Kalamazoo Amp Field Guide: Shielding

Kalamazoo Amp Field Guide:

Shielding an Amp

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!

Shielding inside your amp (and guitar!) can help alleviate most noise induced by external sources (fluorescent lights, motors, dimmer switches, TVs, computers, etc.) If your amp picks up noise when nothing is plugged into the inputs, shielding will help. If the noise only occurs with an instrument plugged in, the noise almost certainly involves your guitar or cable. (If your guitar has any non-hum-bucking pickups, you can get different noises by aiming it in different directions.)
If you just have a 60 cycle hum (low pitched hum), make sure the filter caps are good, and try moving the amp around. If the hum changes as you move it, any form of shielding should definitely help. If the hum stays the same no matter where the amp is or how you orient it, the problem is most likely somewhere in the circuitry; shielding some of the wiring may or may not help. Always try different tubes where possible before spending a lot of time and money of modifying the amp. Some tubes are more prone to hum or picking up interference than others.
There are various approaches to shielding. First decide which overall approach you wish to take. If you go with shielding the compartment (making a cage) you'll probably want to do the whole thing. If you elect to go with shielded wire, you can go one step at a time until you are happy with the results.
Build a cage
One way to shield your amp is to line the area around the components with grounded, conductive material. You'll need to surround the component side of the chassis with metal. The component side of the chassis is the side that doesn't have tubes sticking out of it.
You can use aluminum foil and a good glue, or something a bit heavier (metal screening material will work for most things) and tack or nail it down, or buy adhesive-backed copper foil, which is made just for shielding. If you use glue, test some first on the same type[s] of material you will be attaching the foil to, making sure it will hold sufficiently. I have used Elmer's glue but there are undoubtedly better glues for this. It's not a bad idea to tack the foil down as well as glue it.
After installing the shielding, you need to ground it. Ideally, run copper wire or a ground strap under the shielding, and tack the shielding down over it in several places. If the shield material is heavy enough, you may be able to solder to it, but this may be messy. You could use a washer with a solder lug, screwed down tightly, as long as the cabinet material will hold it (not the best idea on a particle board cab like a Kalamazoo).
Finally, if the back of the cab over the component area is removable, use your grounding attachment scheme to attach a piece of insulated wire long enough to allow easy removal of the back. You probably want to use a ground lug washer here, with a slip on terminal on the wire for easy wire removal.
Shield that wire
As an alternative to shielding the entire amp compartment, you can simply shield critical wiring. Use a good quality wire for this. You can sometimes salvage shielded wire from organ and juke box amps; otherwise just buy some good shielded wire. I prefer foil-shielded wire, but you want the kind with a wire running through the shield as well. This wire is what you solder instead of the foil. Foil is a pain to solder and may not stay soldered. If you don't use foil, I recommend wire with a woven shield. A purely wrapped shield is my last resort, the shielding can open up too easily when you bend the wire. In a pinch you can use guitar cord, but it's usually fat, difficult to work with in tight spaces, and cheap ones have lousy shielding capabilities, anyway. Cheap cable will also probably result in degraded treble tones.
It's not a bad idea to tape or heat sink the end of the shield to make sure no stray, grounded shield wire touches anything else.
The most important shielded wire is usually from the input jacks to the first preamp tube. If the amp has input resistors like the Kalamazoo and most other amps, you can do either of the following procedures.

Replace the resistors with shielded wire, grounding the shields at the jacks. This will result in slightly better tone, but you probably never want to use both jacks at the same time if you do this. (The resistors act as passive mixing devices to isolate the inputs from each other.)
Cut lengths of shielded wire to match the resistor leads, remove the center conductor (but not the insulation), slip the remaining cable over the (unsoldered) resistor leads, and ground one end of each piece of shielding after soldering the resistor leads back in place. The jack end of the shield should solder to the ring (ground) connector. The other shield can be grounded wherever convenient, or you can solder a piece of wire to each of the shield ends closest to the resistor. Be very careful that the shielding (and jumper wire, if any) cannot make contact with the resistor leads, as that would ground out the input, rendering it useless.
Remove the input resistors. Resolder one end to the jack, but with the resistor very close to the jack. Solder the center conductor of the shielded wire to the other resistor lead, very close to the resistor body. Cut off the excess lead at both ends, ground the shield at the jack, and connect the other end's center conductor to the preamp tube socket where the resistor was. You should definitely use a heat sink for soldering close to the resistor body (hemostats should be fine).

If this isn't sufficient, you may wish to try shielding one or more of the following component[s] and wire set[s].

Shield the wires to the volume and tone controls. Wiring to ground (usually black or brown) needn't be shielded. If capacitor leads here are very long, you might shield them as well. Longer wires definitely require good quality, low capacitance cable to avoid bleeding off highs to ground through capacitance between the wire and the shield.
Shield the filament wiring. If this is run as twisted pairs, you can use good quality, shielded mic cable (make sure the inner conductors are large enough). It might be easier to pacify this sort of hum with DC.
Add shielding to more of the circuit, starting with the preamp and moving toward the output stage.

Last updated: 23 March 2006
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