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#11
March 14th 04, 03:40 PM
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In article , Uwe
writes: in article , N2EY at wrote on 3/13/04 15:42: In article , Uwe writes: When I increase the plate voltage to 325V my 2. grid runs about 280V and the plate current increases to about 65 mA. I don't really dare to do that without knowing the max values on that tube. I guess I could lower the 2. grid voltage by increasing the value of the resistor connectd to it. I don't know if that would lower my plate current to what you get. Uwe, How are you tuning up the transmitter? Are you adjusting the plate capacitor for minimum current? 73 de Jim, N2EY Jim, Sofar I have connected a scope to the antenna and tuned for max waveform eg. highest voltage. That is also the setting where my dummy load with bulbs is brightest. The original instructions for the AC1 explain how to tune but use expressions like 'turn the capacitor clockwise' which is useless to me when I am not using their exact part. The tuneup procedure (adapted to your setup) is as follows: 1) Set both output capacitors to maximum capacitance (plates fully meshed) 2) Close the key and adjust the "plate" capacitor (the one nearest the tube in the circuit diagram) for minimum plate current. Maximum output should occur at the same time. Release the key. 3) Unmesh the "load" capacitor about 10 degrees or so 4) Repeat Step 2. The plate current minimum should be higher and there should be more output. 5) Repeat steps 2, 3, and 4 in order until you get maximum rated plate current at the minimum setting of the plate capacitor. They have an alternative way to tune by inserting a mA meter into the key lead but I do not get the little dips in current they are describing. Then there is something wrong with your output circuit. What capacitors and coil are you using? The original design had a 365 pf variable "plate" capacitor and a similar capacitor for the "load" adjustment. The coil was different for each band. What components are you using in the output circuit? Plate tuning capacitor. coil, load capaticor, plate RF choke, coupling capacitor? Any of them could be the cause of your problem. For me it is more of a linear increase in current from the highest capacity setting (lowest current) to the setting where the cap is all open and the current is highest. That says to me that your plate C is too low. The way things are the max output occurs more or less at the lowest anode current of my power supply (about 35mA at 200V B+). Which should happen somewhere in the midrange of the plate capacitor adjustment. Why do I tune for minimum current?? Because that's when the impedance of the load circuit is closest to matching the tube's output. 73 de Jim, N2EY |
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