Back in the 'good ole days' of AM, I seem to remember that you never went above 1000pF, even at 160, because it bypassed the audio signal applied to the plate for modulation. I happen to have a 1200pF, 10kV, 10kVA rated capacitor set aside for my next amplifier: it came with the right price tag (nothing!). 73 Peter G3RZP