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Fender Twin Reverb Reissue distorted reverb

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  • #1

    Fender Twin Reverb Reissue distorted reverb

    This one is a doozy.....

    The situation/problem:
    -The volume on amp is on at least 3, perhaps 4.
    -treb/mid/low are on "normal" levels.
    -volume on the guitar (telecaster with custom "vintage/hotter" pickups) is full on
    -no footswitch plugged in
    -no pedals used, guitar straight into vibrato channel
    -start to bring up the reverb knob from 1, end up around 4. Sounds perfect initially, but after a few minutes of playing heavy chords on E and A (therefore, most signal voltage entering amp), the reverb will start to heavily distort/major fuzz (electrical, not muff). there can be light or major popping depending on if I ease into said process or slam into it. If the volume on the guitar is taken down a little bit before it goes to hell. the problem doesnt really occur, ie once the problem happen it stays that way.
    -i can turn the amp off and let it cool down, and repeat this process as much as i want.

    Parts that have been changed:
    -ALL tubes are new and of very good quality
    -reverb transformer replaced with merc magnetics
    -C14, C15, R30, R31, R74
    -pan visually inspected, small green/black wires tugged on. it all looks good inside it.

    metering other resistors/caps is somewhat futile while they are in the circuit. i have not removed the pcb from the chassis to look for bad soldering.

    What in the heck is going on in there? anybody smarter than I have any other parts to look at? usual suspects? please help!!
    Tags: fender, reverb, twin
  • #2
    Was the problem happening prior to tube replacement???
    Does anything happen to the dry signal as well when the problem occures??? That is, if you turn down the reverb there is nothing wrong with the dry signal.

    I'm thinking perhaps the driver tube itself or the driver tube bias circuit.

    If you have a scope you can check to see if the distortion is pre tank or post tank or recovery. That would be good info to have.

    Have you taken voltage readings??? May not work since the problem goes away when you remove signal. But you should do it anyway for common trouble shooting sake.
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "If you're not interested in opinions and the experience of others, why even start a thread?
    You can't just expect consent." Helmholtz

    Comment

    • #3
      Agree, the driver tube bias seems a likely suspect. Monitor V3 cathode voltage and try to replicate the problem; may need to put chassis back in cab, in order for things to heat up as normal. Pete.
      My band:- http://www.youtube.com/user/RedwingBand

      Comment

      • #4
        The problem definitely happened before changing the tubes. The tubes that were in there were probably close to 10 years old and were stock originals. I replaced v3 with a Jan 12at7wc. I would say the degree of distortion lessened after changing the tubes, but that doesn't surprise me. The dry signal is flawless, once I turn the reverb back down to 1. Don't have a scope, wouldn't know how to use it. Where should I meter for cathode voltage? The sound of the distortion makes want to think it's a dying/dead cap. C13?

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        • #5
          V3 cathode voltage is at the hot end of R25, 7.8vdc according to schem.

          Have you tried an alternative tank, just to eliminate it?

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          • #6
            I have not tried another tank. dont have one readily available. my plan at the moment is to replace any/all resistors and caps in the reverb section first. then if that doesnt fix it, replace the tank. then push amp into hudson river if it still doesnt work.

            Comment

            • #7
              Even if you don't have a scope you can check for weird voltages. It might help you to narrow it furthermore.
              In this forum everyone is entitled to my opinion.

              Comment

              • #8
                another brief question: when i opened this amp up a couple weeks ago, i noticed that both the yellow wires, both marked K, have been removed. anybody know what these wires went to?

                Comment

                • #9
                  Originally posted by gromrock View Post
                  another brief question: when i opened this amp up a couple weeks ago, i noticed that both the yellow wires, both marked K, have been removed. anybody know what these wires went to?
                  Been removed, or just not hooked up? are they just dangling with bare lead ends? I have no idea what those wires are for since I'm not looking at the amp.
                  "Take two placebos, works twice as well." Enzo

                  "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                  "If you're not interested in opinions and the experience of others, why even start a thread?
                  You can't just expect consent." Helmholtz

                  Comment

                  • #10
                    cut out and removed completely. the bummer is fender doesnt have a "wiring diagram" anywhere so I have no idea what they would of went to once they left the pcb. from what i remember, I think they are tapping off one leg of the PT.

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                    • #11
                      Check out
                      http://www.webphix.com/schematic%20h..._%20manual.pdf
                      Everything on the PT is vital for the amp to function (unless it's an export and they are unused primary taps), so I can't think that the yellow wires have come from there. Pete.
                      My band:- http://www.youtube.com/user/RedwingBand

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                      • #12
                        like i said, I dont completely remember what they were ganged up with. I remember the traces coming from the K pads going to a QD blade on the pcb. i want to say i remember them both going to either CP33 or CP34. trouble is i can see them on the layout, but cant find CP33 or 34 on the schematic.

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                        • #13
                          I'm looking at an image o the 65 Twin RI PCB on google, there is nothing attached to CP33/CP34. PT primaries go to CP3/CP4.

                          The yellow wires that I can see appear to be cathode wires, perhaps hence the "K". If connected to the power tubes, these wires go to ground.

                          Comment

                          • #14
                            so not neccesary unless it is an export model? its just odd that there are remnants of wire in the K holes (no laughing). oh well. they must solder all wires on the pcb in the factory, then once they go to assembly, they cut out whats not needed, ie. domestic vs export? makes sense from a mass production standpoint.

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                            • #15
                              Export models still have the yellow wires running from tube sockets to the PCB. To what do the pin 8s of the 6L6s connect?

                              Comment

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