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Fender Twin Reverb w. Master Volume: terrible distortions

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  • #1

    Fender Twin Reverb w. Master Volume: terrible distortions

    I've got this amp on the bench for a full retube. I swapped all tubes out and this amp sounds absolutely terrible. It creates lots of unwanted fart like, disharmonious distortion, on the lower strings even creating octaves (like an effect pedal).
    Before I did anything I had to clean the chassis inside out as there were spider webs and lots of dust all over.

    The bias is ok and all tube socket pins have correct voltages.

    My question is how to go about trouble shooting this. Could this be a cap job or full re-built? The customer wants to restore this amp and is willing to spend what it takes (up to a certain point).

    Thanks for any help.
    "Tubes are less likely than semiconductor devices to be destroyed by the electromagnetic pulse produced by nuclear explosions and geomagnetic storms produced by giant solar flares."
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  • #2
    Originally posted by kka View Post
    It creates lots of unwanted fart like, disharmonious distortion, on the lower strings even creating octaves (like an effect pedal).
    These are typical symptoms of bad filter caps. I'd start there by testing them or at least by reading the ac ripple voltage on the B+ line.

    Comment

    • #3
      How did it sound with the old tubes it came in with? Did you fire it up before doing ANYTHING to it? I'm not disagreeing with Bill, caps sound likely, but you could also have a bad tube. Just being new is no guarantee a tube is OK.
      Education is what you're left with after you have forgotten what you have learned.

      Comment

      • #4
        Originally posted by Enzo View Post
        How did it sound with the old tubes it came in with? Did you fire it up before doing ANYTHING to it? I'm not disagreeing with Bill, caps sound likely, but you could also have a bad tube. Just being new is no guarantee a tube is OK.
        Yes, I played amp before swapping tubes and same thing if not worse.
        "Tubes are less likely than semiconductor devices to be destroyed by the electromagnetic pulse produced by nuclear explosions and geomagnetic storms produced by giant solar flares."

        Comment

        • #5
          There are numerous places where ground connections to the chassis rely on some sort of fixing to create a good connection to the chassis, eg the control panel brass plate, input and speaker jack sockets, RCA reverb and footswitch sockets.
          Chassis corrosion around the fixing hole, dry (or otherwise failing) solder joint, or just a loose fixing, can cause a poor ground connection, resulting in problems such as this.
          Also retension the tube socket contacts, if not already done.
          Pete
          My band:- http://www.youtube.com/user/RedwingBand

          Comment

          • #6
            Also... It's mentioned that the amp is a Twin Reverb with master volume and that the owner wants it "restored". Is this a classic BF type amp that has been modded or is this a later model that came stock with a master volume? Bad modifications can certainly cause problems. I'm only asking because WRT Twin Reverbs most players don't trouble with restoration unless it's a classic.
            "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

            • #7
              I will address corrosion and bad ground connections after I have replaced the filter caps. This amp is a 1972 built and I thought the Master was original. However after looking at schematics my amp here has the (still original) Mallory filter caps of the non master version (3x 20uF 500V 2x 70uF 350 V) built in. I just disconnected all 5 caps from circuit and none of them measured up to specs with my capacitance meter. I will replace all 5 as a first step. Could I please get some recommendations on what brand of caps I should buy? The customer wants everything high quality. He is not interested in a "restored original", he wants a player's amp that sounds great.

              There is someone selling a rather expensive recap set with German F+T caps. Would those be a good option?

              F&T recap kits for your vintage Fender amp | Vintage Fender Amp Repair



              Thank you all for your help.
              "Tubes are less likely than semiconductor devices to be destroyed by the electromagnetic pulse produced by nuclear explosions and geomagnetic storms produced by giant solar flares."

              Comment

              • #8
                Sometime in late '72 the master volume started showing up on Fender amps. Yours may be a one of these amps. If the faceplate has a master volume designation I'd say your amp is stock. This may be the schematic you need.

                http://www.webphix.com/schematic%20h..._nomstrvol.pdf

                Don't buy a cap kit like that unless you hate money. Also, you can't be sure that what you need is in that kit since your amp doesn't match the schematic. You DO need 500V caps though. And those are in somewhat short supply with limited options. Still, make a list of ALL the electrolytic caps in the actual amp and buy from an electronics distributor or amp parts supplier. Match, as close as possible, the uf rating and meet or exceed the voltage rating and you'll be fine. Since the big filter caps are only made by a couple of manufacturers anymore, quality is a moot point. They are all equally up to the task. My preferred distributor is Mouser. They do have the Vishay/Sprague ATOM caps in 500V rating. But they're silly expensive. I don't know if they carry the F&T branded caps. Still, shop around and buy the caps you need for the lowest price. They will almost certainly be F&T, ATOM or some house branded cap sold by Ruby or Mojo or some such. AND...

                Since your waiting for caps anyway, it only makes sense to clean and snug up connections and test grounds while you wait!!! It's something you'll want to do no matter what. Even if new caps fix the amp without this service, it's still something you'll want to do for good measure.
                "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

                • #9
                  Thanks Chuck. I had that schematic already. I'm going to order the caps and clean the amp.
                  "Tubes are less likely than semiconductor devices to be destroyed by the electromagnetic pulse produced by nuclear explosions and geomagnetic storms produced by giant solar flares."

                  Comment

                  • #10
                    Originally posted by kka View Post
                    I had that schematic already.
                    Ok. After I read this:

                    Originally posted by kka View Post
                    However after looking at schematics my amp here has the (still original) Mallory filter caps of the non master version (3x 20uF 500V 2x 70uF 350 V) built in.
                    I got the impression that you didn't have a schematic for a master volume Twin Reverb with this filter arrangement.
                    "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

                    • #11
                      No problem, thanks very much for looking out for me ;-)
                      "Tubes are less likely than semiconductor devices to be destroyed by the electromagnetic pulse produced by nuclear explosions and geomagnetic storms produced by giant solar flares."

                      Comment

                      • #12
                        BTW, I've got 3 different schematics. None of them are exactly for the amp I've got here. The one you sent a link to has got only 1x 20uF 500V cap, but mine has 3 of those. So mine is like a non master AB763 but with a MV installed. The MV knob is original and looks factory installed to me. Anyways, I've got what I need and will just cross reference should I need to.
                        "Tubes are less likely than semiconductor devices to be destroyed by the electromagnetic pulse produced by nuclear explosions and geomagnetic storms produced by giant solar flares."

                        Comment

                        • #13
                          The schem I linked has the three 20uf 500V caps. There just not bunched together. If you follow the HV rail in the schem you'll find them.
                          "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

                          • #14
                            Found them thanks. Then you were right, this is the schem. I need. Thanks again
                            "Tubes are less likely than semiconductor devices to be destroyed by the electromagnetic pulse produced by nuclear explosions and geomagnetic storms produced by giant solar flares."

                            Comment

                            • #15
                              Reading this thread I missed out on the date of the last sweet tone. Was it, maybe, before the some mysterious mod..?
                              In this forum everyone is entitled to my opinion.

                              Comment

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