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Akai M7 tape machine to Mic pre

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  • Akai M7 tape machine to Mic pre

    Can someone please explain to me what the EF86 tube is amplifying? I'm confused about this. As of yet, I've plugged a mic into the thing and the signal is so low I cannot hear it, at all. It is bypassing the EF86 and entering at the 12ax7 grid. However if I plug my phone into it and play some music it works great. I'm guessing the EF86 is necessary for amplifying the low impedance mic signal to a reasonable level. I just don't get what's going on with the grid of the EF86. Any help appreciated!
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  • #2
    When nothing is plugged into J3, the grid of EF86 gets signal from Pg1 via sw1. What does Pg1 connect to?
    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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    • #3
      J3 tells us. PG1 must go to the heads. J3 says head output, and the signal in the shielded lead from PG1 heads right to it. (Pun intended)

      Note for the MIC input to work, the R/P switch has to be in the record position, the schematic shows it in playback. And lookee! With the switch in record, the MIC jack is connectd directly to that 12AX7 grid. Is that what you did?

      The grid of the EF86 is wired to that head signal. The head signal jack is wired as an output, not an input, though you could rewire the jack as input. But that head amp circuit is only passed on through the amp with the R/P in play mode. So to use both, we'd need to eliminate the R/P and wire across the switches needed to get everything we want working at the same time, and not others.

      Pull V4, that is the bias oscillator, and will do nothing good for you. get rid of it.
      Education is what you're left with after you have forgotten what you have learned.

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      • #4
        Originally posted by Enzo View Post
        J3 tells us. PG1 must go to the heads. J3 says head output, and the signal in the shielded lead from PG1 heads right to it. (Pun intended)

        Note for the MIC input to work, the R/P switch has to be in the record position, the schematic shows it in playback. And lookee! With the switch in record, the MIC jack is connectd directly to that 12AX7 grid. Is that what you did?
        Yes I hardwired the Mic input to V2's grid. I also hardwired the 1M after the coupling caps of V3 (pentode output tube) to the headphone output jack. This caused oscillation cause now the output of the amp is directly wired to the input of the EF86. I think I'll ditch the shorting jack for the headphone out and use a simple mono jack. Then I will add a switch that takes the EF86 in and out of circuit. Any drawbacks to doing this that you can see?

        I need the thing to work regardless of R/P mode so I need to make sure the SW1 tripe DPDT switch is replaced or rewired.

        Originally posted by Enzo View Post
        Pull V4, that is the bias oscillator, and will do nothing good for you. get rid of it.
        Will do

        So to clear the EF86 issue up, it is there to amplify the playback signal from the tape?
        Last edited by lowell; 04-15-2016, 02:39 PM.

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        • #5
          yes.

          eliminate the R/P and wire across the switches needed to get everything we want working at the same time, and not others.
          As I said.
          Education is what you're left with after you have forgotten what you have learned.

          Comment

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