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Very interesting project. Great pictures.
I’m curious about the point at which the audio is applied.
As Neil pointed out, high level (plate) modulation would be a better way to go.
However, working with what you have I wonder if screen grid modulation would have less affect on the carrier frequency compared to applying it to the control grid along with the oscillator output.
Depending on the turns ratio of the audio transformer there may or may not be enough audio voltage to modulate the screen grid.
The Lafayette KT-195 I have was designed with screen grid modulation. The modulator plate feeds audio to the screen grid through a series R/C network. B+ is applied to the screen grid in a normal fashion. Simply moving the audio from the control grid to the screen grid might work with attention to the blocking capacitor.
The KT-195 dosen’t have a separate oscillator and final output stage and as such there is some carrier shift with modulation. But where you have separate stages I would think there would be less FM’ing.
I modified the KT-195 for plate modulation and noticed an improvement. You can find the schematics at THIS LINK.