I am working on GF-11 transmitter restoration and having problems with
CW keying (ON-OFF keying). The original design has provisions for MCW
mode, but not for ON-OFF keying.

I tried various keying tricks with control grid bias and screen grid
bias keying in output stage (pair of 837 tubes in push-pull), and the
net result is small frequency shift of about 100Hz.
The original circuit does not have a buffering stage from oscillator
output to PA input, and anything I do in PA stage changes the loading
for the oscillator output, which in turn shifts the frequency of the
oscillator circuit.
Any suggestions?

I also have one, or possibly two spare plug-in coils for GF-11 Tx, if
somebody needs.

73,
Vlad, kb9olm (email: kb9olm AT yahoo DOT com).