Design Error 2 revealed

Hi to all who tried,

Dimitrij got close, posting the exact scenario but not the upshot.

" Mess up something with acoustic feedback so that the amp squeals. Panic and flip the standby switch to kill the squealing."

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  1. When the"Standby Switch" (s/b) in the B+ supply opens, the amp does not suddenly stop, it continues to operate with a new topology.

  1. The new topology includes a 2H inductor in series with the output tranny so at mid to high audio frequencies it becomes the dominant load with a very high impedance.

  2. The role of the output tranny becomes redundant and if the amp is being driven hard at the time - a large AC voltage appears across the 2H choke.

  1. Operating the s/b with full drive at 5kHz produces about 5kV from the plates of both 6L6GC tubes to ground.

  2. 5kV AC is enough to break down plastic insulation between pins 2 and 3 ( heater & plate ) of the 6L6 tubes, their octal sockets or even inside the output tranny.

  1. Far as the guitarist knows, nothing is wrong UNTIL the s/b is flicked back on and the supply fuse blows because there is now a little carbon bridging the insulation.

The s/b switch is in the wrong place.

.... Phil

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Phil Allison
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