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  • Loud Hum in 1974 Twin Reverb

    Hello, people, I did a bad thing today and am feeling quite a bit of anger towards myself. I thought I could fly close to the sun and replace my 1974 silverface Twin Reverb's filter caps. I have experience soldering so I did that and it all seemed to be fine and dandy, so I closed it up, set up my amp, turned it on, and there was no sound. I probably got up to a master volume of 6 or so with a little volume in the normal channel, before I turned off the amp and looked in the back to see that the speaker cable wasn't plugged in, and neither were the reverb input and output cables.

    When I fixed it and turned on the amp again, there was an unbearable loud hum. I would assume if I really screwed up and overloaded the output transformer, it just wouldn't work anymore and there would be no sound (which is also what I need to tell myself to keep sane). There is no weird smell and I did not hear anything explode. The output transformer still gets hot, as do the power tubes, which all light up bright as usual. I'm wondering if there could be a short inside or something? What causes this loud awful hum?

    I don't believe it has anything to do with the reverb, as the hum continues when I turned the reverb to 0. I did happen to take out the reverb tank thing while put the amp back together, because I'm an irredeemable idiot and wanted to see what it was. Shook it and it made a noise. The hum is relative to the volume of the amp, and though I don't know if I can judge it perfectly with the loud hum, the tone seems to have gone shoddy. Just very thin.

    I scoured the forums and haven't seen anyone with a situation as dumb as mine. Please help me, I am desperate and am considering a visit to an amp tech, something I swore I would never do. PLEASE DON'T TELL ME TO GO TO AN AMP TECH, IT WILL MAKE ME CRY.

  • #2
    First thought? Did you put one or more of the caps in backwards? Double check the orientation of all of the caps you replaced.

    Oh, and welcome to the place!
    "Yeah, well, you know, that's just, like, your opinion, man."

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    • #3
      I'm just speculating until the smart guys chime in. Fenders are designed so if you have nothing plugged into the speaker jack the output shorts to ground, protecting the output stage. Did you mess with anything inside the main chassis or only in the doghouse underneath the chassis? Is it possible that you shifted some wires that are now picking up hum? Perhaps there is a bad connection or component among the filter caps. Does the channel volume have an effect on the hum or just the master? Did the tubes get moved so the hum balance is no longer properly adjusted? I'd imagine you probably have a simple problem and it will get sorted out soon.

      BTW, there's nothing wrong with going to an amp tech, but at least you went for a challenge. Good luck and I hope your amp is better soon, sometimes it helps to think of mistakes as learning opportunities.

      P.S. The Dude's reply wasn't up when I opened the window, I am not doubting his knowledge.

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      • #4
        Originally posted by Richard View Post
        ......P.S. The Dude's reply wasn't up when I opened the window, I am not doubting his knowledge.
        Oh, feel free to doubt away! I've been reconing speakers all day. I'm about half loopy from speaker glue right now.
        "Yeah, well, you know, that's just, like, your opinion, man."

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        • #5
          Originally posted by The Dude View Post
          First thought? Did you put one or more of the caps in backwards? Double check the orientation of all of the caps you replaced....
          That would be the first thing I'd check. The first two caps should be in series, not parallel. Here is a picture of a Super Reverb showing the two first stage caps in series.


          You can click on the image to make it larger.


          Click image for larger version

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          • #6
            Originally posted by The Dude View Post
            Oh, feel free to doubt away! I've been reconing speakers all day. I'm about half loopy from speaker glue right now.
            What, no carbon filter mask???

            Click image for larger version

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            Or, you could go the 'firefighter' route.
            (Chick not included)

            Click image for larger version

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            • #7
              Like Richard said, the speaker jack is the shorting type, so don't worry about your OT.
              You replaced filter caps and now have hum, so the work you did is your #1 suspect.
              Aside from what has been mentioned, if you replaced bias filters, they should have their + end grounded.
              "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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              • #8
                Okay, so I just opened the amp again. All of the capacitors are facing the correct way, included the two weird ones where the positive goes to ground. In the doghouse, I have the one on the left that goes the other way, it's all good. I had my dad take a look at it (because he knows nothing about guitars) and he had me plug in a different guitar and the hum went away. Normally, I play with a Japanese Telecaster, so that guitar and my little garbage baby Squier Affinity both cause boatloads of hum. When I switched to my Les Paul, I pinned the master volume and normal volume at ten (while the entire amp was apart and I was making sure not to touch anything sketchy and kill myself) and there was a small, barely audible hum. The chassis was on the cabinet, and when I strummed, the whole thing shook and I shut it down, but hardly any hum.

