[Radioroslyn]

Let me rephrase what I posted before. The problem as I see it is that your eye tube's grid need to be at 0 volt to be fully closed and 160v fully open. That is a range of 160v. What you have is a range of 82v (78-160v). You are starting out with the tube 1/2 open to start out.

You can't drop the SG voltage on the 6K7 to 0 because no matter how much voltage you put on the CG I don't think you can cause the SG to pull enough current to drop the SG voltage down to any thing close to 0. Even if you could then you would have an IF amp that would have very poor gain.

Originally this tube was designed to be used with it's companion the 6AE6 to ampfile the avc voltage and control the grid voltage for the 6AF6, but SC decided not use it nor have an avc amp circuit so there would be enough voltage swing to cause the full operation of the eye opening and closing
https://frank.pocnet.net/sheets/127/6/6AE6G.pdf

By dropping the plate voltage all that will do is give you a dimmer glow from the tube not changing the angle of deflection.

Terry