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  • OK Then

    Champ 5F1 circuit built for 1 watt output, duly notated inside the cabinet and inside the chassis by the power tube socket. Tested and verified that indeed it produces exactly 1 watt into reactive load. (2Vrms into 4 ohms)

    I'd like to restore it back to stock but don't know where to start. Here is the relevant info:

    >Mercury FTCP-59 power transformer
    330-0-330, 6.3v no CT, 5v
    >Mercury Tone Clone FTCHO-5 output transformer
    single 4 ohm tap

    >Rectifier is a good, strong Ken Rad 5Y3
    315 VAC from the PT to rectifier (loaded)
    350 VDC out to first filter cap
    313 VDC at second cap
    273 VDC at third cap

    >6V6 Sylvania 6V6GT
    Pin 3 - 338.5 V
    Pin 4 - 313.8 V
    Pin 8 - 18.5 V

    >12AX7 RCA longplate
    Pin 1 - 163.7 V
    Pin 3 - 1.1 V
    Pin 6 - 183.1 V
    Pin 8 - 1.3 V

    Filaments are getting 6.7 VAC on each tube. Trying to figure out where the builder reduced output power.
    Attached Files
    Last edited by Tone Meister; 01-30-2019, 04:33 AM.

  • #2
    There's only a handful of components in that thing. I'd just start left to right on the eyelet board and see what doesn't match the factory layout. Shouldn't take long at all.
    "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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    • #3
      Verify that R8 is indeed 470 Ohm. Other than that I would need to verify the primary impedance of the output transformer. I think the stock OT has a 5K primary.
      WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
      REMEMBER: Everybody knows that smokin' ain't allowed in school !

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      • #4
        What's the value of the NFB resistor?

        Does your amp match one of the schematics posted? If so, which one? (they differ by a cathode bypass resistor on stage 1)
        If it still won't get loud enough, it's probably broken. - Steve Conner
        If the thing works, stop fixing it. - Enzo
        We need more chaos in music, in art... I'm here to make it. - Justin Thomas
        MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

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        • #5
          Originally posted by The Dude View Post
          There's only a handful of components in that thing. I'd just start left to right on the eyelet board and see what doesn't match the factory layout. Shouldn't take long at all.
          Yeah, but it's not on a board, it's all point to point and the work is very sloppy. I did find a 220K from the 6V6 grid to ground that I'm wondering about. Wouldn't that limit curremt draw on the tube?

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          • #6
            Originally posted by Tone Meister View Post
            Yeah, but it's not on a board, it's all point to point and the work is very sloppy. I did find a 220K from the 6V6 grid to ground that I'm wondering about. Wouldn't that limit curremt draw on the tube?
            That is the grid leak.

            It's supposed to be there.
            If it ain't broke I'll fix it until it is...
            I have just enough knowledge to be dangerous...

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            • #7
              Originally posted by eschertron View Post
              What's the value of the NFB resistor?

              Does your amp match one of the schematics posted? If so, which one? (they differ by a cathode bypass resistor on stage 1)
              The V1 bypass resistor is 1K in this amp, not 1.5K. Also, the NFR is 1.4K and not 22K. R8 is indeed 470R.

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              • #8
                The other thing to look for would be series or parallel high wattage resistors in the OT secondary. It could just have built in power soak.

                Edit: I was typing during your last post. If the NFB resistor is 1.4K instead of 22K, you'll need to change that. That's a lot of additional NFB.
                "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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                • #9
                  Originally posted by The Dude View Post
                  The other thing to look for would be series or parallel high wattage resistors in the OT secondary. It could just have built in power soak.
                  What is the value of the volume pot?

                  nosaj
                  soldering stuff that's broken, breaking stuff that works, Yeah!

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                  • #10
                    If the NFB resistor is 1.4k (and where did we get that odd value?) instead of 22k, then there is a tremendous increase in NFB signal. That will slow down a power stage. Just disconnect that resistor and see if the output grows.
                    Education is what you're left with after you have forgotten what you have learned.

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                    • #11
                      Just looked closely. No power soak resistor(s)

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                      • #12
                        Originally posted by Enzo View Post
                        If the NFB resistor is 1.4k (and where did we get that odd value?) instead of 22k, then there is a tremendous increase in NFB signal. That will slow down a power stage. Just disconnect that resistor and see if the output grows.
                        I measured it. I was thinking that measuring 1.4K is better than relying on the stripes for actual value, at least in this case. After all, we have a (deliberate) reduction in output power. I'll disconnect that NFR and report back.

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                        • #13
                          Peeled back the heatshrink and the NFR is 22K. Disconnexted it measures ok.

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                          • #14
                            If you upload a good pic or two, maybe somebody will spot something.
                            "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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                            • #15
                              Originally posted by The Dude View Post
                              If you upload a good pic or two, maybe somebody will spot something.
                              You asked for it, ha ha.

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