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Self Osc Problem
On Tue, 22 May 2007, Jack Schmidling wrote: I am building an exciter for my 8000 rig. 6AG7 VFO, 6CL6 buffer and 807 final. It would help to get more details from you. What frequency is all of this for? Link-tank output? Or pi-network? Are the coils of the tank/pi-net according to specification (from some plans)? Do you know that the VFO is on frequency? Are you doubling in the 6CL6? I am plagued with the problem that resonance in the final does not agree with peak power out. Can you removed B+ from the VFO and twist your driver and final tune knobs and get dips in plate, grid current (signs of self-oscillation)? Do you generate grid bias in the final from a grid power supply or rectified drive power? I have had some experience with matching networks between the final and the load having slight disagreements between peak output power and maximum dip in plate current. Not to worry if the difference is slight. Most important: find out if the final is self-oscillating (remove drive and rotate final tuning knob to see if there is any wiggle in plate current at any point). The power peak (watt meter or light bulb) is a considerable twist south of the plate current dip. What is a considerable twist? five degrees of arc? or 45 degrees of arc? What is your load? Dummy load? Looking at the output on a scope shows nothing strange about the wave shape as the amplitude increases beyond the dip. All sine waves should look like sine waves. I have tried neutralizing with coupled links between the input and output tanks but it makes no difference nor does anything else I have tried. This can be very difficult to do. If your frequency is HF or low HF you shouldn't have to do this. Most old commercial HF tube rigs did not have neutralization and they worked fine. Any ideas would be welcome. You do have the screen grids grounded through good RF bypass capacitors, right? Once I forgot to add these to a 6146 amp and it self-oscillated at 160 meters (it should not do that) until I put in a good screen to ground bypass capacitor. Another area where you have to look out for a mistake is if you have the coil/inductor at the grid input of the final located where it can pick up the magnetic field from the final plate output coil/inductor, which can contribute to making the final just a self-excited oscillator. So, the next question is: if you get a dip in the final plate current at some position of the plate tuning capacitor, then does that final plate current change if you also tune the grid input capacitor? If yes, then your final has a configuration to make itself a self-excited oscillator, and the next thing is to look for how feedback can be contributing to that. Jack K9ACT -- PHOTO OF THE WEEK: http://schmidling.com/pow.htm Astronomy, Beer, Cheese, Fiber,Gems, Sausage,Silver http://schmidling.com |
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