Re: Spark plugs on old Vauxhalls
"Apart from the pretty blue haze inside the valve - you could also produce Amplitude Modulation on the transmission by talking (shouting?) at the valve."
Proper valve designs had spring loaded metal retaining caps. They did pretty much stop microphony.
But it was a known problem of poor quality EF86 pentodes with no caps, where the grids could wobble, especially if someone moved the amplifier.
Biting the hand that feeds IT
