03-06-2019, 06:52 PM
(03-06-2019, 08:43 AM)radioman Wrote: Did you get the crystal specially cut for this frequency or was it just one you already had ?Hi Andy,
Also, are modern crystals OK for use in valve circuits like this or are they liable to be overdriven and consequently damaged ?
the crystals are in reality ceramic resonators I have a supplier here who had various frequencies in stock ranging from 190kc up to 1600kc that were close to MW band-plan frequencies. Fortunately these resonators can be pulled by about 3-4 kc and are never "spot on" when it comes to the frequency they are specified for, so what I do is I try out several until I find one that is close enough to a desired frequency and then pull it to exactly that frequency. At 20p a piece these are not expensive and work quite well.
I have successfully used this specific resonator rated at 1580kc as a 1584kc reference. I also have used an 800kc resonator for the 801 MW frequency. Once they have been set up to oscillate at a specific frequency they are quite stable but of course not as good as a crystal but far better than an LC tank.
On the subject of RF power handling of the crystals/resonators I have never had any issues with these resonators, possibly also because I have a variable resistor that sets the amplification or feedback level of the oscillator thus preventing too high an amplitude at the resonator by limiting the RF power fed into it.
I had only one occasion when the high input power of a valve circuit became a problem. That was when I built my FM stereo encoder using valves. Here I used a very delicate 38kc crystal and this would start up and then break with a faint "pling" sound. I killed three of them until I found a solution. This was to place two anti-parallel 4148 diodes in front of the crystal. These diodes effectively limit the amplitude of the RF signal coming from the anode of the oscillator valve. Since adding the amplitude limiter the delicate 38kc crystals work without being destroyed.
I think I posted the circuit of my stereo encoder somewhere higher up in this thread, you can check teh circuit there.
cheers
Semir
