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Millett "Starving Student" hybrid amp

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  1. Wiljen
    bad mosfet with the gate already burnt up. if you overload the mosfet it will burn through the shield on the gate and all sides will read 48V. Replace it first before you put any more tubes in it.
  2. Wiljen
    On a side note, anybody else built a SSMH using a 12at7 for pushing planars or other impossible to drive cans? I built one using a pair of RCA Blackplate 12at7s from 1963. With 32ohm headphones like the B&W p5 I can only use the first 5% of the pot but with the big planar magnetic I can use 50%-60% before it is beyond listenable. That little amp is easily the most potent I have for the Fostex.
  3. skajohyros
    Thanks mate. Any idea how I burnt the MOSFET?
  4. Wiljen
    Check all your wiring, and make sure you don't have something cross-wired. You either have a wiring problem that is pushing too much current or voltage into the mosfet and burning it open so the voltage then flows freely on all legs, or you had a bad mosfet that was open to begin with. The good news is mosfets are dirt cheap, the bad news is tubes are not and 48V on the wrong pin will turn a nice tube into an expensive flashbulb. A wiring problem will probably be measurable even without a tube in place, so I would first put in a new mosfet and check the voltage on all legs. If they measure as expected - Ground to source, nothing on gate and Load at drain, then put in another tube, If you see 48V across all legs of the new mosfet, then you definitely have a wiring problem.
    Last edited: Dec 27, 2017
  5. skajohyros
    Bloody he'll. Can't get my hands on any irf510 here.. immediately.
  6. Wiljen
  7. skajohyros
    I'm in Greece. 20€ shipping.
  8. skajohyros
    @Wiljen thanks for your help. Finally paid €10 shipping from Poland to Greece for 40 cent MOSFETs.
    Not the prettiest amp on the block but it's working.
    Last edited: Dec 31, 2017
  9. Wiljen
    Sounds good is what counts, none of my DIY projects will win any beauty contests either. :beerchug::ksc75smile:
  10. skajohyros
  11. Wiljen
    The SSMH with a pair of tube savers makes a great starter for tube rollers. I have no less than 12 different tubes that work and all bring their own differences to the sound. When you add different brands into the equation, you could easily have 100 different tubes to choose from.

    6211 <-- Favorite of mine (Sylvania 6211 from 1966 production) Higher mu than 12au7 with same specs - really drives a Fostex without being too much for B&W P5.
    6414 <-- Another low mu AT or high mu AU depending on how you look at it. These have 10,000 hour rated heaters so should last very well. (1/2 inch taller than standard if that matters to your enclosure)

    I avoided others that would probably work as well due to differing heater current requirements etc. With a few resistor swaps you could open up another 10-15 varieties.
  12. skajohyros
    Very useful list. I will definitely look into some of the cheaper options just to keep the good times rolling.
  13. Nathan P
    Finally finished mine!

    Took some troubleshooting with grounding and isolating signal connectors (notice the wooden spacer spliced into the back plate. That wasn't always there. :D) to get the noise floor down, but it's dead silent now and sounds amazing with the GE 5963 tubes I picked up.



  14. Nathan P
    Can anyone comment on whether this design has the heater current capacity for 12BH7a tubes?
  15. Wiljen
    The short answer is technically yes but realistically with the power supply constraints and the fact that the mosfet is tied to the heater, it is a poor choice and will limit the lifespan of the device. Also worth noting the 12BH7a tubes are high enough mu that anything below 600 Ohm will likely be so overdriven that it will not sound very good. With you being as close as you are to me, I'll offer a couple of my better choices in low Mu 12at7 equivalents to try if you want to increase gain without changing heater specs.
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