[MrFixr55]

Try this.

You always need a cap across the pentode to snub parasitic oscillations. Try a 0.005 uF cap across the output.

You can then add the usual tone control circuit in the 1st audio stage by replacing the tone switch with a tone potentiometer with the center pin to chassis ground and a 0.005 uf cap between the plate of the 1st audio stage and the full CW pin of the pot. This will allow you continuous treble attenuation.