change to 390k.
    This is because, 19J6, 12au7, and 14n7 are all twin triodes, and the 14n7 runs at the same voltage as the 12au7... but this is just inductive logic.

    anyway, the tubes are cheaper than 12au7's as far as I can tell... and from they seem to sound quite nice from what I am reading.

    How wonder how hard it would be to make an adapter from 14n7 to 19j6.
    I'd need a reverse socket thing... to plug into the 7pin tube socket... the rest should be easy.





    Ok, this is a short description of how I did it. However, as I said before, I'm not an electrical engineer and there are a couple of things I don't (yet) understand about the circuit.

    If you look at the schematic you can clearly see that there are two parts to the circuit:

    a) the tube voltage gain stage (increases the volume of the signal)
    b) the MOSFET output current gain stage (increases the power of the signal)

    For simplicity I'll refer to just the right channel part numbers. Naturally the left channel works in exactly the same way.

    Capacitor C2 isolates the DC bias voltages and currents from these two stages.

    Bias voltages and currents for the tube stage are set by resistors R1 (the plate load resistor) and R5 (the cathode resistor). The way to establish the value of those resistors is by drawing a load line and calculating the values. You can read how it's done here. After reading that, you might want to read this post by Dsavitsk.

    Theoretically any triode (or maybe even any pentode) would do I guess.

    Now the interesting part of making tubes work with the SSMH is the MOSFET output stage. There are two bias voltages and currents you want to provide to the MOSFET

    a) The drain-source voltage and current
    b) The gate-source voltage

    As to the drain-source voltage and current, you can see that the MOSFET drain and source are in a series circuit formed by the power supply, the MOSFET and the tube heater. The current in that circuit will be the tube's heater current, a value you can find in the tube datasheets; whereas the voltage will be the power supply voltage less the tube heater voltage.

    So in the 19J6 version, the MOSFET is biased at 150mA (the 19J6 heater current) and 29 volts (48V - 19V).

    In order so keep things simple, you want to find a substitute tube that has a heater voltage close to 19V and the same heater current as the 19J6.

    There are several reasons why you want to stick to the same heater current: first, the heatsinking requirements of the MOSFET will grow enourmously with increasing heater current. Secondly, the standard PSU used in the SSHM has a current capability of .35 A, which is just enough to power two tube filaments, each drawing 150 mA; if your substitute tube draws more heater current you'll need to scale the PSU (more $$) . Finally, and this is something I haven't checked in the IRF510/610 datasheet, you want to make sure that the MOSFET can withstand your proposed heater current.

    The heater voltage requirements are somewhat less strict, but you want to stay around 19 V. Too high a heater voltage and the voltage across the MOSFET will be too low for it to drive your cans (it wont have enough room to move, enough voltage to swing); whereas too low a heater voltage and you'll have the same problem as a high bias current: you'd be dropping too much power on the MOSFET and you'll need BIG heatsinks.

    So, for example, using a 6J6 (a 6 volt heater equivalent to the 19J6) is almost impossible: it's heater voltage is too low as you'd be dropping 42 volts across the MOSFET (compared to 29 V in the 19J6 version) and it's heater current is 450 mA (!!!!!). Nor can you easily use 5J6 (4.7 V@600 mA heater) or 6DJ8 (6.3 V @ 365 mA heater) tubes.

    Yes, you could insert a power resistor in series with the MOSFET and the heater to dissipate the extra voltage but the resistor would have to be well ventilated (it'd need to dissipate some 6 to 8 watts) and that still leaves you with the heatsinking required for the MOSFET to operate at 29 volts @ 450 mA !!!

    So, specifically about the 14N7, while the heater voltage is adequate (12.6 V according to my tube manual) it's heater current is too high (300mA). It could be made to work if you check the three points I mentioned above (massive heatsinks, a 1 A power supply and check that the MOSFETs can withstand 300ma drain-source current).

    Last but not least, we just need to talk about the MOSFET gate-source bias voltage. In the SSHM this voltage is set by the tube heater voltage and the voltage divider formed by R2 and R4. You can see that the MOSFET's source pin (the pin tied to the tube heater) voltage to ground is equal to the tube heater voltage. So in the 19J6 version the MOSFET source pin voltage is close to 19 V, as indicated by Pete Millett in the schematic.

    Now, in order to "turn-on" the MOSFET, the gate needs to be a little bit more positive than the source. How much more positive is something I deduced from the schematic. If the PSU voltage is 48 V, then the voltage at the midpoint of the voltage divider formed by R2 and R4 must be half that voltage (since R2 and R4 have the same value) thus the voltage with respect to ground at the MOSFET gate is 24 volts; if the source is at around 19 V, then the gate-source voltage is around 5 volts. Resistor R3 has no influence in this due to the way MOSFETs operate: there's no current flow in the gate-source circuit, so no current flowing through R3 and no voltage drop across it.

    So, I assumed that that voltage had to be preserved. I was aided in this assumption by Dsavitsk's hint that "only one resistor" had to be changed, so I deduced, it had to be one resistor in the gate biasing voltage divider. Then it was a matter of using Ohm's law to calculate the value for such resistor so that if the MOSFET source pin were at around 12 volts, the MOSFET gate pin sat at around 17, thus mantaining the 5 volt difference.

