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Fender Twin of the ProTube (sometimes called the Evil Twin) with very low output

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  • Fender Twin of the ProTube (sometimes called the Evil Twin) with very low output

    Hi gurus.

    This is my first post…and I’m hoping you can help me out. This problem has me stumped. The amp is a Fender Twin of the ProTube era of 2003, sometimes called the Evil Twin. Here’s the schematic from Ampwares: http://ampwares.com/schematics/Twin_...Amp_SchE45.pdf .

    The amp's volume is very low, (less than a Champ). I have put a sinewave/scope on it and the signal looks all OK until the output of V5 to going to Preamp Out. The voltage at the test point at the plate of V5B is a little low (0.9Vac instead of 2.3Vac) but at the voltage at the junction of the Zener D8 and R79 is real low (4-6mV), so almost nothing is going into the PI, V4.

    I am suspecting the JFET Q5 as slamming the output of V5 to ground. The test point at the collector of Q4 is right on -16vdc. This Q4/Q5 circuit must be a protection of some kind, but I can’t see what it’s meant to be doing. Any clues, my guru comrades?

    Any suggestions for what else could be causing this very low volume ? All tubes tested OK. Voltages on power tubes all measured good.
    I am dreading the prospect of removing that main PCB. It’s going to be an hour minimum to remove the board and front panel. To test it, will mean reassembly, and perhaps the need to remove again. The customer will freak when he gets the invoice.

    The Tremolo was also not working, and with Enzo’s help I sorted that out pretty quick. I’m hoping I can impose you’re your combined experience again.

    Thanks guys

  • #2
    Originally posted by christarak View Post
    I am suspecting the JFET Q5 as slamming the output of V5 to ground. The test point at the collector of Q4 is right on -16vdc. This Q4/Q5 circuit must be a protection of some kind, but I can’t see what it’s meant to be doing. Any clues
    Welcome to the forum. It looks like it's protecting the pre-amp out from switch on spikes. Can't you de-solder the FET with braid and push it out from the component side?

    Comment


    • #3
      Also, when you have the board removed, I think it should be possible to have the boards connected and running without having to completely re-install them?
      Originally posted by Enzo
      I have a sign in my shop that says, "Never think up reasons not to check something."


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      • #4
        Had the same problem recently with a "The Twin"

        Those switch muting FETs were bad like yours probably.
        Amp will work for testing purposes without them.

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        • #5
          Thanks Dave. I have done this with a couple of other resistors with this amp, including the Tremolo not working. I was concerned about soldering from above when the solder pads are as close as this, just in case an unseen blob of solder grows underneath and shorts. On the other hand, it would be no worse than what's happening now.

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          • #6
            It is such a shame Drewl that Fender made this service-unfriendly amp layout. Did you find a better substitute FET in Q5? Tank you for taking the time to answer.

            Comment


            • #7
              Originally posted by g1 View Post
              Also, when you have the board removed, I think it should be possible to have the boards connected and running without having to completely re-install them?
              Thanks g1. I'll try it from above first. If not, I'll try it out. Thank you for your suggestion, mate. Appreciated.

              Comment


              • #8
                Success! It was indeed the J113 Q5 JFET. Thank you all for your advice.

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