These instructions assume a basic ability level with a soldering iron.
Caution: Always remember to unplug your reverb tank before changing tubes or touching the electronics! Discharging the filter capacitors is also a good idea – click here for details on how to do this.
Note: This modification will void your warranty (if the reverb tank is still under warranty)!!!
DISCLAIMER: THIS UNIT USES DANGEROUS HIGH VOLTAGES THAT CAN SERIOUSLY HURT (OR KILL) YOU!!!!!! IF YOU ARE NOT FAMILIAR WITH WORKING WITH HIGH VOLTAGE CIRCUITS, HAVE THIS WORK DONE BY A QUALIFIED TECHNICIAN! THE WEBMASTER EXPRESSLY DISCLAIMS ANY LIABILITY FOR BODILY INJURY AND/OR PROPERTY DAMAGE RESULTING FROM THE USE OR MIS-USE OF THE INFORMATION PROVIDED IN THIS SITE. USE THIS INFORMATION AT YOUR OWN RISK!
Whew! Now that the disclaimers are out of the way, let’s get to it . . . .
General Overview
This modification involves replacing the Mixer capacitor C10 with a silver mica capacitor valued at around 390 pf. I used a 500pf capacitor (rated at 500V), because that’s what I had in my spare parts bin! Any value in between will probably work as well. The result is a deeper and richer reverb sound – it’s hard to describe, but I was very pleased with the results.
These instructions also provide some details on how to install a DPDT switch that will allow you to switch from the “stock” capacitor to the silver mica capacitor. After a few weeks of testing, I decided that I preferred the silver mica cap over the stock ceramic capacitor, so I removed the switch and permanently installed the silver mica cap.
The Specifics:
Step 1
Plug in your low wattage soldering iron now – it’s important to have a hot iron to make a good connection!
Lock the reverb pan and unscrew the back panel. Set the back panel and screws aside.
Step 2
Carefully place the reverb tank face down on a padded work surface (old carpeting works well).
Step 3
Unscrew the Tone, Mixer, and Dwell knobs. If you turn the knobs so that the indicator hash mark is at the 12 o’clock position, you will see the set screws. Notice the set screw marks on the pot shafts; this will help in placing the knobs in the correct position when you reattach them later. In this picture, you can see the indent the set screw has made on the Tone pot shaft.
Step 4
Remove the nuts holding the Tone, Mixer and Dwell pots to the control panel. Use a 7/16″ socket to minimize the chances of scratching the control panel.
Remove the nuts holding the Input and Output jacks to the control panel. These are held on by a 1/2″ nut; again, use a socket to minimize potential damage to the control panel. Be careful not to misplace the washers on these shafts – there are two of them (metal and fiber) on each shaft. Be sure to replace them in the proper order!
Step 5
To replace capacitor C10, you will have to get to the rear of the small circuit board that holds the Tone, Mixer, and Dwell pots and the Input / Output jacks. You can see capacitor C10 under the Mixer pot (the one in the middle).
The small circuit board is connected to the main circuit board by two ribbon cables, which you can see in the photo above. You will have to carefully pull the small circuit board towards you – it’s a bit tricky, as you must also angle the rear of the small circuit board upwards to clear the large 500 Volt, 47 uF cap (seen in the lower left hand corner of the photo). Take your time and be gentle! You don’t want to pinch or kink the ribbon cables.
Once you free up the small circuit board, you can rotate it so you can access the solder joints on the back. Here’s a large shot of the work area:
I am holding the corners of the small circuit board with a small heat sink and alligator clip that rest on the chassis – this provides a stable work area so the joints can be easily de-soldered.
Unsolder the joints holding capacitor C10 in place; you can use some de-soldering ribbon or a vacuum desoldering bulb to remove the solder after you have melted it with your iron. The capacitor can then be easily extracted by using a probe or some tweezers after the solder is removed (if it didn’t just drop out!).
Step 6
Insert the leads for your new capacitor in the circuit board holes, and solder them in place. Here’s a comparison shot of the “before” and “after” caps – the image is close to the actual size. My 500 pf cap is huge compared to the original 270 pf ceramic cap, but it fits just fine!
Since the leads on the 500 pf cap were wider than the original cap, a little bending of the leads was required.
Step 7
Put the small circuit board back where it belongs – again, be careful not to bend or kink the ribbon cables! Replace the nuts on the pot shafts, and don’ t forget to place the washers in the proper order on the Input and Output shafts! Don’t overtighten them!
Put the knobs back on – remember to look for the set screw marks on the shaft so you can position the knobs accurately. Reattach the back panel, and you’re done!!
Step 8
Congratulations! Enjoy your new modification! Don’t forget to unlock the reverb spring pan before testing – it’s easy to forget to do that!
Installing an A/B Switch
If you want to install a switch that will allow you to switch between the original ceramic cap and the silver mica cap, do Steps 1 – 5 above. Instead of wiring the silver mica cap directly to the small circuit board, you’ll want to wire up a two position double pole/double throw (DPDT) switch and hook the leads to the board.
Step 1
Get a small two position DPDT switch (most electronics stores carry these) and solder each leg of the ceramic cap to one side of the switch like this.
Solder two wires to the middle lugs of the switch (I used 22 gauge wire, but the size isn’t critical). These leads should be about 6″ long, depending upon where you want to access the switch. Since my hookup was just temporary, I let the switch hang down so it could be accessed through one of the cut-outs in the back panel. For permanent installations, you will want to fasten the switch somewhere.
Repeat the process with the silver mica cap on the opposite side of the switch. Here are some views of the completed switch unit:
The right photo shows the switch selecting the silver mica cap; flipping the switch down will select the ceramic cap.
Step 2
Solder the leads to the connections for capacitor C10 on small circuit board as shown here:
Reassemble the unit as detailed above.
What Does the Mod Sound Like?
To get an idea of what this mod sounds like, check out The Nebulas live! They have done this mod to a couple of their reverb tanks. If you haven’t heard these guys, check them out – they are awesome!