Some info on how we test and match 5654W tubes at Riverstone Audio.
We have tested thousands of the 5654 and 6AK5 family of tubes and have established an internal test protocol for these tubes that allows us to identify the best 5654W tubes for hi-fidelity audio use. We currently (as of May 2017) have over 75,000 NOS 5654W vacuum tubes in our inventory. With this large inventory, we can be very particular about the tubes we select for use in our high quality matched sets.
The 5654 family of pentode tubes (5654W, 5654, 6AK5, 6AK5W, EF95, as well as 6J1 and 6J1P) are characterized by a very close physical spacing between the cathode and grid #1. Because of this close spacing, these tubes have a very high sensitivity to thermal drift (related to thermal expansion of internal tube elements). With such close spacing, the differential thermal expansion between internal structures in the tube can result in transient changes in the electrical response, including thermally-induced changes in plate current, grid current and transconductance - leading to inconsistency and errors in measured parameters. All tubes are first visually inspected followed by initial screening of electrical parameters. Tubes that pass the initial screening are subjected to detailed follow on electrical parameter testing and audio testing. The detailed testing, which includes checking for stability in measured parameters (plate current, screen current and mutual conductance) as well as a life test and audio test, requires approximately 10 minutes per tube (this includes a 200 s to 300 s soak period at a plate voltage of 120V). During the stabilization period, the grid current and screen current is monitored. Next, each tube is subjected to multiple test cycles, which allows us to more carefully examine the stability in the measured plate current and mutual conductance. After testing is completed, tubes are matched as closely as possible into pairs and quads, with a final audio sound test performed on each of the matched sets. For the test protocol we have in place, fewer than 50% of the 5654W tubes we test are selected for use in our matched tube pairs and tube quads.