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70/80 sf twin with a hum.

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  • 70/80 sf twin with a hum.

    Hi guys new to the group. I have a 79 fender twin with the push/pull master volume and it never is making what I believe to be a 120 cycle hum. That's not (100%) sure. I have seen guys talking on this thread by googling this stuff and I have tried pulling all 4 6L6 tubes out and still have hum, have tried putting a different 6l6 in each socket still have hum. Did the same with each preamp as well and same thing. I noticed the other day pulling out the master volume and turning it to a certain point the hum is gone. So I pulled the master taped the wires fired up the amp and still have the hum. It reads fine goes from I believe 900s down to zero when. I measure the ohms. I read in one of these forms this could be a transformer and that's a big problem. I was curious how do I test a transformer and why is that a big problem? My caps don't really seem to bulge a whole lot and seem fine I don't have a good way to test them presently. Thank you for any help. Also when I move all the volume knobs and the reverb the hum is there and as well as a crackle. My 2yr old climbed the amp one day and it did fall on its face. No loose solder joints that I have seen by poking around in the amp. Thanks , Derek Christiansen, Nebraska!

  • #2
    You had all 4 of the 6L6's pulled at the same time and you still had hum?
    Or you only had 1 pulled at a time?
    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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    • #3
      Also, apart from the hum does the amp work normally and does it have good volume?

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      • #4
        Originally posted by g1 View Post
        You had all 4 of the 6L6's pulled at the same time and you still had hum?
        Or you only had 1 pulled at a time?
        I had all 4 power tubes pulled and still had hum.

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        • #5
          Amp has been my name amp for 5+years works great and quiet as could be til it dropped. Started getting louder past 6months. 2yrs ago had a tech from nashville go through it and retube and bias it. So tubes are 2years old but they did go through 2 falls.

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          • #6
            I think you may be dealing with a few different sources of hum here.
            With all power tubes removed, the master volume should not have any effect. And the hum with the power tubes pulled may not be through the speakers, but a physical hum from the chassis or transformer.
            Some of these have both a 'hum balance' and an 'output tubes matching' control on the rear panel.
            Set volumes and master to zero. Adjust 'matching' control for minimum hum.
            Turn master up full. Set 'hum balance' for minimum hum.
            Does that help?
            Originally posted by Enzo
            I have a sign in my shop that says, "Never think up reasons not to check something."


            Comment


            • #7
              Originally posted by g1 View Post
              I think you may be dealing with a few different sources of hum here.
              With all power tubes removed, the master volume should not have any effect. And the hum with the power tubes pulled may not be through the speakers, but a physical hum from the chassis or transformer.
              Some of these have both a 'hum balance' and an 'output tubes matching' control on the rear panel.
              Set volumes and master to zero. Adjust 'matching' control for minimum hum.
              Turn master up full. Set 'hum balance' for minimum hum.
              Does that help?
              I tried the hum balance control but that had no effect. I don't have a way to match tubes back so pry will end up taking it in. Really wanted to learn how to do this stuff but it's a bit over my head. Thanks for tips guys!

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              • #8
                Common Sources of hum in Fenders:
                a) power supply - are those caps original? if so, I would replace them.
                b) bias supply caps - are those original and are they the white Mallory caps - if so they should be replaced.
                c) grounds - check them all. especially the grounds by the power transformer and the grounds on the power tube sockets.
                they may look good but still may me bad. I would definitely re-solder the grounds to the power tubes sockets!
                also check the grounds under the volume controls.

                My experience has shown that the hum is most likely grounding when the master volume is involved.

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                • #9
                  Originally posted by Derekcdc View Post
                  I don't have a way to match tubes back so pry will end up taking it in.
                  Does your amp not have the control on the back called 'output tubes matching' ?
                  Originally posted by Enzo
                  I have a sign in my shop that says, "Never think up reasons not to check something."


                  Comment

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