[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?

[mikethedruid]

Without seeing exactly what you are talking about it is hard to say exactly; but, from your description, and the fact that the tube works at all, it sounds like what you are describing if the "getter." The getter is installed in a tube to burn off and consume the very last of the oxygen which might remain inside the tube after the vacuum is drawn. It usually has a tiny amount of mercury and a filament on it, and when the tube is finished being made, a current is run through it which heats up the mercury (or other metal) to high incandescence, vaporizing the filament and allowing it to combine with any remaining oxygen atoms. This is what leaves the silver you see on the inside of the glass envelope. It is a normal part of the manufacturing process.

[mikethedruid]

Here is a video about the manufacturing process for vacuum tubes. They use a different type of getter in it, but it gives a very good idea of the process.
[Video: https://www.youtube.com/watch?v=5WCWejeRR_s]

[falcon123]

and ive learned yet another tube radio detail. i do thank you

[morzh]

And to check the filament all you have to do is to take the Ohmmeter to the pins ##2&7.

[mikethedruid]

Mike, do you remember, when we were kids, they used to sell these chepie "test them yourself" tube testers that basically only checked the filament for continuity? They had a neon bulb and resistor, so the current they passed through any tube wouldn't blow the filament, and if the filament was good the bulb would light up. Beyond that basic test they were worthless, couldn't tell you if the tube was weak or not, so if it was on its last legs, and could never function properly, it still gave a good reading.

[morzh]

Yeah.....but at least it would tell you if the tube should glow or not

To tell the truth even a good MC tester does not guarantee that the tube that tests good will work.
A couple of tubes I bought on eBay for my Mac amp tested good with no gas, but blew the fuse upon warm-up.
I take it the tubes, especially id DC-coupled like it is in unity-gain outputs, they also should be tested for Cathode leakage.
The sound was overwhelming.