A long long time ago, far away in another galaxy, I remenber as an apprentice building an amplifier with our Apprentice Master, Mr. James Plaster (ex Sgt. REME) of sacred memory, whom I truly feared. He was a fierce, red faced man, much given to explosions of anger: "Drilled a bleedin' hole through your hand, boy? You won't be doing that again in a hurry!". He departed this life over 45 years ago. We had trouble with harmonics of unknown origin creating noise, which spoiled the performance. The Master said we could filter out the harmonics and we built a device with a shunt coil in series ( ? I think?) with some condensers. If I remember it right, this had to be 'tuned' so as the impedance of the unit was zero at the ripple frequency of the harmonic, and this would 'short out' the high frequency volts. I think we put it across the supply. Mr. P worked out the number of turns with log tables. I only remember the incident because some cretin electrocuted me with with one of the 500v paper condensers wired to my locker as a 'joke'. I literally 'flew' across the shop like Peter Pan. The amp used thermionic-valves of course, none of them useless, new-fangled transistors for Mr.P! The harmonic was measured by the electronics guy at, I'm sure it was, 6 times 50hz = 300hz and there were others far higher than that but faint. Anyway, I moved on to the machine shop soon after, and Mr. P threw one too many wobblies and 'popped his clogs' with a H.A. I'd forgotten it all until the 'harmonic' post appeared. Feel free to shoot me down if I remembered it all wrong.
Alan
