[Boatanchors] Colpitts Oscillator Tuning Range Question

[wa9wfa]

Hi, I have a 6C4 tube Colpitts oscillator running on the bench and have a question about oscillator amplitude over a 600 KHz span.  The parts I'm using are similar to this W1DX design.  The oscillator design looks normal and typical to me.  I am looking at the relative amplitude of the unloaded (and maybe this has something to do with it) oscillator and measuring it with a turn or two of wire on the oscilloscope probe placed near the circuit rather than direct connect.

The relative amplitude at 3.945 Mhz is 1.3 volts p-p and decreases to 0.5 Vp-p at 4.565 (the range is needs to work over). Should the amplitude change that much? I would have expected the oscillator output to be relatively consistent over its tuning range. This doesn't seem desirable as the HF oscillator feeds the HF mixer, so the mixer output amplitude at 455 KHz would drop proportionally as I tune higher in frequency. System gain would be X2.6 higher at 3.945 as it would be at 4.565. Do I understand mixer output amplitudes correctly?

I would attach the schematic if allowed.  If you want to see the oscillator schematic, please email and I’ll forward it to you.  

Does anyone have some insight here on amplitude vs frequency? Thanks. 73 Scott WA9WFA