On larger plate supplies the soft start circuit used can make a big difference in the level of protection offered. While there are countless ways to design a soft start circuit, I see them all falling into two basic categories. Time based and voltage based to control the delay between soft start and full on. The vast majority are time based. Instantly powering the primary trough a wire wound resistor usually around 100 ohms. It then shorts the resistor through a second contactor after some pre-determined amount of time to apply full power.
Here is a great opportunity to add protection beyond just the rectifiers and filter caps. This is especially useful when building a new amplifier or testing a tube of unknown condition. On the typical soft start if we have a short anywhere on the HV line, it's still going to apply full power after about a second. If we trigger the contactor that shorts the soft start resistor from the voltage across the primary of the plate transformer, we added more protection.
Since the voltage across the primary is being fed through the 100 ohm resistor, it will take time for the primary to reach 220 volts as the secondary charges the filters through the rectifiers. You can use a small relay with a 220 volt coil connected the primary to fire a larger mercury contactor that shorts the soft start. By adding a resistor to the relay coil or tightening the tension on the spring of the 220 volt relay you can add more delay if needed.
Now if there is a short on the HV line on power up, the primary voltage will be forced across the 100 ohm soft start resistor. This will hold the primary voltage low, keeping the 220 volt trigger relay de-energized and prevent the full power from ever being applied into the short. Adding an appropriate slow blow fuse in series with the 100 ohm soft start resistor will remove all power and save the resistor.
With the voltage triggered soft start you can have a shorted tube installed on power up and still protect rectifiers, filter caps, current meters, glitch resistor, meter shunts, and the plate choke just to name a few. The standard soft start would fail to protect all of these components in a case like this. Something to seriously consider if your plate supply is big enough to cause this damage.