can you see the shape of the broken off key on the bottom of the tube?
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Thanks vinylzone and millercarbon, on rca vt231 you can see where key may have been on the other tube not so much. If my memory is right I think my Mullard fx2's have numbers but none of the 6sn7's are numbered. I even pulled out some matching tubes but there is no consistency in orientation of keys and plates. I may have to junk the Sylvania black glass if there is no way else to tell. Any other ideas, thanks for trying.
If you have the original key from that tube it should align to the tube like a puzzle. IOW, there should only be one way to attach it back. At least that was the way it was with my KT88 that had the same damage. However I can see that the 6sn7 would be more challenging. Mine are all stamped the same BUT they are new not NOS. If you cant be sure the its cheaper to chuck it than repair the amp from a defective tube
I've done this.
With strong light, magnifying glass, careful look by young eyes, see where the positioning tab was broken off the tube center post. Scratch a vertical line in the tube base where the tab was. Line up the scratch with the key slot in the tube socket, insert.
The whole center post snapped off? Still have the piece? Figure out where the tab would be if the broken piece was re-attached, scratch the outside of the base like above.
You can use a keyway replacement from Tube Depot,
The filament pins of 6sn7 are 7 and 8, you can use a DMM ( digital multimeter ) to identify the filament pins by using ohm meter, other pins will show open circuit except pin 7 and pin 8 have resistance reading.