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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. Racing

    Racing Tele-Meister

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    Nah. The pic works just fine. In a worst case scenario,copy n paste.

    No reason to redraw the schematic. Pic show what i point out. You see a long grey wire leading halfway cross the PCB?
    To the right in the pic it ends as a small solderjoint. Just left of that solderjoint is the machines center starground,as i guess is quite evident.
    The grey wire is basicaly soldered directly at the solderjoint for the rectifierbridge negative,which is turn leads for that starground on the PCB.

    To make it even more logical,i recap.
    Absolutely nothing is to be installed between the point of where the bridge negative or the PT´s centertap hit the circuit,be it a PCB based point or chassis,and the grounding of the first electrolyte.
    Nothing.

    Simple as that.

    [​IMG]

    The purple marked area is the forbidden zone. If you ground anything but the negative/CT and minus for the first lyte in there...the amp WILL hum. To a larger or lesser degree. Period.
     
  2. jimi00

    jimi00 TDPRI Member

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    I have decided, based on advice from another forum, to change C101, C102 and C103 values to 200uF, 350V. I will keep you informed on the outcome of this change.

    While I'm at it. I read that KT66 should be a plug-and-play replacement for 6L6GC. I tried it a month ago and it worked for a few minutes before blowing a fuse (can't remember which fuse). It was a Valve Art KT66 (chinese-made) and instantly put it away in fear of having done something wrong. Are these tubes interchangeable?
     
  3. Racing

    Racing Tele-Meister

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    I can 100% absolutely assure you that replacing those e-lytes with ones of larger capacitance will amount to zero if the ones in there ain´t injured in any way-or worn beyond belief.

    Many italian amps back in the day used larger capacity cans for whatever reason for example...and they hum just as much as you agerage Fender or whatever. That larger capacitance sometimes can be beneficial from the aspect of instantanious availability of power is one thing.. (read "stiff" powerstage).
    Getting the amp hum free though is something else,and that is a matter either of that something´s broken or lack of design-or understanding thereof.

    If anything inrushing current can,and most likely will with those completely pointless capcitances mentioned,blow you fuses though.

    From previous pics it looks like you already replaced the cans with some fresh F&T`s ?
    They´ll do just fine in that case.

    Challenge the grounding path of the amp. That is where you´ll remedy your issue.

    1/Study the pic above. Compare to how your Champ is put together. Check where safety ground is installed vs that point and check where the CT for heater does too.
    2/If you want to,AFTER you´ve settled the issue with ground vs e-lyte,install a 1/4w trimpot of 500-1000 Ohms to the two leads of the heater circuit.
    CT MUST be located and cut. Insulate with a bit of shrink sleeve and put it away-so it can reach ground ever again.
    The two "outer" legs of said trimpot vs each lead of the heatercircuit. Viper of pot is run to a groundlug.
    This way you have in the easiest of manners converted the amp to adjustable zero heater circuit.
    2/ Twist any and all wiring that carries AC current and twist it tight. Yep. even the three prong that carries incoming current.
     
  4. printer2

    printer2 Poster Extraordinaire

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    I was going on the idea that the amp originally did not hum and there was a problem with it. As far as reducing power supply hum in a SE amp like the Champ I would not think twice about using 200uF as long as it does not blow a fuse or the diodes. I think the 47uF value in this amp is too low for a SE 6L6 amp that uses a voltage doubler. A voltage doubler has two capacitors in series which effectively cuts your capacitance in half. So rather than having filter capacitors of 47uF they are more like 22uF caps. Since a single ended amp does not have a push-pull output transformer that helps cancels out hum you, well get more hum.
     
  5. Racing

    Racing Tele-Meister

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    Agreed. However,canceling hum takes rather little IF the design is on par.
    Voltage doublers or not.

    Likewise for hiss. I´ve lost track of the number of times overly amounts of hiss has been related to bad electromechanical contact and/or ground.
    As late as last night i fooled around with an older 18w that showed more hiss than it should. Turned out that the old novals for the preamp..yeah well,one of the pins in the No1 sockets was broken within. Replaced the socket..dead silent in idle.

