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Fender Reverb Head Scratcher

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  • Fender Reverb Head Scratcher

    I'm working on a 135 watt Twin Reverb. I recapped it and installed a preamp out jack
    as per the customers request. Also the reverb wasn't working.

    Troubleshooting the reverb.

    I can hear the crash if I shake the tank but, otherwise the reveb isn't working.

    I tried another known good tank, different cables and swapped the tube. I clipped in a known good transformer. Nothing brought the reverb back. I tried injecting signal at the grid of the reverb driver tube and heard nothing except the local spanish station which didn't get louder if I turned up the reverb. All voltages around the driver tube were close to spec. I measured a 5 volt drop across the cathode resistor. Around 16 volts across the reverb transformer primary. I tried clipping a 500pf cap across the one that couples the output of the channel to the driver grids. I checked continuity between the caps back to the plate of the preceding stage.

    When I checked the voltage at the driver plates I could hear a pop and reverb if I turned the reverb up and touched the voltage probe to the plate, the reverb got louder.

    I think the transformer and tank are OK. The driver tube seems to be conducting. But signal isn't getting to the grid of the tube...although if I touch the grid there is no pop, just the mexican music that doesn't get louder if I turn up the reverb.

    I'm stumped. Any help is appreciated. Hope that all made sense, I'm tired.

  • #2
    Not an expert opinion by any means, just a guess...
    "Around 16 volts across the reverb transformer primary" I don't know the circuit, but if it's a single ended circuit like most Fender reverbs then isn't 16 volts far too little?
    Second thing would be to change the cable going from the transformer to the tank.

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    • #3
      Have you checked the driver tubes resistors? I once had a bad cathode resistor and it made just the popping noise you're talking about. Changed the resistor and reverb was back. Just an idea.
      See this thread: http://music-electronics-forum.com/t6302/

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      • #4
        Yeah. I measured it's resistance from the cathode to ground. Also it's voltage drop.

        Comment


        • #5
          I had the EXACT same problem with the EXACT same amp a month ago! As it turned out, the 500pF cap that feeds the input of the reverb drive tube (V3) had a broken lead that I couldn't readily see. I went through the EXACT same troubleshooting procedure you did, and ended up with the same level of frustration. Hopefully, this is your problem.
          John R. Frondelli
          dBm Pro Audio Services, New York, NY

          "Mediocre is the new 'Good' "

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          • #6
            I must know if you ended up having the exact same problem, i had a very similiar problem a few months ago as you two.
            Guitar amplifier repairs at AudioWorks
            713-89-Fix-It (893-4948)
            http://www.audioworksrepairs.com

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            • #7
              Originally posted by jrfrond View Post
              I had the EXACT same problem with the EXACT same amp a month ago! As it turned out, the 500pF cap that feeds the input of the reverb drive tube (V3) had a broken lead that I couldn't readily see. I went through the EXACT same troubleshooting procedure you did, and ended up with the same level of frustration. Hopefully, this is your problem.
              Thanks,

              I did try clipping another cap across that one.

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              • #8
                The only things that you haven't checked are the driver tube socket and wiring.

                If you touch the other end of the grid wire at the fiberboard, do you get the radio signal?

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                • #9
                  Yep.

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                  • #10
                    Is the pan input transducer open?
                    Building a better world (one tube amp at a time)

                    "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

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                    • #11
                      Nope.

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                      • #12
                        Looking back over your first post, I just saw the bit where you say you've only got 16V across the RT primary. You need about 400ish+
                        Building a better world (one tube amp at a time)

                        "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

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                        • #13
                          Sorry. I meant the voltage drop across the primary.

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                          • #14
                            Okay - so what are the idle voltages?
                            Building a better world (one tube amp at a time)

                            "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

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                            • #15
                              Right around what's shown in the schematic. A bit higher.

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