To use smaller transistors and LEDs, increase the inductance and resistance, and reduce the capacitance, proportionally.
What kind of core did you use?
People have build JTs from crap like ferrite beads before. Again, just because it works, doesn't mean it works well. In particular, ferrite beads are specifically made for high impedance and high loss, exactly the opposite of what's desired here: energy storage, low losses and (most likely) freedom from saturation.
In my experiments, I found that too much series base resistance causes switching to be very sloppy. This is represented by Rb in my prototypical schematic up above. I never found a case where performance was better with resistance here.
You can imagine it this way: the base must have some (small but limited) amount of current flow, because the base draws current (especially in saturation). It must also have a low dynamic impedance, because it requires voltage control (referring to another thread: BJTs are voltage controlled). This combination can only be provided from the supply if we use a very small series resistor. But that would deliver far too much current for our purposes (with the high hFE of this transistor, it would simply turn on forever and destroy itself; it might still oscillate, but the peak current would be excessive, and it will turn on again much sooner than desired).
If the supply voltage were very low, right around the threshold (Vbe), this would be a quite acceptable means of operation. In fact, since the act of switching on provides power to the base, it can even remain operating to even lower voltages, once started. (For example: note the DC voltage on Cbb in the simulation!)
But at elevated voltages, this isn't going to cut it. So, suppose we dropped the voltage with a resistor, and filtered it with a capacitor, so it acts like the base has a nice low voltage, stiff supply. Well, that's what I've got here.
You can also extend the principle to much higher voltages: in the old days, one or two transistor SMPS were all the rage. Everything from the Apple II/E (I think?) to consumer VCRs (and maybe still DVD players and such today?).
Here's a (somewhat backwards) example:
I say backwards, because the loop is noninverting: the FJPF13009 is normally biased only through the 1M resistor (so it "ticks" at a fairly low rate, and draws very little supply current under startup or short-circuit conditions), and current flow through the optoisolator causes increased base bias, increasing the repeat rate (the switching itself is essentially a monostable timer, so it charges up to an ampere or two peak, regardless of load; the frequency of pulsing is all that varies). This does complicate the feedback circuit, which uses three transistors in addition to the TL431 regulator (which is
practically a transistor, anyway, I mean, come on, right?
). These provide a constant current sink (allowing full opto current at low output voltages -- so it is able to start up more quickly, and more tolerant of heavy loads), and invert the level (because the TL431 doesn't have a noninverting input to complete the negative feedback loop).
Other examples use a MOSFET instead of the FJPF13009, which doesn't switch off as easily (there's no gate current, and saturation isn't as easily controlled), so a current sense transistor is used below it. That looks like this:
http://seventransistorlabs.com/tmoranwms/Circuits_2010/Fast_DCDC.pngTim