    I guess what i´m trying to say is that it can often pay off to question the design per se.

    [​IMG]

    As i replaced the OEM multi e-lyte i opted to redo the grounding per above. As you can see with regular multistrand wire. This machine sports a CT and the grounding of first e-lyte runs for the M4 stud furthest away in the pic. IOW,redirecting ground with a wire works just fine.
     
  6. jefrs

    jefrs Doctor of Teleocity

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    No! no! no!

    The PT must be earthed by those screws.
    Loosening them is a very very bad thing: danger - no earth.

    lifting the PT and the OT off the chassis by a sheet of plastic breaks the induction from the transformer into the chassis. There's an alternating magnetic flux in the core of the transformer which will induce a current (hum) into the adjoining sheet of metal. Some chassis have holes in them that serves the same purpose as lifting the transformer. It does not need to be lifted far, a couple of millimetres is enough, magnetism is a "weak force".


    Never! never! never! use an ungrounded amplifier!
    Are you trying to kill yourself?

    If you do not know what you are doing, put your tools down and take it to a technician!
     
  7. jimi00

    jimi00 TDPRI Member

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    Red Knob, success!

    Big update on this thread! I have finally received the 350V 220uF caps. I have replaced C101, C102 and C103, and guess what? When I powered it on the first time, for a moment I thought I made a mistake in re-assembling because it had to stick my head IN the speaker to hear anything . THE AMP IS DEAD QUIET AS FAR AS 100 HZ GOES!! This is fantastic news for all the Champ 12 lovers out there. A very slight 50 Hz hum is still to be heard inches away from the amp but this is so faint that it would not even be a concern for studio or recording use.
    The only point of caution I would give concerns the size of the new caps which are twice as wide as the original. Don't let that fool you, they fit perfectly in length.
    I will see how the amp deals with the extra capacitance in the long run, and will keep you informed is anything new happens.
    Thank you everyone here and in the other forums for providing me with the right insight in solving this problem.
     
  8. printer2

    printer2 Poster Extraordinaire

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    What? No pictures?
     
  9. jimi00

    jimi00 TDPRI Member

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    Well, there isn't much to see, really. The three filtering caps are just wider is all.

     
  10. printer2

    printer2 Poster Extraordinaire

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    Just kiddin. :lol:
     
  11. jimi00

    jimi00 TDPRI Member

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    Now that this is cleared, my next step would be to install a pot to allow biasing and usage of other tubes than just the cathode biased 6L6. I will open another thread for this...
     
  12. Racing

    Racing Tele-Meister

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    Again.
    If fresh e-lytes improved your issue the old ones were worn. There´s absolutely no way in hell a slight overkill in capacitance will amount for the change described. Zero.

    Regardless. Congrats on moving ahead and for a better sounding amp!

    ´N for the record..i STILL think you should look into the grounding issue. Most amps can be improved upon from that respect.
     
  13. jimi00

    jimi00 TDPRI Member

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    I beg to differ, Racing. The replaced e-lytes were BRAND NEW. They could not have been worn. The reason why the extra capacitance is beneficial has also been covered. Champ 12 users have been plagued with this defect for ages. I would love to hear from another Champ 12 user to see how his mileage varies with this solution in hand. This would allow us to make sure this solution is reproducible.

    Thank you for your suggestions.

     
  14. Racing

    Racing Tele-Meister

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    Might be.
    In that case a solderjoint,defect lyte or whatever along those lines has been the culprit as far as i´m concerned. Again..there is no way that the added capacitance got rid of the hum. Zero
    FYI you can get an amp dead silent in idle with way less capacitance than the OEM champ of yours sport.
    Just take a look at any older tuberectified unit. Rarely any of them sport more than 32uF for the first hit. More often than not..less. In fact there´s an old untold law that states that more than 50uF for the first hit for a tube rectified amp will kill the rectifier-and that kind of sets the limit doesn´t it?
    If you design a tuber correct it will turn silent in idle with way less e-lyte than most imagine or even propose,and THAT is my point. Differ all you wish.

