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Thread: Mesa Boogie F30 - Phase Inverter drawing voltages down

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    Mesa Boogie F30 - Phase Inverter drawing voltages down

    Hey guys,

    I've got a Mesa F30 that's got me stuck. Redplating EL84s and low voltages all across the board. Here's my readings at the phase inverter and the output tubes:

    V4 1-178 6-178
    2- 0 7 - 0
    3 - 2vdc 8 - 2vdc

    EL84
    2 - 1.6
    7 - 290
    9 - 187

    Now with the phase inverter removed, my voltages are more in line with where they should be

    V4 1 - 340 6 - 340
    2 - 0 7 - 0
    3 - -19vdc 8 - -19vdc

    EL84
    2 - -15
    7 - 398
    9 - 349

    The one thing that's jumping out at me is the 0vdc on the grid. Shouldn't I be seeing some voltage there? I've checked all of the resistors and coupling caps in the output section, as well as tested and swapped the tubes

    F_30.pdf

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    Supporting Member eschertron's Avatar
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    Not sure why the phase inverter would be messing with the bias (cause of the red plating) unless the coupling caps were bad.

    Well, now that I look at the bias, point E (PI cathode bias) comes off right near point F (power tube grid bias). Maybe something there. Does a different tube in the PI fix things?

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    Supporting Member mozz's Avatar
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    Feed to Pin 2 of the output tubes should be -15, not 1.6. If not coupling caps, you have power supply problems.

    Edit to clarify.

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    Last edited by mozz; 02-14-2019 at 01:49 AM.

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    Quote Originally Posted by mozz View Post
    Pin 2 of the output tubes should be -15, not 1.6. If not coupling caps, you have power supply problems.
    Schematic says : -11V.

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    The first thing that struck me was the coupling caps, which I've heard were a problem in these amps. I swapped them for Orange Drops but it didn't cure the issue. I've tried different tubes in the phase inverter with no difference. The -15 on the EL84s is there without the phase inverter, but goes to 1.6vdc with it

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    Not sure why the phase inverter would be messing with the bias (cause of the red plating) unless the coupling caps were bad.
    With this special PI circuit and bias supply, the PI current influences the power tube bias voltage.

    I recommend to check the voltage at point E for +3V. If higher there is too much PI current. Does it change in mute mode?

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    Last edited by Helmholtz; 02-13-2019 at 10:53 PM.
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    Don't forget the joker g1's Avatar
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    Have a look at this, different model but same bias arrangement:
    https://music-electronics-forum.com/...l=1#post518627

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    Have a look at this, different model but same bias arrangement:
    https://music-electronics-forum.com/...l=1#post518627
    Amazing. It's a conductive board. How he figured that out is beyond me, but i'm glad he did. I've currently got the bias/cathode circuit floating and everything is right where it should be. Thanks for the help!

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