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Fisher tube amp distortion

Discussion in 'Troubleshooting and Repair' started by KilgoreCemetery, Sep 13, 2021.

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  1. KilgoreCemetery


    Apr 12, 2017
    I'm trying to rebuild a Fisher 400-CX tube amp, but I'm having some trouble with the signal while under load.

    What I've done so far:
    Replaced all of the electrolytic/film/mylar capacitors
    Replaced some resistors that had drifted beyond their tolerance
    Replaced the original rectifier diodes with UF4004's
    Cleaned the controls with deoxit
    Checked all power supply voltages and heater voltages. They are a bit higher than what the schematic shows, but since my AC is closer to 120v than the rated 110v from when this amp was made, it's about what I'd expect.

    If I inject a 1kHz sine wave and look at the output on my oscilloscope, it looks fine. However, if I connect a couple of 8 ohm/100W resistors (or a pair of speakers), the bottom half of the sine wave flattens as soon as you turn it up past 1/4 turn on the volume knob. If it were a solid state device, I'd say that there is a problem with the negative half of the driver stage, or the power supply that feeds it. In this case, it affects both channels equally.

    I get a clean sine wave all the way up to the very last stage. The sine wave is clean going into pin 2 of the second half of a 7247 tube, but comes out on pin 3 as distorted. I've double-checked the resistors around it and tried disconnecting the Center Channel, but they were all within tolerance and it made no difference.
    Output tube voltages.jpg

    Attached Files:

  2. Ylli


    Jun 19, 2018
    That's a preamp - it is not designed to drive speakers.
    KilgoreCemetery likes this.
  3. KilgoreCemetery


    Apr 12, 2017
    Where did all this egg come from and why is it on my face?

    Looking back, the signs were all there. I just missed them. Thank you, Ylli
    Harald Kapp likes this.
  4. Audioguru


    Sep 24, 2016
    Antique toob amplifiers used a huge output transformer for each channel that this preamp does not have.
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