[ARC5] BC-453 fet BFO

[J Mcvey]

Before I describe this, please know that I don't have much in the way of test equipment at home where I am doing this project.I am taking the bc453 into the shop on Monday and hopefully will be able to set it up for drift tests.
I did build the fet colpitts BFO from some stuff I had here. I worked pretty well on the table but proved problematic inside the BC453.
The first issue was interaction between the tube grid and the oscillator at resonance, I surmise  that the high amplitude ( about 125 vpp) has enough leakage back to the grid to destabilize the fet oscillator. Doing a little back and forth and setting the plate ( original BFO can) a tad off peak resonance  cleared it up.
Second, since the fet runs all the time ( powered by the 24V heater supply) and  there was enough leakage that there was always some residual  BFO tone with the switch off.The coil I used was encased in ferrite material, so I though the leakage would be minuscule, but apparently not minuscule enough.It did seem work better in the "off" mode when the VT could not detune it which brings me to the third observation.
The drift was still pretty bad in the "BFO on" position! However when the fet oscillator was still providing tone via leakage  in the "off mode" the waterfall trace straightened out to a more vertical position.Also, as the set warmed up , the original plate tank adjustment must have drifted to the point where the tube driven BFO/fet combo went into oscillation rendering the BFO in the "on" position useless.
Someone suggested building the fet oscillator iside the BFO can. That may be the most practical solution if:I can't fix the VT BFO drift.I can find a fet with a Vdd of 200V 


 


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