5f1 Champ too bright

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by gregory347, Jan 26, 2012.

  1. gregory347

    gregory347 Member

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    Got a real nice tweed champ clone built that is just too bright when pushed. It has a 10" speaker in a slightly oversized cabinet. I have tried a few speakers and all sound good at lower volume clean to medium crunch. When I push the amp into overdrive (9-10 on the vol) it gets real bright and the highs are harsh and unpleasant. Can the voicing of the amp be tweaked or is this just the way the champ is supposed sound.
     
  2. thosk

    thosk Silver Supporting Member

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    Is V1's cathode resistor by-passed? The original Fender Schematic shows it un-bypassed, but every original 5f1 I've seen inside of has it by-passed with a 25/25 cap. My homebuilt 5f1 clone sounds just as you describe without the by-pass cap. I tried a few values(un-bypassed, 4.7, 10) but I like 25/25 the best. Increases the gain/volume as well.
     
  3. gregory347

    gregory347 Member

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    Thosk, thanks for the reply. I will check that out.
     
  4. wyatt

    wyatt Member

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    Make sure you are using a NOS 5Y3GT and not the Sovtek. The voltage drop on the latter is not correct and will drive up you plate voltages and make everything stiffer and harsher.
     
    Last edited: Jan 26, 2012
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  5. gregory347

    gregory347 Member

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    Thanks. Have a nos CBS 5y3 in it.
     
  6. guitarcapo

    guitarcapo Senior Member

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    Usually the bypass cap will increase gain and make the amp even brighter in my experience.
     
  7. stratman_el84

    stratman_el84 Member

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    One big factor in how bright/dark the amp is, are the type of coupling caps used. I've found that for myself and my ears, orange-drop caps make a Champ too bright. I prefer Mallory 150-series coupling caps in a tweed Champ style amp.

    Strat
     
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  8. phsyconoodler

    phsyconoodler Member

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    Champ and bright are not really used to being in the same sentence.Most 5F1 champs are dark sounding.I haven't built any that were too bright.
    Wonder what is going on inside that amp????
     
  9. thosk

    thosk Silver Supporting Member

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  10. gregory347

    gregory347 Member

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    installed g10 greenback and that helped with the harsh fizzy treble when overdriven. cant get good pics indoors, will try outdoors tomorrow
     
  11. gregory347

    gregory347 Member

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    Here it is and its big.[​IMG]
     
    Last edited: Jan 28, 2012
  12. guitarcapo

    guitarcapo Senior Member

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    I bet if you clip out that tiny black cap to the far right (leave just the resistor) it will darken up.

    You might also try disconnecting the negative feedback loop (the wire coming off the speaker) or increasing the rsistor that it's attached to.

    Weird because most Champs I have played sound dark, especially when you dial down the volume. I'm always trying to do tricks to get better cleans. Maybe you should check that your voltages aren't too high?
     
  13. Alvis

    Alvis Member

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    When I get done playing with coupling & bypass cap values ,speakers and all, I usually add a passive tone control to my tweed champ projects , mostly for cutting highs when playing harmonica (or teles.....)
     
  14. guitarcapo

    guitarcapo Senior Member

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    I have a tweed Champ that I've modded like crazy. I love the sound now. I ended up with bypass caps on every gain stage but only 3.3uF. I also put two extra toggle switches on there. One adds or takes away the first bypass cap. The other selects between two different treble bleed caps across the volume pot. I also disconnected the negative feedback loop...a few other mods...I think I altered the input resistors down a bit from 68K to 40K

    The amp has pristine gorgeous cleans when you turn down...but when you crank it, it roars with smooth buttery distortion. Lots of compression and pinch harmonics. Ireally should make a schematic because at this point it's the perfect class A 6V6 six watt amp. And it's so modded that it would classify as an original circuit.
     
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  15. gregory347

    gregory347 Member

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    How do I do this mod?
     
  16. gregory347

    gregory347 Member

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    I was thinking about putting a switch on the v1 bypass cap. Anything I should know?

    Also, can I put the negative feedback on a switch. If so what type/rating di I need?
     
  17. guitarcapo

    guitarcapo Senior Member

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    Adding a tone pot might take up some space and require drilling etc. You might consider replacing the volume pot with a push/pull pot. then all you would have to do is pull the pot to engage a small cap to ground somewhere on the signal chain to roll off your highs
     
  18. guitarcapo

    guitarcapo Senior Member

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    Very simple. Just run the bypass cap to a simple single throw single point toggle that engages it or removes it.

    You can do the same thing with the negative feedback or even add a pot in series to the resistor to add resistance, therefore slowly gradually removing it from the circuit. something like a 50K or 100K pot would work.
     
  19. jnickel

    jnickel Member

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    From the pic of the chassis interior it appears that the NFB circuit is disabled. The positive lead from the speaker jack should be going to a 22k resistor which is missing. Then it goes to the load resistor and back to pin 8 on the V1 to complete the NFB circuit.
     
  20. Alvis

    Alvis Member

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    I used concentric pots for volume /tone on the ones where I couldn't drill another hole
     

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