                So, while this is a break in the case, it means I can't really use my only good guitar? I'm a single coil kind of guy, I need to be able to use my tele, but the hum is like that of a hardcore band who all have their JCM 800s cranked, and this occurs on the twin when the volume isn't even high. Should I change pickups? I know the Japanese pickups are notoriously poor quality compared to the guitars. I was thinking of getting a Lollar Special T in the bridge.

                Not only is there the hum though, but the sound is thin with the tele. The Les Paul had a nice warm sound, and it was the amp, the amp was giving it vibrant tone, but the tele sounds dead and limpy with the blaring hum. I need to be able to use this amp clean and quiet, and it was undoubtedly much quieter before I did anything, and at least with the tele, it sounded better. With the Les Paul, it sounds pretty great, but the Les Paul SUCKS.

                So uhh, could changing the pickups help me here? Love you people.

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                • #9
                  Hi Emmitt....

                  Yes, by nature, the single coil pickups are noisy... "hum" if you will. Have you tried stepping away from the amp... maybe a few feet, turn 90 degrees, try a different instrument cable? Sometimes, the cables with thin conductors and poor shielding add to the noise. Try these ideas and see if it makes a difference. Oh... no fluorescent lights if you can avoid it!

                  Tom

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                  • #10
                    Originally posted by EWaffle View Post
                    The chassis was on the cabinet, and when I strummed, the whole thing shook and I shut it down, but hardly any hum.

                    I'm a single coil kind of guy, I need to be able to use my tele, but the hum is like that of a hardcore band who all have their JCM 800s cranked, and this occurs on the twin when the volume isn't even high. Should I change pickups?

                    Not only is there the hum though, but the sound is thin with the tele. The Les Paul had a nice warm sound, and it was the amp, the amp was giving it vibrant tone, but the tele sounds dead and limpy with the blaring hum. I need to be able to use this amp clean and quiet, and it was undoubtedly much quieter before I did anything, and at least with the tele, it sounded better.
                    The tone from the cab with the amp chassis not in place will be weird.
                    The open slot above the baffle allows a lot of the sound to cancel out, leading to a thin weedy tone.
                    So don't write off the tone from the Tele / amp with it set up like that.
                    However, the hum may be louder too.

                    Kinman pickups sound proper, get rid of most of the hum and make my single coil guitars usable in places where there's horrid hum levels with regular single coils http://www.kinman.com/guitar-pickups/telecaster/

                    If another guitar sounds ok and not hummy, then the amp may be fine; put it back together and re-assess.
                    If still concerned, take some detailed photos of the work you've done, post them on a hosting site like photobucket etc, and put links to them in your reply.
                    Last edited by pdf64; 07-13-2015, 02:16 PM.
                    My band:- http://www.youtube.com/user/RedwingBand

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                    • #11
                      Just in case....

                      If you have these guitars set up in some way that they all have individual cords then try the cord used for the quiet guitar with the other guitars. This was already alluded to I spose, so I'm just seconding that.

                      OTOH it is telling that the humbucker guitar is fine and the singles are not. It may just be room noise. Even if it hasn't been a problem before. Button up the amp and take the whole rig into another room.

                      And, just thinking out loud, you did replace the filter caps for a reason. If that reason was hum then perhaps it was never the amp humming but the very same problem you're having now. Which appears not to be amp related. Not that there's any harm in changing out the old filters in the amp. If they were original I'm sure they were overdue anyway.
                      "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

                      "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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                      • #12
                        I have a variable temp. soldering iron and that thing sends huge hum through the mains. Are you sure that the hum is a new problem? Possibly the power where you were working on your amp is dirtier than where you normally plug in or there are other things that are causing interference. At least we know your repair was successful!

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