We adopted the habit of checking high-power glass tubes for gas before applying high voltage to them in an amplifier. The vacuum can degrade over decades on the shelf. If it does, even a tiny amount of gas in the vacuum can break down and cause an arc inside the tube. Generally not a nice thing to do to your power supply, or to the front-panel meters. Those meters are hard and expensive to replace, and there is NO protection circuit for them in the original design. Adding protection diodes is a good way to avoid expensive grief from blown meters.
Most folks won't have a high-voltage breakdown tester to perform this test. Applying a few thousand volts between the grid and cathode will reveal any gas contaminating the vacuum. The tube should show little or no leakage current up to around 3000 Volts with the cathode cold.
The gray coating on the tube's anode surface is a so-called "active getter". When the tube's anode gets hot enough, any gas molecules that get close enough to that surface become chemically bonded to it. Keeps the vacuum 'scavenged' of stray gas. But you have to operate the tube at a normal temperature to 'clean' the vacuum this way. A long shelf visit can allow tiny traces of gas trapped in the tube's materials to enter the vacuum and create this problem.
Conventional wisdom recommends running old tubes with just the filament, and no high voltage for a few hours before trying to operate it at normal voltages. Simply disconnecting the plate cap will accomplish this in a SB-220. Just use EXTREME caution, and make sure the plate-cap connectors can't brush against anything else when they're disconnected from the tube's plate terminals.
This amplifier CAN and WILL kill you if you fail to observe proper procedures.
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