Ref. post 33.
Experimental crystal oscillator built and measurements made.
The crystal itself is an ex-WD 500 Kc/s unit: it's oldest one I could find in my stock. I have no idea of the cut of this crystal, but I doubt very much that it is a AT or BT cut: this crystal is too old for those types of cuts; it might be an X-cut. Here is a close-up of that crystal unit:
First attempts were with a triode oscillator: screen grid strapped to anode. But I could not make that arrangement stable without the crystal installed. So I configured it as a tetrode with a by-passed 270 Ω resistor in the cathode: this resistor is simply there to protect the valve if the cct. fails to oscillate. This tetrode configuration was unconditionally stable - so measurements were made - see attached drawing.
When the 'scope probe was attached to the crystal current sensing resistor, it was necessary to slightly adjust the anode tuning capacitor to compensate - this is to be expected, of course.
The 10 Ω resistor in series with the crystal is only for the purposes of indirectly measuring the r.f. current through the crystal.
As for the assembly - it'll win no prizes in a constructor's competition - but it's not meant to!
This is obviously a 'one-off':
The brown knob (seen in picture 1 above) is simply there as a convenient way to adjust the anode tuning capacitor.
For the HT @ 200-v., the r.f. voltage across the 10 Ω crystal current sensing resistor was approx. 26 mv. p-p (9.3 mv. rms, approx.). Hence, the r.f. current through the crystal is in the region 1 mA. (Note: the r.f. resistance of the crystal alone will be in the region of 100 ~ 200 Ω: that's why a value of 10 Ω was chosen for the sensing resistor).
All r.f. waveforms were measured with a 300 MHz Tektronix 2465 'scope plus low capacitance X10 probe (7 pF); all d.c. voltage were made with a Fluke 77 D.M.M.
Al. / Sept. 7, 2014 //
Experimental crystal oscillator built and measurements made.
The crystal itself is an ex-WD 500 Kc/s unit: it's oldest one I could find in my stock. I have no idea of the cut of this crystal, but I doubt very much that it is a AT or BT cut: this crystal is too old for those types of cuts; it might be an X-cut. Here is a close-up of that crystal unit:
First attempts were with a triode oscillator: screen grid strapped to anode. But I could not make that arrangement stable without the crystal installed. So I configured it as a tetrode with a by-passed 270 Ω resistor in the cathode: this resistor is simply there to protect the valve if the cct. fails to oscillate. This tetrode configuration was unconditionally stable - so measurements were made - see attached drawing.
When the 'scope probe was attached to the crystal current sensing resistor, it was necessary to slightly adjust the anode tuning capacitor to compensate - this is to be expected, of course.
The 10 Ω resistor in series with the crystal is only for the purposes of indirectly measuring the r.f. current through the crystal.
As for the assembly - it'll win no prizes in a constructor's competition - but it's not meant to!
This is obviously a 'one-off':The brown knob (seen in picture 1 above) is simply there as a convenient way to adjust the anode tuning capacitor.
For the HT @ 200-v., the r.f. voltage across the 10 Ω crystal current sensing resistor was approx. 26 mv. p-p (9.3 mv. rms, approx.). Hence, the r.f. current through the crystal is in the region 1 mA. (Note: the r.f. resistance of the crystal alone will be in the region of 100 ~ 200 Ω: that's why a value of 10 Ω was chosen for the sensing resistor).
All r.f. waveforms were measured with a 300 MHz Tektronix 2465 'scope plus low capacitance X10 probe (7 pF); all d.c. voltage were made with a Fluke 77 D.M.M.
Al. / Sept. 7, 2014 //
