A high crystal current: my own belief is that a crystal current of the order of 10s of mA harks back to the 1930s when the piezo-electric effect was effectively still in its infancy. With that, crystals would have been chunky affairs, not only capable of handling such currents, but the low crystal 'activity' (due to the type of cut, impurities, surface irregularities from the grinding process, etc.) would have required a circuit arrangement that would have required such high currents to be produced in the crystal in the first place. A triode would have been the natural choice - with its attendant high Ca-g, providing a much higher feedback current than with a tetrode or pentode - and more power would have been required from that oscillator to drive successive stages (typically triodes), compared to a much more efficient all-pentode arrangement. Net result: a high crystal current by requirement; a crystal capable of handling such a high current.
However, the technique of inserting a light bulb (typically 2-v., 60 mA) in series with the crystal was still being advocated up until 1974 in the ARRL Handbook of that date and the RSGB H/B (4th. ed.) of 1971 (and other RSGB publications of about that period). However, I note that the latter does specifically refer to X-cut crystals using that bulb technique. But from the 1930s to the 1970 is a very long time in terms of radio / electronics technology, and during those later years, the X-cut was obsolete (superseded by more efficient cuts, such as AT and BT, amongst others). And that, in turn, generates one question: why the persistence of still referring to X-cut crystals in Amateur Radio publications during that later period? I can only suggest that Radio Amateurs are notorious for being extremely reluctant to dispose of anything that one day just might be useful (here, I speak from personal experience
): hence, one metric of any Radio Amateur's enthusiasm for his hobby was (and perhaps still is) the size of his 'junk box'! 
An auxiliary comment is appropriate: I do not know the cut of the crystal used in my experiment. If I had had a known X-cut type, I would have used that. And - just as an aside - I did try a 400 kHz HC6/U type: circuit operation was much the same; the crystal current was of insufficient amplitude for a reliable measurement to be made. (And that itself is highly relevant here.)
Finally (if it isn't too obvious), in this monologue, I have made a few assumptions - and a bit of 'educated guesswork'. So if anyone feels that they need to add or correct anything thus written, please do so: your comments will be welcome.
For me, the furtherance of Science is more important than the protection of one's ego.
Al. / Sept. 2, 2014 //
However, the technique of inserting a light bulb (typically 2-v., 60 mA) in series with the crystal was still being advocated up until 1974 in the ARRL Handbook of that date and the RSGB H/B (4th. ed.) of 1971 (and other RSGB publications of about that period). However, I note that the latter does specifically refer to X-cut crystals using that bulb technique. But from the 1930s to the 1970 is a very long time in terms of radio / electronics technology, and during those later years, the X-cut was obsolete (superseded by more efficient cuts, such as AT and BT, amongst others). And that, in turn, generates one question: why the persistence of still referring to X-cut crystals in Amateur Radio publications during that later period? I can only suggest that Radio Amateurs are notorious for being extremely reluctant to dispose of anything that one day just might be useful (here, I speak from personal experience
): hence, one metric of any Radio Amateur's enthusiasm for his hobby was (and perhaps still is) the size of his 'junk box'! An auxiliary comment is appropriate: I do not know the cut of the crystal used in my experiment. If I had had a known X-cut type, I would have used that. And - just as an aside - I did try a 400 kHz HC6/U type: circuit operation was much the same; the crystal current was of insufficient amplitude for a reliable measurement to be made. (And that itself is highly relevant here.)
Finally (if it isn't too obvious), in this monologue, I have made a few assumptions - and a bit of 'educated guesswork'. So if anyone feels that they need to add or correct anything thus written, please do so: your comments will be welcome.
For me, the furtherance of Science is more important than the protection of one's ego.Al. / Sept. 2, 2014 //
