Kalamazoo Amp Field Guide:

Modifying Guitar Amps for Safety

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 GUARANTEE

These 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!

De-zapping your amp

Before messing with your amp's guts, you should drain any high voltages left laying around. These voltages are generally stored in the power supply capacitors. These are the big capacitors with values generally higher than 5uF, even in wimpy amps. Original power supply caps in older amps will have more than two wires (usually 3 or 4) coming out of one end.

The standard method for draining voltage is to hook a jumper cable (wire with alligator clips) from the power tube plate (Tube V2, pin 7) to ground. (Note: thin wires may melt!) Leave the wires hooked up for a few seconds, longer can't hurt. If you have a voltmeter (VOM, DVM) you can check to see when the voltage gets to 0. You should then use the jumper to ground each preamp tube plate for a few seconds (Tube V1, pins 6 and 1).

If you're in a hurry, you can always jumper the power supply cap leads to ground. Use a large screwdriver and keep your fingers and the rest of your self clear; this will throw a hefty spark. It's not the best way to discharge the cap, but it works.

Make it Safe to Play (Grounding)

If you don't do anything else to your old tube amp, do this crucial safety mod!

The Kalamazoo (like most, old amps) is sensitive to which way it's plugged in (no ground polarity switch, capacitor from one side of line cord to chassis). It is a most excellent idea to add a newer, grounded line cord (three wires, three plug prongs).

These instructions will work for almost any, old guitar amp, except that the switch and fuse may be located in other spots, and the power lamp may not be hooked to the AC line. The concepts are the same for a Reverb 12, but the layout is very different. I'll get pictures up on that when I have a chance.

  1. Lose the old cord
    With the unit unplugged and the filter caps discharged, cut or desolder the old, two-wire, line cord from the fuse and switch. While you're at it, get rid of the cap from the switch to chassis ground (leave that ground lug, though!) Save the cap; if it's still in good shape you may be able to use it for something else. Pull the old cord out through the hole in the chassis. If you can't get the strain relief to let go of the cord, you'll need to cut it.

    [click for image]

  2. Hook up the new cord
    NOTE: If you use a computer power cord or other cord based on the international wiring color code, BROWN = HOT, BLUE = NEUTRAL, and GREEN/YELLOW STRIPE = GROUND. For standard power cords in the USA, BLACK = HOT, WHITE = NEUTRAL, and GREEN = GROUND.

    Run the new cord (with strain relief on it) through the hole the old line cord used. (Ream the hole out if necessary.) Solder the green wire to the ground lug from which you disconnected the capacitor. Connect the black power cord wire to one lug of the fuse. Run a wire from the other fuse lug to one lug of the power switch. The other power switch lug is connected to one of the lamp leads, and one of the primary (black) leads of the power transformer. The other power transformer (black) lead and the other lamp lead should go to the white power cord wire. Slide some heat shrink tubing over the white wire, solder this last set of leads together and apply heat to the heat shrink tubing. (In the drawing, the blue splotch where the white wire meets two black wires represents where the heat shrink will go. While heat shrink is best, a wire nut is acceptable if the amp won't be moved much.)

    If you can't find a strain relief to fit the cord, there are a couple of alternatives. Make sure the edges of the hole are as rounded and edge-free as possible, and either:

    1. put a grommet in the hole, run the cable through, and fasten a wire tie tightly on the cord on each side of the grommet, or
    2. as a last resort, tie a knot in the cord inside the amp before soldering it.

Your Kalamazoo is now much safer to play.

Thanks!

Special thanks to Justin Belshe <JBelshe@AOL.com> (who would like to thank Tim Swartz <TimTube@AOL.com>) for lots of ideas on rebuilding this amp.

Last updated: 14 September 2005

Copyright Y2K 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