Fender Bassman Repair Part 4

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  • Опубліковано 20 січ 2025

КОМЕНТАРІ • 8

  • @markg1051
    @markg1051 2 роки тому

    Re the feedback cap, although there is no DC as you said on the sec of the output transformer, there is on the grid side of the phase inverter.
    This is a pretty high impedance point which bypassing the cap would throw off the phase inverter bias and the whole section would stop working, probably kill the sound altogether.

    • @timkrauseelectronics9245
      @timkrauseelectronics9245  2 роки тому

      That cap comes after the input cap. Where is the DC coming from?

    • @markg1051
      @markg1051 2 роки тому

      @@timkrauseelectronics9245 from the 12AT7 grid, supplied by the 1 Meg resistor attached to it.

    • @timkrauseelectronics9245
      @timkrauseelectronics9245  2 роки тому

      @@markg1051 So obviously it wouldn't affect the phase inverter, since it's coming from the phase inverter. That means we're trying to keep DC from reaching the speaker. Would that tiny amount of DC current really affect the speaker?

    • @markg1051
      @markg1051 2 роки тому

      @@timkrauseelectronics9245 No and yes, the secondary, speaker and the 47k FB resisor would drag the bias voltag on the grid down and alter the bias of the PI which would change its operating point considerably.
      Edit - the "No and yes" should be opposite. No, it would not effect the speaker and yes for the phase inverter.

    • @markg1051
      @markg1051 2 роки тому

      Even with a high ohm voltmeter, typically 10 Meg ohm if measuring the voltage at the junction of grid, 1 Meg and input cap you will load it slightly and not measure the true dc on that point. The best place to measure this bias is at the point where the two 1 Meg resistors attach to the cathode circuit, that way you won't change the operating bias point but you can see any changes if either of the grids are loaded in any way.