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Fender Red Knob Champ 12 with 100Hz Hum

Discussion in 'Shock Brother's DIY Amps' started by jimi00, Feb 9, 2013.

  1. jimi00

    jimi00 TDPRI Member

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    Hello,

    Being the proud owner of a Japan-built Squier Telecaster, I have been lurking this forum for quite a while. But the time has come to post at last about a problem I have been having for years.

    My amp is a Fender "Red Knob" Champ 12 and, since I play in a quieter environment than before, the 100Hz hum is driving me nuts. This amp was bought second-hand and has, as far as i know, always had this humming noise:

    -The humming is 100Hz and starts as soon as the 6L6GC power tube warms up
    -The hum is absent if the 6L6GC power tube is removed
    -The hum is steady with or without an input instrument and whatever are the values of the tone and volume and reverb control settings
    -The hum is steady with or without the 12AX7 preamp tubes
    -I have changed the four power caps to no avail
    -I have tried moving the wiring with a chopstick but the noise remains perfectly steady

    I am under the impression, given the poor quality of some of the solders on the power tube leads and on the output stage present on the circuit board that indelicate service was brought to this amp before I bought it.

    I am joining pictures and the amp schematic and would be grateful if someone could point me in the right direction.
     

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  2. printer2

    printer2 Poster Extraordinaire

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    Bad cap or bad solder joint in the power supply section.
     
  3. marshman

    marshman Poster Extraordinaire Silver Supporter

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    I would try a different 6L6 tube, if you have one, just to be sure, but it doesn';t sound like a tube problem.

    Bon Chance. You've come to the right place for some help.
     
  4. printer2

    printer2 Poster Extraordinaire

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    Doubt it would be a n output tube problem but it is something you can eliminate by swapping out a new one. The schematic shows a master volume control just before the output tube. If it is turned down all the way (shorts out the signal to the tube) and you still have hum then it is normally the voltage supply to the power tube, either the plate or the screen is getting AC on it.

    Another thought is this amp has a reverb section that returns the reverb signal to the input of the power tube. It could be introducing the hum into the power tube when the master volume is turned down. I would think the master volume being turned down would also load down the reverb signal and turn it down but without the amp in front of me I can't say for sure. Try pulling V2, the second triode returns the reverb signal to the output tube. If the hum remains then it is not the reverb or the rest of the preamp.
     
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  5. dlaroe

    dlaroe Tele-Meister

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    The board to tube socket wiring is absolute *******. Those thin and stranded wires break everytime I pull my Champ 12's chassis. I'd double check all those connections, especially on the power tube.

    Does it happen on both channels? The "master" is only active on the dirty channel. I think different stuff is grounded by the LDR's for each channel, but I could be wrong.
     
  6. marshman

    marshman Poster Extraordinaire Silver Supporter

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    Jeff Bobers' column in PG recently was about the Champ 12...next time I go down to the library, I'll check it out...I do recall he said there was somehing odd about the reverb in that particular amp.
     
  7. jimi00

    jimi00 TDPRI Member

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    Thank you all for your responses.
    I forgot to mention that swapping the 6L6 with a new one doesn't solve it.
    And that the hum remains constant if I switch to the overdrive channel.
    I seem to recall that the odd thing about the reverb is that it receives it's input from the speaker output; don't know if that rules the reverb out as the culprit but the reverb control doesn't affect the hum.
    May these details provide you with more clues?
     
  8. jimi00

    jimi00 TDPRI Member

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    How can I check that it is the plate or screen getting AC?
     
  9. printer2

    printer2 Poster Extraordinaire

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    Usually you measuring across the power supply capacitors with a meter on DC to determine the DC voltage. If the merter is set to AC you will measure the AC voltage.



    The reverb gets the drive from the amp's output so fender did not have to build a driving circuit for it. The bad thing about it is that you can not use much reverb otherwise the circuit starts feeding back on itself. The recovery section of the reverb could still be a problem point as it could work but if may still introducing hum into your output tube. Again, pulling V2 will rule it out if the hum does not stop when you pull it. Basically you will have no active circuitry feeding the output tube and if you still have hum it is coming from the PS.
     
  10. jimi00

    jimi00 TDPRI Member

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    The hum does not stop when V2 is pulled out.

     
  11. jimi00

    jimi00 TDPRI Member

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    I'm sorry. I'm a newbie to tweaking. Could I please have a precise indication of places where I should put the meter probes to get a measure?

