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? Maybe its to protected when 60/100w switch is thrown. Thinking that it failed by shorting then burned itself open. Found a Mark II picture randomly on the internet with exact same diode configuration, same brand, same installation technique ...from the factory. It appears as if a model conversion or 2 have been attempted prior to me with smoke and fire as a result.