Tone Control Bypass Mod For Vibro Champ Amp

The Vibro Champ is done.  I finished it up with a couple of mods.

The main mod is a switch to bypass the tone controls.  It's well known that black- and silver-panel Fenders (1963 onward) have a tone control circuit that 'scoops' midrange and reduces gain.  A popular mod is to bypass the tone controls - to get a gain boost and to make the tone more midrangey like a tweed Fender amp.

I didn't want to give up the stock tone, which is great for cleans and moderate distortion.  But I love the tweed tone as well.  After studying the schematic, I cooked up a way to wire a push-pull DPDT switch to get either tone.

The picture above shows a new 1 meg volume pot with a push-pull switch on it.  I don't drill holes for mods if they can be avoided.  This pot let me keep a stock look, but get the added functionality.  I wired the switch as a subassembly and then installed it in the amp.

Here's a notated picture that explains the mod.  (You can click for a larger version). 

The input comes through the input jack on the top right via the shielded cable to the first triode of the preamp tube. This is stock, and you can see that wire going to the tube.  Right near the input (the grid of the tube), there is a white wire that runs to the circuit board.  That's the plate - it gets DC voltage via the white wire, and also passes the amplified signal to the circuit board - see the bright green arrow.

Here's where the mod starts.  In the stock amp, the signal then goes to the tone stack.  At the point where the white wire joins the board, it connects to a 100K 'slope' resistor and a 250pf cap.  The cap is the small black one.  (Changed the stock ceramic cap for a silver mica).

We want to split the circuit here so we can use the switch to choose the stock circuit, with tone controls, or a 'tweed' circuit that bypasses the tone controls.  What we need to do is lift the stock wiring to the tone circuit up off the board.  Again, see the red arrow in the picture.  The resistor and cap are still together, but they don't connect directly to the white wire as they did originally.  Instead, they go to a new shielded wire that goes up to the bottom left tab of the switch.  See the red arrow on the switch.

The point where the white wire connects at the green arrow now runs through a new shielded wire up to the center tab of the switch.  (See the green arrow on the switch).  So what we've done is just split that connection so that when the switch is pushed in, the signal passes through it like usual and thus goes through the tone circuit.

The switch has internal connections between the center lug and the top and bottom lugs on each side (the sides are independent) that are connected depending on the position.  With it in, the center and bottom lugs are connected (have continuity).  With it pulled out, the center and top lugs are connected. 

Now we have a point on the switch - marked by the green arrow - where the amplified signal comes out of the tube, and is bypassing the tone controls (whoo hoo).  We want that signal to go though a coupling cap, through the volume control and back to the second triode of the preamp to be amplified again before it ultimately goes to the output tube.  Just like a tweed Champ is wired.

So we just put a .02 uF cap across the switch at the top.  When the switch is pulled out, the signal is directed up to the cap, and then over to the other side of the switch, and to the center lug of the switch.  From the center lug, a green wire (blue arrow) runs up to the 'input' side of the volume control.  Make sense?

On the same (right side) of the switch, the bottom lug goes to the center ("output") lug of the treble control (white arrow in the picture).  This is the stock wire that formerly ran to the input of the volume control.  With the switch pushed in, the signal coming in from the treble control goes to the volume.

It's harder to explain in words than it is to visualize.  You can trace the signal from the input and then "see" where it goes in the picture with a little vizualization.  Switch in, stock circuit.  Switch out, tone controls bypassed.

And with them bypassed, the amp becomes a little monster!  It's got TONS of gain.  Really great for blues or rock tones.  And since it's a Vibro-Champ, you can have tremolo on that overdrive to boot.

The other thing I did was to tie all the preamp grounds together to a big (10 gauge) cable and run that to one point.  That's the dark blue arrow - I hadn't soldered it to the chassis in this picture.  That helps cut a little noise. 

The amp had the grid lead that goes to the output tube twisted together with the B+ line that runs to the preamp tube.  Bad idea.  I just untwisted them, and I relocated the coupling cap to a position right on the output tube.  And also shortened up a couple of other leads - including the ones to the preamp tube.

Here's the Weber 8A-125 speaker installed.  It's a close fit, but it works. 

This thing has a lot of bass and mids for an 8 inch speaker.  It really transforms the amp.

Here's the little monster sitting innocently on the workbench for a final test.

It's crazy loud for such a small amp, and it's great to have the stock and tweed tones available with the pull of a switch.