    Or...put another way.
    Last redknob i did..also hummed. AC based hum( a Twin). I did NOT increase capacitance one yota,didn´t even replace the caps-no need, i just reinvestigated the amps grounding setup/path..and again got a dead silent amp in idle by the use of approx 6" of wire.

    Then again..you might very well know better than me.
     
  15. printer2

    printer2 Poster Extraordinaire

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    In the SE amp I built I found the best bang for the buck in dropping hum by adding capacitance is to do it in the screen supply. Tried it singly and in combination on the other stages and none other compared with making this stage smooth.
     
  16. Donelson

    Donelson Tele-Afflicted

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    I'm no amp expert, but I bought one of these Champ 12 amps brand-new around 1988. It had the hum as mentioned by the OP. Very noticeable in a quiet room. I returned it to Manny's a few days after I bought it and they told me that they all were like that, so I kept it. I had that amp until 2005 when I sold it. During that time I tried another identical one that a friend had bought. It had that same hum. So, there is something inherent in the product that causes this hum.
     
  17. Silent

    Silent NEW MEMBER!

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    Hi jimi00,

    The annoying hum in the Champ 12 is endemic. Your solution is right. The first cap of the filter must be 100uF. I’ve tried it with several Champion 600 and VHT Special 6 and it always works. The wire of Racing reduces the hum but doesn’t avoid it completely in class A amps, that’s my experience.

    In the Champ 12 C101 and C103 work together at 22uF (47/2) and 700V. So two caps of 200uF in series are 100uF. I think it’s not necessary to change C102.

    I tried it in my Champ 12, but I just could find 100uF/350V caps, so it’s not enough. It’s less humming but not completely silent. So you’re my man: Where have you got these 200uF/350V caps? Please, please…
     
  18. jimi00

    jimi00 TDPRI Member

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    Hi, I found the caps here : http://bit.ly/YvpUvW
    BTW, the amp has been running without a problem since the mod. Please provide feedback if the solution works for you too. Glad to be of help.
     
  19. Racing

    Racing Tele-Meister

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    I recap.
    Adress the grounding layout.

    For the sake of argument. I´ve currently just modded the living daylights out of a very old "toaster".

    [​IMG]

    This is a tuberectified unit where first hit is set correctly vs the powertransformers centertap.
    First hit in this case,OEM,is 22uF. (After my modding it´s 47uF)
    The amp is dead silent in idle. I kid u not. Another FACT is that if first hit of a tuberectified unit is set above 50uF chances are that you´ll fry your rectifier in notime flat.

    I´ll return the question,what is so hard to take to heart about a correct grounding layout?
    I recap..i´ve modded more redknob Fenders than i care to remember this way.

    The -58 vintage amp in the pic btw is nowadays a 3 channel unit of which 2 of the channels entry stages are EF86 equipped. Ie;with a rather massive difference in first stage amplification (read-where hum and hiss will be way more pronounced than with a regular 83/AX7 stage)
    Further it is setup to roar. Ie; it is run rather hard as far as the stages running for the phaseinverter and then mainly controlled by a PPIMV setup. (A master set to work between the phaseinverter stage and the powertubes).
    The thing is... no matter the channel,if you turn the volume of the guitar down to zero the amp goes dead silent. No matter how far the gainlever is turned.
    ...as will ANY amp if setup correctly with a reduced amount of gainstages,of which the Champ is one.
     
  20. MaxRite

    MaxRite TDPRI Member

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    Hi Everyone,
    I'm a new member of the forum
    I'm no tech, so I'm here merely to seek for help
    Hope you don't mind if I take advantage of your precious advices
    In particular, about jimi00's mod

    I'm taking my Champ 12 to a tech to replace, guess what?.......Yes, you're right, the input jacks! ;-) So since I'm opening the amp I thought I might try the caps mod

    Has anybody else tried it?
    Jimi, have you had any problem since?
    Can I leave C102 alone, as suggested?
    Also, are C101 C102 C103 the power capacitors?
    If yes, is it true, as I read somewhere else, that: "Higher capacitance in the power supply will tighten up low end a bit" ?

    Thanks a lot
    Max

     
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