02-09-2014, 04:00 PM
I have only had one reply to the question I posted on another forum:
Lawrence.
"There will be no current through the crystal as a result of cathode-grid diode action, as a crystal is an open circuit for dc. Cathode-grid diode action will, however, develop a dc bias level at the grid (charging the cathode-grid capacitance and discharging it through the resistor from grid to ground--the grid leak resistor).
But your main interest is the 6F6 as oscillator and RF current through the crystal.
The circuit you describe is a a tuned-plate, crystal-grid oscillator, an analog of the tuned-plate, tuned-grid oscillator in which the crystal acts (in its parallel-resonant mode) as a parallel-tuned circuit between the tube grid and ground. Positive feedback for TPTG/TPXG oscillator occurs through the 6F6's plate-to-grid capacitance--0.7 pF in a metal 6F6 (and its transmitting-rated alter ego, the 1621); more for a glass 6F6. The oscillator plate circuit must be tuned to a frequency close to, but above, crystal resonance for the phase of the feedback to be correct for oscillation.
RF voltage at the 6F6 grid makes the crystal vibrate. This vibration--and hence the stress on the crystal--is maximum at crystal resonance and increases with the RF voltage across the grid. Because the crystal is acting as a tuned circuit, and a tuned circuit excited by RF has RF current circulating through it, RF current flows through the crystal.
We should note that although a maximum for crystal current is generally specified, and that exceeding this maximum can result in crystal fracture, at least one source (James J. Lamb in his April 1937 QST article "A Practical Survey of Pentode and Beam Tube Crystal Oscillators for Fundamental and Second Harmonic Output") declared that RF current through a crystal is actually a derivative value and potentially misleading, as--as we know from an understanding of piezoelectricity--it's applied voltage that causes a crystal to deform (and possibly shatter) and not applied current. Lamb considered crystal current potentially misleading because away from resonance a crystal acts, in conjunction with its electrodes, as a quartz-dielectric capacitor. A frequency-nonselective current indicator, of which a panel lamp is one, in series with the crystal sums the effective values of all current through it and therefore may over-indicate if harmonic, LF or VHF parasitic-oscillation energy, or other off-crystal-resonance currents are present.
Lamb also mentions, however, that a lamp-in-series-with-the-crystal current indicator is therefore conservative as a means of indicating possible danger to the crystal.
In a TPXG oscillator, the greater the plate-to-grid capacitance of the oscillator tube, the greater the feedback and the generally greater the voltage across the crystal, and therefore the greater the current through it. Plate tuning therefore also affected the grid voltage, and hence the crystal stress. Potentially most dangerous to the crystal was its use in a TPXG oscillator used stand-alone as a transmitter--that is, coupled directly to an antenna. Loss of appreciable plate loading during tuning or as a result of sudden disconnection of the antenna could suddenly greatly increase feedback and destroy the crystal.
Not long after the series-lamp-as-crystal-current-indictator became popular, its users realized that the large resistance increase from cold to warm to hot in the lamp could compromise keying quality, resulting in yoop, a relatively slow frequency shift across multiple dots and dashes of Morse code elements. This led to builders equipping such lamps with a shorting switch; adjust the oscillator for crystal safety with the lamp unshorted, and then operate the transmitter with the lamp shorted for best keying.
Crystal current indication with a lamp would all but disappear rapidly after World War 2, as increasing attention to the crystal oscillator as a frequency standard rather than as a producer of appreciable RF driving power (and increasing use of RF power tubes that needed less drive for a given output power than prewar tubes) resulted in crystal oscillators generally being so low-power that the danger of crystal stress from overcurrent was all but banished by improved circuit designs.
And so this final note on the circuit you've found: Tiny crystals of modern manufacture (think HC-49), and even older HC-6 crystals, are likely to be destroyed when used in circuits like your 6F6-6L6 transmitter, especially crystals resonant at 7 MHz and above (because they're so thin). Even pre-World-War-2 we see, over and over, beginner's transmitter designs that went to great lengths to encourage the use of 160 or 80 meter crystals (relatively hard to fracture) rather than 40-meter ones."
