01-11-2013, 08:51 AM
Here's an interesting issue:
In checking the LO, I found that even on the lower bands, the output waveform is not good. It's the ugliest cathode follower I've ever seen.
Looking at the actual oscillations at the plate, waveform shape is a nearly perfect sine. However, looking at the cathode follower, it's more like a half wave rectified signal. All but the top 10% or so of the lower half of the waveform is not there. But it's not a clean clip, it's more like a slight upward slant at the bottom of the waveform.
I can't find or figure any reason for this, other than a bad tube. It speaks to inadequate bias, but the voltages are very close to spec. I've replaced the 500 ohm wirewound in the cathode circuit with a 500 ohm flameproof, but no difference. Checking other components in the circuit I found no problems.
All of this, added to the drop off on the lower end of the high bands, I can only think of the tube, but perhaps there's something in what seems to be a very simple circuit that I'm missing.
In checking the LO, I found that even on the lower bands, the output waveform is not good. It's the ugliest cathode follower I've ever seen.
Looking at the actual oscillations at the plate, waveform shape is a nearly perfect sine. However, looking at the cathode follower, it's more like a half wave rectified signal. All but the top 10% or so of the lower half of the waveform is not there. But it's not a clean clip, it's more like a slight upward slant at the bottom of the waveform.
I can't find or figure any reason for this, other than a bad tube. It speaks to inadequate bias, but the voltages are very close to spec. I've replaced the 500 ohm wirewound in the cathode circuit with a 500 ohm flameproof, but no difference. Checking other components in the circuit I found no problems.
All of this, added to the drop off on the lower end of the high bands, I can only think of the tube, but perhaps there's something in what seems to be a very simple circuit that I'm missing.
