I monitor the output signal on my scope and notice that right after key down
the waveform contract just a bit and the tone changes pitch.

Uwe-

I can think of two reasons for the "chirp".

You did not mention your receiver. It is possible that the strong signal from
your nearby transmitter causes the receiver to change pitch.

I assume your transmitter is crystal controlled, since that is similar to the
6V6 homebrew transmitter I started with back in 1955. By increasing voltage,
the crystal current increases. There is an effect due to heating of the
crystal that might cause a chirp.

In the case of my old transmitter, I found that I could get more power output
by changing the 6V6 to a 6L6, which has the same base diagram. However, one
thing that happened along the way was that crystal current increased to the
point that one of my crystals fractured and stopped working.

Looking back, I realize that the increase in power by changing tubes and
increasing voltage, may not have made a significant difference. If you double
your power, the received signal only goes up half an S-Unit. Improving your
antenna can make a bigger difference!

I don't know about the waveform contraction. I doubt it is from overload of
the scope, but it might be due to the tube being a little "soft" due to low
cathode emission in the 6V6.

73, Fred, K4DII