Kalamazoo Amp Field Guide: Converting a Reverb 12 to Three Wire Power Cord
DISCLAIMER:Tube amplifiers contain potentially lethal, high voltages even after they are unplugged, that may cause personal injury or death. Do not attempt to repair, modify, or work on any amplifier unless you are absolutely certain you know what you are doing.
NO GUARANTEEThese mods are all things I have tried, someone I know has tried, or are recommended by people who work on amplifiers for a living. Nevertheless, if you try any of these, you assume all responsibility for anything that happens, whether the amp explodes, you get zapped, or the amp suddenly increases in value because everyone falls in love with it. The glory, the pain, whatever, they're all yours. If you can't live with that, don't mess with the amp!
First let's look at the original wiring. There are several things to note.
- Most of the wiring goes to a terminal strip, though one power switch lead goes to the fuse.
- The power switch wiring runs by signal wiring - this is bad practice which results in hum and noise being coupled into the signal wires an components.
- There is a two wire outlet (unseen in the photo) on the back of the amp; this should be removed in the three wire version, and a small slide switch fits perfectly here.
The following instructions assume you remove the old outlet and put a slide switch in its spot as a new power switch. If you prefer to continue using the old power switch, you may do so, but your amp will likely be noisier than it should be. In this case you should replace the old outlet with a newer, three wire outlet, or at least disconnect the old one.
Step by Step
- Unplug the amp. Never work on an amp's power wiring while it's plugged in!
- Remove the fuse. Otherwise, heat on the fuse holder terminals may weaken the fuse.
- Desolder the wires from the fuse holder.
- Desolder all wires from the terminal strip except the blue one. The blue wire is a ground wire and needs to stay there.
- Desolder the death cap from the terminal strip. (You no longer need it.)
- Desolder the wires from the power switch (on the treble pot).
- Pull the power switch wiring out.
- Desolder the wires to the outlet on the back of the amp.
- Drill or punch out the rivets holding the outlet in place.
- Remove the outlet. (You no longer need it.)
- Remove all excess solder from the terminal strip.
- Remove the old, two wire power cord (line cord). Using pliers (preferably duck bills, but any good, wide pliers will work), grip the strain relief inside the chassis so that one jaw of the pliers is on the small part by the cord. Squeeze together and push down against the chassis with the pliers. When the strain relief starts to go through the chassis, you may have to let go with the pliers and push by hand, or try again with the pliers, but with a bit more height above the chassis. Remove the cord and strain relief. (You no longer need these.)
You have now undone the old work, time to start on the new!
- Insert the new strain relief or grommet. The three wire cord will be larger than the two wire cord you removed. If you;re using a strain relief, you'll need to make a larger hole. I prefer to insert the largest grommet that fits the hole, and use wire ties to keep the cord in place (described below). If you use a strain relief, insert the cord into the strain relief first, then place the strain relief in the hole. (Make sure you leave the right amount of cord after the strain relief!) If you use a grommet, make sure the power cord fits into it, then pull the cord out, insert the grommet into the hole, and then insert the power cord into the grommet. Pull the cord through until you can touch the tip of it to the fuse holder's terminal closest to the front panel, with the cord as you wish it to end up. Holding the cord in place, put a small cable tie around the cord inside the chassis, next to the grommet, tighten the cable tie thoroughly, and snip the end off. Repeat with another cable tie outside the chassis.
- Strip the power cord's outer sheath to a point just short of the terminal strip (see photo).
- Strip the end of the power cord's hot wire (black) and solder it to the fuse holder terminal strip on the end of the holder.
- Strip the end of the power cord's neutral wire (white) and insert it into the rightmost terminal strip. Insert one of the power transformer's primary leads (black) into this same terminal, along with one of the lamp leads. Solder these.
- Bolt the new (slide) power switch into the holes vacated by the power outlet on the back of the amp. You could use any type of switch, but you can buy slide switches that fit perfectly. Use a switch rated for 250V at 1A.
- You'll need two pieces of 18 gauge wire, or a 10 inch piece of two wire cable (such as your old power cable!) to connect the switch. (I used yellow and black wire from another project.) Connect one pole of the switch to the other fuse holder terminal with one of these wires. Connect the other wire to the left-most terminal of the terminal strip. The other lamp lead and power transformer primary lead (black) also go here. Solder these down.
- Strip the end of the green wire of the power cord. Solder this to a ring connector that fits one of the power transformer bolts. Remove one of the power transformer's nuts and washers, put the ring connector over the bolt, replace the washer and nut, and tighten them down good. This is your safety ground; get it right! If you want to use Lok-Tite[tm], nail polish, or something else to hold this nut in place, go for it. Just don't let anything get between the ring washer and the chassis!
- Replace the fuse in the fuse holder.
Now check your wiring. Hook a meter across the flat prongs of your new power cord's plug. With the switch off, you should have an open circuit (infinite resistance). With the switch turned on, you should see a couple of hundred ohms. Leave the switch on but remove the fuse. You should see an open circuit again. Turn the switch back off. If you saw behavior other than described here, you have a problem; check your wiring and solder joints (and your fuse).
Connect one of the meter's probes to the ground lug of the power cord plug. Connect the other probe to each of the other prongs in turn. You should see an open in each case. If you see anything else, you ave a safety problem.
Last updated: 01 September 2005
Copyright 2004 Miles O'Neal, Austin, TX. All rights reserved. Miles O'Neal <roadkills.r.us@XYZZY.gmail.com> [remove the "XYZZY." to make things work!] c/o RNN / 1705 Oak Forest Dr / Round Rock, TX / 78681-1514