(26-08-2014, 06:40 PM)John M0GLN Wrote: . . . but I'm still puzzled as to how a control grid can supply those levels of current.
A bit more research into this topic has revealed that the current flowing through the crystal - and thus through the indicator bulb in series with it - is an r.f. current, not d.c. (Obvious, in retrospect
) Therefore, the d.c. grid current of the oscillator valve is quite a different current altogether. And that explains why it appears that the grid current of the osc. valve appears to be so very and contradictory high when first meeting this technique.It also seems that this technique was once popular with X-cut crystals operating in series-resonant mode, in which the r.f. current could become dangerously high (for the crystal). Modern crystal oscillators tend to use the AT-cut in the parallel mode, in which the crystal current is much lower and these cuts produce a more robust crystal with a far superior temperature co-efficient too.
So, in conclusion, the technique seems to be one that has now become obsolete. And that may explain some of our (or perhaps just my) puzzlement about it.
Nuff said then: I learn something new every day in this vast field of Radio & Electronics!
Al.
