They come from different sources. Some are new but the original boxes were destroyed. We are constantly adding to this list. If you don't see a tube listed please Email or call. These tubes are tested and meet or exceed the specifications for that particular tube.
Every effort is made to insure that weak or low gain tubes are discarded.
We always Test these Tubes before shipping them to you. Tubes are being tested and added to stock everyday. If you don't see a tube listed please Email or call. All the tubes we sell are tested prior to shipping, even though the suppliers test them, we retest them again. This Model 707 measures Dynamic Mutual Conductance.