[falcon123]

ive got a 7f8 tube thats a bit different than the other used 7f8's i have, yet seems to work the best   of all of them with regard to sensitivity. the odd thing is i think its got a broken filament. on pin 2 which is a heater pin,there appears to be what i'd liken it to as 'BASKET BALL HOOP". IT sticks straight up about 3/4 of the way up the glass envelop. the tube has a nickel plated or stainless base and im guessing its sylvania. all the 7f8's i have,have this though most are square not round hoops and the hoops are closed by what looks like a wound filament. the round one appears to have endured an arrested landing at 160 knots as the hoop is distorted  and the filament is open. it works just fine but as i aligned the if with it  im sure theres a reason why nos 7f8's dont work as well. (i know about scraping the pins) the hoop isnt shown in the tube data sheet and ive no idea what it does or what its for, can anyone explain what this does and how it being broken effects the operation?