SOB HOB 29 V8/V9 diode

Discussion in 'Amps and Cabs' started by Boogie-mun, May 10, 2015.

  1. Boogie-mun

    Boogie-mun Member

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    May 10, 2015
    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.
     
    Last edited: May 10, 2015
  2. Boogie-mun

    Boogie-mun Member

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    May 10, 2015
    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. Will install bigger diodes, can't hurt.
     
    Last edited: May 10, 2015

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