6 October 2013

JCA22H Crunch/Clean and Bright Switches

Next up! Adding the SLO100 style Crunch/Clean and Bright/Normal switches to the Crunch channel of the JCA22H.

These work by dropping in place of R5 which is just to the side of the Crunch channel gain control of the amp. The output of C2 feeds into the On-On DPDT switch via the purple wire on the left. When that switch is down the signal goes through that brown 475K resistor mounted vertically on the switch and then goes across to the next switch. This switch is an On-On SPDT with another 475K resistor. The signal passes through this second resistor and back to the DPDT were the white cable feeds the signal back to the PCB and to the Crunch gain pot. At this point there is an added 39K resistor connected between the signal and ground also by the DPDT switch. The two 475K resistors in series and the 39K resistor to ground form a "potential divider". This potential divider decreases the level of the signal going into the Crunch gain control. Reducing the gain of the channel at that point. 

When the switch is flipped up into Crunch mode the first 475K resistor is bypassed and the 39K resistor is no longer connected to the signal. This means there is now only a single 475K resistor in series with the Crunch gain control and no potential divider. Increasing the gain of the channel. 

The bright switch is very simple. It switches in and out a 470pf capacitor across the second 475K resistor. That resistor is always in circuit unlike the other resistors. The capacitor creates a way for high frequencies to bypass the resistor so the tone is brighter. Simple! 

This idea was directly stolen from the SLO100. If you are going to take ideas from any amp... you may as well do it from the SLO. Enough other people have!

The last switch on the right is the bright switch for the Overdrive channel. All this does is switch the bright cap that is usually across the pot (C6). I swapped both gain pots for 500K's so the bright cap in my amp is 1000pf. When the switch is down the amp is in its 'normal' mode so the capacitor is allowing high frequencies to partially bypass the Overdrive gain control. When the switch is up the cap is disconnected. In the SLO100 this is called the switchable Haynes mod.

This Bright switch is different to the other. The other bright switch is across a fixed resistor which means in effect it always works. The Haynes mod however doesn't. As you turn the gain pot up and decrease the resistance the signal is passing through, there will be a point where the frequencies not skipping through the cap will really be just as loud as the ones that are. At this point the cap is no longer adding any benefit to the highs, so switching the cap out will not dull the amp. For this reason this kind of switch works better at lower gain settings where the effect of the cap is much more noticeable.

The other DPDT switch we will leave for another day!

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