Here is a list of the tools you’ll need to get through this job:
Electric drill or Screwdriver with Phillips head
And here are the caps and the resistor you’ll need:
33K Carbon Comp Resistor
Before we dive in, check out the sound clip below of what my normal channel sounds like before I make the mod. I am plugged into input 1 with the volume on 3, the treble on 6, and the bass on 4.
Let’s get started! First thing’s first, we need to use our drill or screwdriver to remove the top back panel from our cabinet. Once this is done, use the drill or driver to loosen the chassis screws and remove the chassis so you can access the tone stack on your normal channel.
Next, before we get too excited we have to drain the filter caps so that we can safely work with them. There are a couple of ways to do this, but the way we’ll do it today will involve using those alligator clips we talked about earlier. Connect one of the alligator clips directly to the chassis, and connect the other to the highlighted point in Image 1A below.
Note : Either of the highlighted leads or the actual solder point will work for this, but it is easier to get the alligator clips to bite onto one of the leads.
Before we go further, just to show exactly what we are doing, let’s use our multimeters to see what kind of voltage is in our caps before we drain them, and then verify that they have been properly drained; then we can begin tinkering around with the circuit. Grab your multimeter and place the ground pin on the chassis. Then place the hot pin on the point circled in red in Image 2A below (this is essentially the same place your alligator clip should have been placed).
Note : While working on your chassis, orient it so that the front panel is facing away from you. This will put all the caps and resistors we will be working on, on the far right of the chassis. Now you should have your bearings straight, so take a look at the image below…
My initial test shows around 80V on the multimeter...we will definitely want to drain these caps before digging in. First, in order to safely drain the caps, we must make sure the amp is NOT plugged into the wall. Next, we need to flip the STANDBY switch to the ‘on’ position (do not flip the POWER switch, this is very important). Wait about 30 seconds with the STANDBY switch flipped ‘on’ and then reapply your multimeter to the ground and hot points discussed above. You should notice a significant drain in the caps at this point. Once it gets down to zero (as you can see in Image 3A below), the circuit is safe to work on.
Note : It is best to leave the alligator clips in place and the standby switch flipped on throughout the duration of this modification, so that the caps are constantly being drained and there is no risk of them recharging themselves.
Now we can start with our serious tinkering. Let’s begin with the .1uF cap highlighted in Image 4A below.
Heat the eyelet closest to the front panel of the amp, and use a pair of needle nose pliers to pull the lead out of the eyelet (Image 5A).
Once this is done, do the same thing with the .047uF and the 250pF caps highlighted in Image 6A below. For now we are only removing the leads in the eyelets closest to the front panel of the amp.
Now let’s do the same thing where the .047uF, .1uF, and 100k resistor meet. This point is highlighted in Image 7A below. This time we will be removing all three leads from the eyelet. The result is pictured in Image 8A below. (Both caps should be completely freed and removed from the circuit, while the resistor is still connected by its other lead).
At this point we are free to finish removing the 250 pF silver mica cap, as well as the 100k resistor. After you’ve done this, your circuit should look something like the one in Image 9A.
Now it’s time to use our desolder pump to remove the old solder from the eyelets. We do this for a couple of reasons : One, this makes it easier to insert the leads of our new components into the eyelets, and two, it's best to have a nice fresh solder joint...for posterity’s sake.
To “arm” your desolder pump, press the top trigger down (this may not apply to all desoldering devices, so please refer to the literature that accompanied yours). Then we will heat the solder in one of our eyelets. Once the solder is hot and fluid, push the button to release the pump and suck the solder away from the eyelet (Image 10A). Repeat this process on all the eyelets from which you removed a component.
Now it’s time to start replacing those components we removed with our new ones! Start by replacing that 250pF cap with our 500 pF silver mica. To do this, you’ll want to hold the cap over the eyelets and bend the leads accordingly. Once they are bent, you can eyeball the length you’ll need for a good connection, and trim them to length with a pair of wire cutters (Image 11A). Once you have a good length, insert the leads into their respective eyelet holes and solder them in (Image 11B).
Continue doing this with the rest of the caps and the resistor. The .047uF and .1uF caps will be replaced with our Mojotone Dijon .022uF caps, and the 100k resistor will be replaced with the 33k resistor. Be mindful to insert all of the necessary components into each eyelet before soldering, so that a solder joint is only made one time per eyelet.
You should now have a circuit that looks something like Image 12A…
Before testing out your new normal channel, make sure you remove the alligator clip you applied before the start, and flip the standby switch back to the 'off' position. Now you can plug the amp into the wall, flip on the power switch and let it warm up for a second. Then, as you know, flip the standby switch and start to rock.
Below are two sound clips: the first is a standalone clip of what my amp sounds like after the mod, and the second is a comparison clip beginning with 'before' the mod, and ending with 'after' the mod (the same amp settings are used in both clips -- volume 3, treble 6, bass 4).
I can already tell a difference in the treble response even with the same settings. It also has a little more break up than it did prior to the mod. And once I started messing with the tone controls I can really tell how differently they interact with one another.
We hope this mod makes for a nice couple hours of electronics work and many many hours of tweakable playability! As always, thanks for tuning in and we’ll see you next time!