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fender hot rod deluxe slight hum

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  • fender hot rod deluxe slight hum

    hi folks im new here and recently got into a hot rod deluxe the amp had a few issues that i have fixed thus far but still have some problems the amp seems to lack a tight bottem end and when the volume is turned down on tne clean channel all the way i hear a slight hum that goes away @ the 2 mark on the vol setting and then starts to return at 3 and gets louder as volume is increased any ideas? iv installed new mached pair of sovtek 5881wxt power tubes and biased at 75 mv at the test point also all new preamp tubes replaced the screen-grid 470ohm resistors as the old groov tube 6l6 were bad one causing the resistor to fail fixed broken ribbon connector from the center preamp tube to the main board also it dont seem like the filter caps are holding voltage as they read only 3-4 volts dc only seconds after powering down the amp any help is greatly appricated
    Last edited by jack von; 08-22-2014, 03:49 AM.

  • #2
    Sounds like something is drawing way too much current. Make some more voltage measurements and post them here.
    Measure all the HT voltages on all the plates.
    What is the voltage across the choke?
    Check those high voltage diodes on the 6L6 board.

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    • #3
      My check on hum is to firstly to decide if it's in the power amp or preamp. Plugging a shorted plug into the 'power amp in', or just plugging your guitar into that socket and turning the volume down on the guitar will cut off the preamp and kill the input to the power amp section.

      If no hum, then you're looking at the preamp.

      Pull V1 and see if it hums. No hum means the fault is around that tube.

      The cap voltages can drop pretty quickly when all tubes are inserted. The power tubes still continue to conduct for a while after the power is removed and will quickly drain the B+ and Z nodes. Try the same test with all tubes removed and you should find that the cap voltages hold up for much longer.

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      • #4
        ok yeah pluged into power amp in and no hum il do more tests and i did have a problem with one of the ribbons in the preamp section that was causing the static crackling sound .

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        • #5
          The fact that you can "null" the hum by setting the volume at 2 indicates it is something before the volume control and perhaps being picked up, rather than created.
          You mentioned working on the ribbon cable, in my experience these amps are real picky about the exact bend of the ribbons. Did you put it back to the same shape bend as it was to begin with?
          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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          • #6
            That null thing means there are two sources of hum that are out of phase, one before the volume control and the other after. Their relative levels cancel out at 2 on your amp.
            Education is what you're left with after you have forgotten what you have learned.

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            • #7
              have not had chance to get back into it yet but going to try tonight after practice any one know what voltage should be across choke? and how to properly check it im no electronics engineer lol and thanks in advance for the patience and advise.

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              • #8
                Schematic says about 1 volt.
                Education is what you're left with after you have forgotten what you have learned.

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                • #9
                  ok thanks ENZO il check and is that dc voltage ?

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                  • #10
                    Well yes. Look at the schematic. One end is at +431v and the other is at +430v, so 1v across it.

                    Don;t expect your amp to have exactly 430v on anything.
                    Education is what you're left with after you have forgotten what you have learned.

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                    • #11
                      ok voltage was at 1.45 on the choke and iv found some more bad solder joints fixed them now hum is gone and amp is running great thank everyone for the help you guys are some smart people many thanks again for all the help.

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