Configuration: My 6L6 with the tank circuit disconnected and instead using the tank circuit from an old Knight T-50 Transmitter (refer to attached pic).Max Output power = 6.7 watts
Frequency varying. On key down the freq would start out at 7043.4 Khz and then drop to 7042.5Khz over a period of 7 seconds. Thereafter the freq would abruptly change back to 7043.4Khz. As the key is kept down the freq would gradually stabilize at 7043.8Khz after a period of 30 seconds. During this 30 seconds the frequency would oscillate slowly around these two max and min points and then settle down and stabilize. If the key is then lifted and then immediately key-on the settle down point is still quicker but still takes time to settle down. Note that the freq varies both up and down. ie not just up or down but both.
Tried moving the RFC in the plate circuit away from the voltage divider for the screen voltage. This had no effect.
Added a different capacitor across the screen voltage divider no difference.
Crystal noted also to get warm. Only gets warm on key down.
However changing the crystal to another one oscillating at 7050Khz does not seem to improve matters.
I blew cold air onto the crystal and it immediately started to oscillate similar to key down and then settled down much the same as the initial key down period. Indicating that temperature change in the crystal is causing the frequency change. I can definitely feel the crystal getting warm. Is there somethng wrong with the biasing of the contol grid? Why is the crystal getting hot?
Is it that the screen voltage is too high? In this circuit the screen acts as the plate for the oscillator. Note that the oscillator is stable when the Anode plate is not loaded up.
Measured Plate voltage Key up = 337volts
Measured Screen voltage Key up = 219 volts
Measured Plate voltage Key down = 319 volts stableMeasured Screen voltage Key down = 197 volts stable and not changing with freq change of the oscillator.
I detuned the tank circuit and thus took the load off the plate. The key down screen voltage is now measured as 202volts. ie a 5 volt difference to the loaded voltage. Frequency is now more stable for sure. Crystal does not appear to heat up.
Question? Why is the crystal also operating so far off frequency? Would expect it to oscillate on 7040Khz yet it is in fact oscillating 3 Khz above the resonant frequency. This must indicate some heavy reactance in parallel with the crystal.
Tried disconnecting the Anode plate voltage and left the screen voltage in place. Result. The oscillator was perfectly stable with excellent shape.
Measured voltages with Anode disconnected.
Screen Voltage Key up = 219volts
Screen Voltage Key down = 173volts
Note that the 173 volts is a full 219 - 173 = 46volts drop.
Compared to the situation with the Plate connected. 219 - 197 = 22 volts drop.
Oscillating frequency now dropped to 7042.69 Khz.
Cathode current with only screen grid connected = 11.77mA
Next I will try a voltage divider of 12.5k / 12.5 k which will drop the screen voltage significantly.
Measured screen voltage key up = 162 volts
Measured screen voltage key down = 119 volts
ie voltage drop = 162 - 119 = 43 volts
Cathode current with only screen grid connected = 7.68mA
Now reconnect the Anode Plate voltage and do the tests again with lower screen voltage.
Frequency now stable, however the power output has dropped as would be expected.
Max power output now 4 watts (as opposed to with higher screen voltage). However significant chirp at this point. Although the freq is basically stable. Indicating room for improvement on chirp. Power needs to be backed off to abou 3 watts to reduce chirp.
Power = 4 watts
Key up anode voltage = 339volts
Key down anode voltage = 325volts
Key up screen voltage = 162volts
Key down screen voltage = 146 volts
Cathode current keydown = 32.8mA
Screen current keydown (Anode disconnected) = 8mA
Power input = 325*32.8mA = 10.66 watts
Efficiency = (4/10.66)*100 = 37.5%. I reckon I should be able to do better.
Conclusion must be to try to find a crystal that can better stand a higher voltage.
Next consider adjusting the bias on the control grid.
No comments:
Post a Comment