Lawrence.
"There will be no current through the crystal as a result of cathode-grid diode action, as a crystal is an open circuit for dc. Cathode-grid diode action will, however, develop a dc bias level at the grid (charging the cathode-grid capacitance and discharging it through the resistor from grid to ground--the grid leak resistor).
But your main interest is the 6F6 as oscillator and RF current through the crystal.
The circuit you describe is a a tuned-plate, crystal-grid oscillator, an analog of the tuned-plate, tuned-grid oscillator in which the crystal acts (in its parallel-resonant mode) as a parallel-tuned circuit between the tube grid and ground. Positive feedback for TPTG/TPXG oscillator occurs through the 6F6's plate-to-grid capacitance--0.7 pF in a metal 6F6 (and its transmitting-rated alter ego, the 1621); more for a glass 6F6. The oscillator plate circuit must be tuned to a frequency close to, but above, crystal resonance for the phase of the feedback to be correct for oscillation.
RF voltage at the 6F6 grid makes the crystal vibrate. This vibration--and hence the stress on the crystal--is maximum at crystal resonance and increases with the RF voltage across the grid. Because the crystal is acting as a tuned circuit, and a tuned circuit excited by RF has RF current circulating through it, RF current flows through the crystal.
We should note that although a maximum for crystal current is generally specified, and that exceeding this maximum can result in crystal fracture, at least one source (James J. Lamb in his April 1937 QST article "A Practical Survey of Pentode and Beam Tube Crystal Oscillators for Fundamental and Second Harmonic Output") declared that RF current through a crystal is actually a derivative value and potentially misleading, as--as we know from an understanding of piezoelectricity--it's applied voltage that causes a crystal to deform (and possibly shatter) and not applied current. Lamb considered crystal current potentially misleading because away from resonance a crystal acts, in conjunction with its electrodes, as a quartz-dielectric capacitor. A frequency-nonselective current indicator, of which a panel lamp is one, in series with the crystal sums the effective values of all current through it and therefore may over-indicate if harmonic, LF or VHF parasitic-oscillation energy, or other off-crystal-resonance currents are present.
Lamb also mentions, however, that a lamp-in-series-with-the-crystal current indicator is therefore conservative as a means of indicating possible danger to the crystal.
In a TPXG oscillator, the greater the plate-to-grid capacitance of the oscillator tube, the greater the feedback and the generally greater the voltage across the crystal, and therefore the greater the current through it. Plate tuning therefore also affected the grid voltage, and hence the crystal stress. Potentially most dangerous to the crystal was its use in a TPXG oscillator used stand-alone as a transmitter--that is, coupled directly to an antenna. Loss of appreciable plate loading during tuning or as a result of sudden disconnection of the antenna could suddenly greatly increase feedback and destroy the crystal.
Not long after the series-lamp-as-crystal-current-indictator became popular, its users realized that the large resistance increase from cold to warm to hot in the lamp could compromise keying quality, resulting in yoop, a relatively slow frequency shift across multiple dots and dashes of Morse code elements. This led to builders equipping such lamps with a shorting switch; adjust the oscillator for crystal safety with the lamp unshorted, and then operate the transmitter with the lamp shorted for best keying.
Crystal current indication with a lamp would all but disappear rapidly after World War 2, as increasing attention to the crystal oscillator as a frequency standard rather than as a producer of appreciable RF driving power (and increasing use of RF power tubes that needed less drive for a given output power than prewar tubes) resulted in crystal oscillators generally being so low-power that the danger of crystal stress from overcurrent was all but banished by improved circuit designs.
And so this final note on the circuit you've found: Tiny crystals of modern manufacture (think HC-49), and even older HC-6 crystals, are likely to be destroyed when used in circuits like your 6F6-6L6 transmitter, especially crystals resonant at 7 MHz and above (because they're so thin). Even pre-World-War-2 we see, over and over, beginner's transmitter designs that went to great lengths to encourage the use of 160 or 80 meter crystals (relatively hard to fracture) rather than 40-meter ones."
