Re: What type of capacitor for R.F. decoupling ?
You are looking for a capacitor that is still a capacitor at 40MHz  ie that is selfresonant frequency is greater than 40MHz. That means that its inductance has to be less than 16nH, including the leads that connect it to the cathode/screen/heaters. That is a tough ask  2cm of wire, 1mm diameter has about that inductance, so the wires that connect the capacitor can be the dominant inductance.
Above the self resonant frequency it no longer looks like a capacitor, it looks like an inductor.
