Kalamazoo Amp Field Guide:
Basic Gain/Distortion Mods in a Tube Guitar 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 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!
The mods:First off, are you sure the amp is capable of doing what you want? Is it worth more unmodded? Some times you're better off to start with a different amp. You won't turn a Champ or Kalamazoo into a Mesa Boogie 50 Caliber. But if you want more clean headroom, or earlier or later breakup, these mods can help.
Cathode Resistor Bypass CapA standard, cathode-biased tube stage has a certain amount of negative feedback built into it. This reduces gain and tames distortion a bit. You can mitigate these factors by adding a cathode resistor bypass cap. The bigger the cap, the less the negative feedback, the greater the gain, and the greater the distortion capability. Past a certain point, though, the distortion may get ugly. A large cap that works in a hi fi rig may not be appropriate for guitar. (OTOH, Marshall used up to a 350uF bypass cap on their first stage, so feel free to experiment.) A bypass cap increases perceived bass response, and reduces hum to boot. I usually prefer something between .068 and 1uF on gain stages, but 25uF to 50uF (or even 100uF) for a single ended output tube stage such as the Model One and Model Two have. This topic is also covered in the page on hum mods (http://www.rru.com/~meo/Guitar/Amps/Kalamazoo/Mods/hum.html#crbcaps).
Use a non-polarized (non-electrolytic) cap if possible for a better tone. I know, this sounds like cork-sniffing, but try it yourself. A non-polarized cap really sounds better here; the polarized caps add a subtle form of distortion since they react differently to positive and negative pulses in the waveform. Up to a 1uF or so cap is easy to find in your favorite flavor; above 1uF they start to get big rather fast, and expensive as well.
Headroom/Breakup Changes (Load Resistor)Increasing the load resistor (or plate resistor) value results in a lower plate voltage. This in turn yields earlier breakup and higher gain. Decreasing the value of this resistor raises the plate voltage, resulting in more headroom and lower gain. Don't decrease the value too much, or you end up with feedback through the plate resistors to the previous stage, unless you decouple the stages further through additional power supply filtering.
Last updated: 23 March 2006
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