     
  12. printer2

    printer2 Poster Extraordinaire

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    On the schematic you want to measure from ground to points; A (C25), C (C102), across C103, and from the top of C103 to point B (C101). Measure the DC and the AC voltage across each capacitor. Please be careful.
     
  13. printer2

    printer2 Poster Extraordinaire

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    Just found a couple of picture of the amp, the preamp cap is on the circuit board and the power amp section is in the corner by the fuse. The preamp voltages Point A is not necessary you will have to get to the component side of the board and it might be more trouble than worth.

    http://stevepedwards.com/wp/?m=201301

    Capacitors to check.

    [​IMG]

    [​IMG]
     
  14. jimi00

    jimi00 TDPRI Member

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    Thank you so much for all these details.
    Here are the measures.
    GND->C 532 VAC / 240 VDC
    C103 545 VAC / 246 VDC
    C103->B 550 VAC / 249 VDC

     
  15. printer2

    printer2 Poster Extraordinaire

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    Point C, across C102, 240V DC, schematic says 242VDC

    C103 is the bottom capacitor in your plate supply, 246V DC

    C101 248V DC, the top half of your plate supply, add the two together and you get 495V DC, schematic says 496V. You are right on the money on your DC voltages.

    Now your AC voltages. They seem wrong, way wrong. I am thinking you might be on the wrong scale or missing a decimal point or something. Possibly 50V? 5V?

    C101, C103 are in series and supply the power transformer, the schematic says they should be a combined 486V DC.

    C102 is your screen resistor and should be 242V DC.

    I think you may have
     
  16. jimi00

    jimi00 TDPRI Member

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    I measured again and again and AC Voltages vary a bit but stay on the same scale:
    GND->C102 545 VAC
    C103 555 VAC
    C103->C101 560 VAC
    The meter even indicates HV in one of the top corners which I reckon stands for High Voltage. (Of course I did try lowering the scale on the meter and it goes way over).
     
  17. kleuck

    kleuck Tele-Holic

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    Red Knobs....are just plain crap when they become old.
     
  18. jefrs

    jefrs Doctor of Teleocity

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    The hum goes away when the 6L6 is pulled because there is now no output to the speaker.

    The hum is steady because it is not being amplified by the previous gain stages, this is good. This is why the 12AX7 removal has no effect.

    You have changed the power capacitors. This is the obvious one.

    Poor solder produces buzzing not mains hum although it can make mains hum worse.

    A small amount of mains hum can be normal on a guitar amp. How much is acceptable is debatable.

    Sometimes the fault lies with the mains supply itself, noise on the earth pin. Check that if possible, try a known-good socket, maybe in other premises.
    No pedals, straight into the amp, eliminate all FX (ground-loops).

    Examine the amp's lead dress and chassis earthing. You want twisted-pair on the HT and heaters. You want star-earth. Do do not earth twice. If wires cross then they cross at right-angles. You want wires are kept down against the chassis and around its edges.
    Example, it the input jack is earthed to the chassis at the socket then the signal coax shield to V1 should be connected to the jack but not at the board end, or you get a loop - but this would be amplified and you are not apparently suffering that.

    Imo mains hum most likely site between the power transformer and the rectifier. Keep cables short and twisted pair here, and hard down to the chassis. Same from rectifier to smoothing caps. Keep other stuff well away from this area.

    Power transformer: should be right angles to the output transformer and/or choke (if applicable), sometimes these can be re-arranged, rotated about face. PT might need to be raised off the chassis by a sheet of Paxolin (thick blank PCB), still earthed by the screws but induction now de-coupled from the chassis.
     
  19. jefrs

    jefrs Doctor of Teleocity

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    Looking at the pics, I've got to say that the lead dress is a complete plumber's nightmare.

    The OT and the PT ought to be at opposite ends of the chassis, as far apart as possible.

    The wires ought to look tidy, not a plate of spaghetti.

    Who designed this **** :confused:

    It is fixable :)
    - probably stuck with transformer locations so expect better rather than perfect.
     
  20. jefrs

    jefrs Doctor of Teleocity

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    This is not an old amp so the white muck on the solder is most probably lead-free packing up (it oxidises, one of its accelerators for that is passing electricity through it).
    Simples: re-flow with tin/lead, sorted.

    Use a solder sucker to remove muck. Use a lead-free temperature iron (they are hotter) or it won't wet/flow and do not bridge to next point (careful).
     
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