Princeton Switch on Tweed Champ Increases Volume - Why?

Discussion in 'Shock Brother's DIY Amps' started by itsGiusto, Jul 20, 2020.

  1. itsGiusto

    itsGiusto Tele-Meister

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    I recently added a "Champ - Princeton" switch on my homemade 5F1. It basically takes the amp from being a 5F1 to a 5F2A by adding in a tone control. See Sw2 in this schematic.

    [​IMG]

    It sounds great, and I'm very happy with it.

    The only thing that's weird that I don't understand is that when I switch it on, it actually boosts the gain of the amp, especially if the tone knob is set in the upper 1/3 of its range. So strange, because I always thought the old adage for these passive tone controls is that they're purely subtraction, and will cut gain. But there is a distinct volume boost. Can anyone explain why this happens?

    Note: I know that one of the differences between the 5F1 and 5F2a is the bypass cap on V1A, which CAN cause a difference in gain. I have this bypass cap switchable, but not on the same switch, so this difference is not responsible for the gain boost.
     
    Last edited: Jul 20, 2020
  2. Nickfl

    Nickfl Friend of Leo's

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    I would guess that what you were hearing is the effect of the bright cap on the tone control when you switch it into the circuit. In 5F1 mode you have no bright cap, so the volume control is attenuating all the frequencies to some degree once you switch it to 5f2 mode that bright cap is letting some of the high-end through regardless of the position of the volume so those frequencies get louder. Its similar to the effect of a Tweed volume control on gain in general, the higher the treble is turned up the more gain the amp seems to have.
     
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  3. itsGiusto

    itsGiusto Tele-Meister

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    Ah, yeah that's cool! So I could test this theory if I completely dimed the volume. If I did that, then maybe the switch-in tone control would have a subtractive effect, or at least would not have an additive effect.
     
  4. James Knox

    James Knox Tele-Meister Ad Free Member

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    Cool!

    Where did you put the switch on the 5F1 Chassis?
     
  5. itsGiusto

    itsGiusto Tele-Meister

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    Prepare to be impressed or horrified!

    [​IMG]

    You can see there are a lot of mods I've done to this amp! All the ones to the right of the volume knob were done a few years ago, varying values in NFB, varying V1A coupling cap value, and varying V1A and V1B bypass cap values (the arrows on the labels indicate the switch positions to get the stock champ sound).

    For the tone control, last November I moved the fuse down to a new location in a new hole I drilled to the left of the rectifier tube, and I put the tone control in place of the fuse. At first I had a push-pull 1M audio pot for that which turned it on or off, but I realized that the audio taper was not good for the tone control - absolutely no change in tone until the last 1/8 of the knob's range. I looked around but could not find a 1M push-pull linear pot, so I just used a normal 1M linear pot and drilled a new hole next to the knob to put the switch to switch in the tone knob.

    To avoid any potential problems with interference from the power supply, I used shielded wire for the wires going to the Champ-Princeton switch.
     
    Last edited: Jul 21, 2020
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  6. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Passive tonestacks always have a subtractive effect. Two versions can/will have a different attenuation. One of the two settings will then be louder than the other, but still less loud than if they were absent.
     
  7. James Knox

    James Knox Tele-Meister Ad Free Member

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    Love it Man! Impressed...

    I’m with you, make it the way you want it. I’m currently building my second Champ. I also have move the Fuse down my the Rectifier tube. I put a regular Carling on/off switch in it place. I didn’t like the on/off/Volume knob. Big source of noise and just turning the amp on and you are already at 3 on the knob. I’m also ditching one of the instrument inputs. I don’t need one that is 6db softer when I can’t just turn the volume down. I putting a NFB Pot there.
     
  8. D'tar

    D'tar Friend of Leo's

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    Now that's 10Lbs in a 5 Lb chassis!!!
     
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  9. jtcnj

    jtcnj Tele-Holic

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    Nice!
    That tone lift is one last mod I have been meaning to add.
    All these Champ threads lately I think I have to shelve the 5e3 and fire up the Frankenchamp this evening.

    "It's alive, it's alive!!"
     
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