    The math is simple:

    Total resistance in the voltage divider
    390 Kohms + 220 Kohms = 610 Kohms

    Total current in the voltage divider
    48V / 610 Kohms = .08 mA

    Voltage at the junction
    .08 mA * 220 Kohms = 17.31 V

    You can also see that Logistic got the same result with different resistor values. The possible implications of choosing different values are something that I don't completely understand. I think you'd want to keep R4 the same value or higher, so that there's no bass roll off in the insterstage RC coupling formed by R4 and C2 but this I get from my incomplete understanding of the circuit; I could (very likely) be wrong.

    Another thing I don't yet understand: does the source-gate voltage have to be around 5 volts ? Can it be higher? How much higher? Could the 12AU7 version work with the same voltage divider as the 19J6 version (e.g. with the gate sitting at 24 volts with respect to ground) ? I can see that this voltage (along with the source-drain voltage) determines how much voltage the MOSFET can swing, but I haven't yet sat down to carefully read the datasheet and run some experiments with the amp [​IMG]

    So, that's pretty much it... it didn't end up as brief as I thought it would be [​IMG]. I hope it wasn't too confusing and that I managed to answer your questions.

    cheers!
     
  8. m11a1
    Great explanation the_equalizer!!

    Cheers
     
  9. tomb
  10. the_equalizer
    Quote:

    Originally Posted by tomb /img/forum/go_quote.gif
    In fact, that makes for a great writeup to post on the web![​IMG]



    Quote:

    Originally Posted by tomb /img/forum/go_quote.gif
    Posted!
    SSMH Variations
    [​IMG]




    Wow, thanks.! .. [​IMG] it might be a good idea for someone more technically savy than I am to revise it [​IMG]

    By the way, to those interested, I think a possible good substitute tube would be the 17EW8. It's a double triode with a 17.5 V @ 150 mA heater. It would be a great exercise to draw the load line, determine the values for the plate load and cathode resistors and adjust the MOSFET gate bias voltage divider.

    I already ordered a couple of these tubes from e-bay, they should be in my hands in about a couple of weeks. But please, don't let that stop you! If you have those tubes handy please design and prototype your own variation on the SSMH [​IMG]... and obviously post it here, please! If it works, we could call it the "head-fi 17EW8 mod" [​IMG]


    cheers!
     
  11. tomb
    Quote:

    Originally Posted by the_equalizer /img/forum/go_quote.gif
    Wow, thanks.! .. [​IMG] it might be a good idea for someone more technically savy than I am to revise it [​IMG] </snip>



    Well, I edited it a bit.[​IMG] Still, it's in your own words and in your opinion - and it worked! So, I don't think it will need a lot of revising, per se. We'll get Dsavitsk to look at, though.[​IMG]
     
  12. timmyGCSE
    wow man, thats a nice writeup indeed.

    I'm torn now..I'm 80ish % of the way through a PCB design for the 12AU7 using the modified Dsavitsk schematic, I think I've got it right. I priced it up this morning using nice parts, kinda spending a bit extra to get some nice bits like full metal TruConnect jack instead of the REAN plastic one, plus the PCB stuff to make the PCB, plus a clear blue hammond case and it all comes to around £50. Which is how much I paid for my original SSMH but that included tubes and PSU, this one wouldn't.

    Or, I can build a mini3 or a SOHA (yes..aware that its obselete but its all pratice on the cheap) for the same cost. I'm tempted by the SOHA actually because I can still make the PCB myself (expressPCB design is already done on headwize) and its different and possibly actually cheaper than the SSMH to make.
    But I dunno, making the modified SSMH seems somewhat appealing bearing in mind how good the SSMH 12AU7 vanilla sounded.
     
  13. nullstring
    thanks the_equalizer, much appreciated.

    So, it sounds like the better solution, if I really wanted to use the 14N7, would be to use double the mosfets.
     
  14. the_equalizer
    Quote:

    Originally Posted by nullstring /img/forum/go_quote.gif
    thanks the_equalizer, much appreciated.

    So, it sounds like the better solution, if I really wanted to use the 14N7, would be to use double the mosfets.




    No problem, glad it helped.

    I'm not sure I quite follow you there with the "double the mosfets" idea. As it is you already have as many MOSFETs as you need: one for the right channel and one for the left channel.

    As I mentioned in my post, the way to get the 14N7 to work would be:
    1.- Draw a load line and determine the value for the plate and cathode resistors.

    2.- Get a PSU that can supply at least 650 mA of current (300 mA for each tube heater + a 50mA safe area)

    3.- Get some REALLY BIG heatsinks for the MOSFETs - You'll be dropping (48 V - 12V) = 36 V @ 300 mA ... let's see that's 10.8 watts per MOSFET (compare to the 4.35 watts per MOSFET in the 19J6 version)

    The gate voltage divider resistors would be the same as for the 12AU7 version, since the 14N7 heater runs at 12.6 V too.

    Finally, I checked the IRF510 datasheet and it can easly work with the 300 mA current (it's rated for a maximum of 5.6 A)

    cheers!
     
    vakumtube likes this.
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