How can I wire up a 3-position bright cap switch for my amp?

[msb69]

I have a Vox AC15CC amp, which has a 120pF bright cap across the volume control.  I would like to add a toggle switch to my amp so that I can select between (1) the 120pF cap, (2) a 68pF cap, and (3) no cap at all (i.e., just as if the bright cap were removed from the circuit board).  What kind of switch do I need, and how should it be wired?

For ease of reference, let's say that the 120pF bright cap is soldered to pads X and Y on the amp's circuit board.

My first step will be to remove the 120pF cap, leaving empty holes at X and Y.

The question then becomes where my switch lugs all connect to . . .

Can anybody help?

[kurtlives]

The easiet way to do it would be with a rotary switch.

Connect the common lug of the switch to X. Wire one lead of the caps to open lugs on the rotary switch. Then connect all 3 leads together and run a wire from all the caps leads to Y.

[Mick Bailey]

A SPDT switch with centre-off (sometimes described as 1-0-1) will work to give you the three positions. Make sure it is NOT specified as momentary in any position. Wire one leg of each cap to the outer terminals of the switch and join the other two legs together as the common connection which goes to one side of your volume pot. Connect the centre lug of the switch to the other side of your volume pot.

Now you have three positions with centre being off - no cap connected, and either side being 120pf or 68pf respectively.

[Stu Diddly]

Hey msb69,

Let me know how this works.  I also have a Vox AC15 CC and I'd like to try this to.  You should post some pics after doing the mod.  What is your purpose for doing the mod?

[Mick Bailey]

Correction - "Connect the centre lug of the switch to the other side of your volume pot" should read "Connect the centre lug of the switch to the wiper (centre lug) of your volume pot"

I overlooked your original X/Y reference, which if I re-phrase my original instruction would give;

Wire one leg of each cap to the outer terminals of the switch and join the other two legs together as the common connection which goes X. Connect the centre lug of the switch to Y.

Apologies for the confusion.....

If it still isn't clear I can mail you a diagram.

[msb69]

Correction - "Connect the centre lug of the switch to the other side of your volume pot" should read "Connect the centre lug of the switch to the wiper (centre lug) of your volume pot"

I overlooked your original X/Y reference, which if I re-phrase my original instruction would give;

Wire one leg of each cap to the outer terminals of the switch and join the other two legs together as the common connection which goes X. Connect the centre lug of the switch to Y.

Apologies for the confusion.....

If it still isn't clear I can mail you a diagram.

Thanks for this info.  Is this what you are suggesting:



Also, will this switch work:

http://www.radioshack.com/product/index.jsp?productId=2062486&cp=&sr=1&origkw=spdt+toggle&kw=spdt+toggle&parentPage=search

[Mick Bailey]

Yes - spot on. The switch also looks correct.

[Mick Bailey]

There's another way I switch a cap to give a top-boost/normal/top-cut arrangement to give a more extreme range. This switches the capacitor from the signal side of the pot (X) to either the wiper (Y) or ground. This is quite good both on amps and effects units, where you want to cater for a wider range of guitar types and can cut some of the harshness out of overdriven sounds on single-coil pickups. I usually use a single 470pf cap, though this is in Marshall preamp designs and this value may be too high for your Vox. You might want to try this with a 120pf cap.

Anyhow;

Using the same type of switch, connect one leg of the cap to the switch centre lug. The other leg of the cap goes to X. Now connect one of the switch outer lugs to Y, and the other outer lug to ground (this will usually be the end of the pot opposite to X).

Either way, I'd be interested to hear how you go on. I've used both methods with excellent results.

If you want to save drilling your amp panel, you may be able to mount the switch on a small aluminium plate discretely mounted out of sight around the back (though I'm not familiar with your particular amp - my AC15 was a 60's one and they may be different nowadays). Caps can be mounted directly to the switch - just make sure there's no strain on the leads. To make a really neat job use heatshrink over the joints and tie wrap the wiring to the switch body.