Billm's Bright/Dark Switch for Blues Junior

Or should it be a presence control? See below. Although most people want to brighten the tone of green-board Blues Juniors to match the crispness of current cream-board amps, there's something to be said for the old tone, especially after the tone stack mod. It has a rich, creamy, warm sound that you never hear from the new amps, and it's worth preserving.

No single component has more effect on this overall tone than the capacitor C35, which connects from the input side of the master volume to ground. The stock value, in both the green and cream circuit boards, is 1500pF. The amps sound radically different because of the circuit board layout--differences in capacitive coupling give each amp its own distinctive voice.

Below is my approach to voicing the green board to sound more like the cream board, but with an option to retain the original tone. I used a subminiature single pole-double throw toggle switch, which is mounted on the control deck between the master volume and mids controls. One side of the switch has the original 1500pF of capacitance, and the other has a 330pF cap. The tone difference isn't earth-shattering, but the bright position definitely has more sizzle.

Build the switch assembly outside the amp. Solder one leg of the bright and dark capacitors to their respective outside lugs of the switch. Solder the center conductor of thin shielded hookup wire to the center lug. Solder the remaining capacitor legs to the shield; keep your connections short and neat.

Solder the center conductor of the hookup wire to the left hole of C35; solder the shield into the right hole.

	The switch is the smallest single pole-double throw toggle I could find. It needs to be close to the master volume because that's where the capacitor it controls is located. Center-punch the location and drill carefully so you don't scratch the chrome. Place aluminum foil, a rag, or paper towel below to catch metal chips.
	I wired the switch to the capacitor location with a short piece of shielded hookup wire The shield side goes into the right hole of C35; the center conductor to the left for noise and hum control. In this picture I have a 1000pF and a 500pF soldered to the "dark" side and a 500pF soldered to the "bright" side, but 330pF sounded more like my cream-board Blues Junior.

If you just want brighter tone for your green board BJr, you don't have to install a switch. Just replace C35 with a lower value. I started with 500pF, but my cream-board BJr was still brighter. 420pF was brighter, but 330pF sounds closest to to the new Blues Junior and helps you cut through the mix.

Update: I've stopped doing the bright switch alone on green board Blues Juniors. As long as you're going to make a hole in the control deck, you might as well add a presence control. For green board Blues Juniors, I build the presence control with a pull-up switch to control the bright switch. The presence control increases the high mids, while the bright switch kicks in at the very top of the frequency range. I use dipped silver mica capacitors for an extra edge in brightness. The presence control also makes the output stage looser, louder, and more responsive. It takes the tone into the brighter blackface range.

The cream board Blues Junior benefits greatly from a presence control too, but there's no reason to do the bright switch--the amp is plenty bright without it.