SF Champ heater wiring mod. Advice please?

Discussion in 'Amp Input - Normal or Bright' started by Shinbones, Aug 18, 2016.

  1. Shinbones

    Shinbones Strat-Talker

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    Hi all,
    I'm planning to run twisted pair wiring from the pilot light to the heaters on my late 70's Champ... the aim is to reduce 50Hz hum (it's a 240V PT). I could really use some advice or clarification on the wiring to the tube sockets.

    I've seen a few pics of this mod and done some reading, so I think I know what I need to do, I'm just unsure on some of the finer details. I'll be using two 100ohm resistors from each pilot light terminal to the chassis to create an artificial CT.

    First up, here's a pic of my Champ's pilot light & PT wiring now (yes, the filter cap has been discharged):
    Champ pilot light.JPG


    The mod I plan to do looks like this (pic borrowed from another forum):
    champ_reroute_heaters.gif

    My question is: Do I need to consider polarity when connecting the new heater wiring pair to the tube sockets? That is, do I need to make sure the same side of the pair goes to the heater pin on both the 6V6 and 12ax7, and ensure the other side of the pair goes to the pins that are (were) grounded to the chassis?

    I'd be really grateful for any advice from anyone who's done this mod

    Thanks,
    Scott
     

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    Last edited: Aug 18, 2016
  2. Stevem

    Stevem Senior Stratmaster

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    Wire up the two resistors so a wire off of there common end can be soldered to the clipped green wire that is still on the chassis.

    Polarity does not matter in a amp like this with only one output tube.

    Keep the wires twisted up as tight as you can when feeding the preamp tubes, and of course when you add the the new wiring to each sicker be sure to cut the wire there now that goes to ground and your done, and NO rewiring gets done to the rectifier tube socket.

    As best as you can lay down the new heater wiring on the chassis and on the two preamp tubes lift the other wires up as far as you can above the new wires.
     
  3. Shinbones

    Shinbones Strat-Talker

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    Thanks for the advice @Stevem. I did the mod a few hours before your post, but think I got it right.
    Disappointingly, I've still got a 50hz hum though.
    I went about the resistor grounding a bit differently from your method, but AFAIK it's achieved the same thing.
    I'll post some pics in a few hours once I get back home... and would welcome constructive criticism on my mod' technique or other suggestions for how to lose the hum.
     
  4. candyapple1964

    candyapple1964 Senior Stratmaster

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    Are you absolutely sure of where the hum is induced FROM ?

    Also from my experience shielding the inputs leads before all the preamp gain can amplify the hum is a better strategy.
     
  5. Shinbones

    Shinbones Strat-Talker

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    I'm not absolutely sure about anything :(. This is a steep learning curve for me... my first step(s) into valve amp modding/troubleshooting.

    The hum is subtle, and is always there regardless of volume setting, so I thought it must be coming in after the input stage. But I'm happy to be told otherwise.

    Here are some pics of the Champ post-heater wiring mod... any suggestions/advice welcome. It's the export (240v) model hence the slightly different switch set up.
    image.jpeg image.jpeg image.jpeg image.jpeg image.jpeg image.jpeg
     
  6. Stevem

    Stevem Senior Stratmaster

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    These silver face amps already shielded wire off of the volume pot and the very short lenght of the wire off of the input Jack to the first gain stage makes very little difference .
    I first preamp tube with wound filiments can be helpful though!

    The main sourse of hum once the filiment wiring mod is done is the output stage and since this is a single ended amp you do not have two output tubes to phase cancle power supply hum.

    Also since the amp uses a rectifier tube you can only use do much filtering which would help hum before you pop the rectifier tube.
    Since these are not high gain amps what you have done along with a 12ax7 with wound filaments should be all you need!
     
  7. Shinbones

    Shinbones Strat-Talker

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    Thanks again @Stevem. Can you recommend a 12ax7 with wound filaments for me to try please? I have an EH 12ax7 in it now - which I think has wound filaments?, but I also have this old girl which might be the origina/factory one(?):
    orig_12ax7.JPG

    I've tried both 12ax7's as well as different 5Y3GT's and 6V6GT's, and and noticed little or no difference in hum. Maybe the subtle hum I have is as good as a SF Champ gets? I can actually feel the 50Hz vibration too when I put my hand on top of the amp... Is this normal?

    Cheers,
    Scott
     
    Last edited: Aug 20, 2016
  8. Stevem

    Stevem Senior Stratmaster

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    Could be, I know the Sovtec LPS model does!
    The tube looks like a original GE, the way to tell is that GE etched the numbers into the glass , no ink stamp.

    Yes the vibration is normal at 50 HZ and is also why power transformers run hotter on non 60hz power for which they where made!
     
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  9. Shinbones

    Shinbones Strat-Talker

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    OK then, I reckon hum-wise it's as good as its going to get. Thanks for your help @Stevem.
     
  10. fezz parka

    fezz parka fezz parka Strat-Talk Supporter

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    Single ended amps are more susceptible to power supply hum. Nature of the beast.
     
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  11. rocknrollrich

    rocknrollrich Senior Stratmaster

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    Absolutely correct.
    Single ended amps don't have the phase cancellation "benefit " that push pull amps do.
    All of my single ended amps are noisy compared to my push pull amps.
    It took me a little time to accept it.
    But it is completely normal.
    And spot on about over filtering a tube rectifier. It's not done, because it makes for an unreliable power supply.
    I'm sorry I have nothing of my own to contribute, but stevem covered it pretty well.
     
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