6L6 diode across pin-3 to pin-8, why?

I have an SOB HOB 29 head with a charred socket pin-8 on a 6L6. There is a diode across plate pin3 to ground via pin8 on 2 of 4 6L6's, but its not shown on any schematics. One was determined to be open with pin8 badly charred. What is the purpose of this and is it necessary? What is a safe replacement size, had a VG10? Thinking that it failed by shorting then burned itself open. Found a Mrk II picture randomly on the internet with exact same diode configuration, same brand, same installation technique ...from the factory.

The diodes are on 6L6 V8 & V9, 2 of 4 tubes NOT disabled by the 60/100w switch that abruptly disconnects the cathode from ground on V6 & V7. The transformer will de-stabilize with the abrupt change in current when switching and may induce a voltage. The diode lets it bleed back to B+ at the plate pin to safely dispose of it.

Yes. It is called ringing and I have seen other amps with diodes off of the primary to dampen it. Music Man comes to mind.....
They also offer some level of protection if the amp is accidentally operated without a speaker load.