Kalamazoo Amp Field Guide:

Reducing Hum in a Tube Guitar Amp


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.


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!

The mods:

Before doing any mods (though I recommend all of these), make sure the caps are all in good shape. Bad caps, especially in the power supply, are the first place to check. If the hum volume remains constant regardless of where the amp controls are set, you almost certainly need to replace the PS caps!

DC bias on heater wires

Many amps try to reduce hum by grounding the center tap (CT) of the power transformer's (PT's) heater (filament) winding. This helps, but isn't always enough. And some PTs (such as that used in the Kalamazoos) don't have filament winding center taps. In either case, you can typically reduce hum quite a bit by floating a higher DC voltage onto the heater lines. (If the heater winding has a CT, you will need to lift it from ground for this to work. If it has no CT, check to see if one of the heater wires is grounded. It almost always is, especially in Kalamazoos.)

The trick here is to use a voltage divider to take get somewhere between 20 and 50 volts DC from the B+ (plate voltage). (Consult a tube manual to make sure you don't exceed the heater to cathode voltage differential. With most tubes, this will present no problem, but you should double check.) Take this voltage at the highest B+ point, the first capacitor after the rectifier. This yields the least hum from the heaters feeding back into the B+.

With a Kalamazoo Model One or Two, the B+ should be in the neighborhood of 250 to 275 volts. The 6BQ5 is the weak link here, allowing no more than 100 volts differential between cathode and heater - which is a lot more than we plan to use. I would stick with a voltage divider ratio of between 10 to 1 (about 25 volts) and 5 to 1 (about 50 volts). You don't want (or need!) much current flow.

[click for image]
R2:22K (25V)
47K (50V)
C1:22u@50V (25V)
22u@100V (50V)
22uF is a minimum, I prefer 100uF.
R3, R4:100 to 330 ohms
[click for image]
Figure 1
Center tapped heater winding
  Figure 2
Non-center tapped heater winding
The values shown are suggested values; feel free to experiment within reasonable parameters for your amplifier.

If the heater winding has a center tap, you simply connect the voltage divider output (where the resistors join) to the CT (figure 1). If the heater winding has no CT, you connect a small resistor from each side of the voltage divider output to each heater winding lead (figure 2). These resistors can be anywhere from 100 to 330 ohms. The main concerns here are heater current leakage and power ratings. With 100 ohm resistors, the AC heater current leakage is negligible (3mA), and it goes down with larger values of resistance. So the big issue would seem to be DC power dissipation.

And it would be, if you had a grounded CT on the heater winding, or if either side of the heater winding is grounded. Make sure there is no heater grounding at all, or you will need much larger (and more expensive) resistors! In fact, if your amp uses ground for one side of the heater connections, you'll need to replace all those grounds with wire from the heater pins on the tube sockets back to the heater winding lead on the PT.

You could add another capacitor across R1 if you liked; I would use the same voltage rating as the main B+ caps.

Some people prefer to just bias the heaters from the output tube cathode resistor; this may work, but you have no control over the voltage as you do here. This also keeps the output tube cathode isolated from the power transformer, which I like.


Thanks to various folks on the www.ax84.com forums for insights into this.

Moving the on/off switch

The on/off switch being on a control in the preamp (tone for Model One or Model Two, Treble for Reverb 12) means you have power wiring in the preamp. This is a recipe for induced hum. Twisting the wires helps, as does shielding, but the best answer is simply to add a new on/off switch elsewhere. Simply decide where to put the new switch, install it, and move the power wiring from the old switch to the new one. You can leave the old switch in place if you like.

On a Model One or Two, I typically put a toggle switch where the fuse originally was, and move the fuse to a new location, under the chassis. You can use the power cord hole and move the power cord, or drill a new hole for the fuse, or use an in-line fuse, or add a bracket, or whatever you like. You could also look into a combination fuse/pilot light. If you want a http://www.rru.com/~meo/Guitar/Amps/Kalamazoo/Mods/standby switch you can combine the power and standby switches as shown at http://www.rru.com/~meo/Guitar/Amps/Kalamazoo/Mods/standby.html.

On a Reverb 12, I prefer to remove the "courtesy outlet" on the back of the chassis, and install a slide switch or toggle switch there for power. With the Reverb 12, it's a good idea to use a hefty DPDT and wire this up for standby as well. (See http://www.rru.com/~meo/Guitar/Amps/Kalamazoo/Mods/standby.html for details; just turn the switch on its side.)

Adding cathode resistor bypass caps

A certain amount of hum is present across any cathode bias resistor. You can reduce this with a cathode resistor bypass cap, sometimes referred to simply as a ``bypass cap''. The larger the cap, the more hum reduction you get, up to some point determined by factors too complex to worry about here. While typical values are from 10uF to 50uF, smaller values (.1uF or lower) and larger values (Marshall used 350uF) are not uncommon.

NOTE: Adding a bypass cap to any stage will increase stage gain and bass response. The stage will distort earlier. If you add a bypass cap, you'll have to balance the hum reduction with the changes to the tone and gain structure of the amp. See http://www.rru.com/~meo/Guitar/Amps/Kalamazoo/Mods/dist.html for more details.

If hum is your biggest concern, start with a medium sized cap such as 10uF and go up or down from there. If tone and gain are more important, start with a smaller cap (.1uF or .068uF) and go up from there as necessary.

You can add a bypass cap to any gain stage, such as V1a on the Reverb 12, and V1b or V1a on a Model One or Model Two. You can add a bypass cap on the output tube for the Model One or Model Two; this will have very little effect on gain but will help reduce hum. The second gain stage of the Reverb 12, V2a, already has a bypass cap. I would not add a bypass cap to a phase inverter or tremolo oscillator. You may add one on a reverb driver or recovery stage (the Reverb 12 already has these, on V3b and V3a). Theoretically you shouldn't need a bypass cap on a push-pull output stage, but they're common there, anyway.

If possible (as defined by your space and monetary constraints), use non-electrolytic capacitors for bypass caps. They really do sound better. For anything up to 1uF, this is an easy choice; after that they start getting bigger and more expensive than many folk want to deal with.

If you use an electrolytic, try to stay reasonably close in terms of working voltage. A 25V cap is more than enough for any 12A?7 gain stage, and a 25V or 50V has plenty of margin for a 6BQ5. You can use caps with higher ratings, but this isn't best practice.

The one place you probably won't hear a difference in hum from a cathode resistor bypass cap is on a cathode-biased push-pull output section (such as the Reverb 12 uses). Hum through the cathode there should cancel in the OT.

Lead dress (where the wires go)

Make sure the heater wires are twisted tightly and run high above the rest of the wires, since everything else is down by the chassis (where they belong, in my book). This will cut down on hum coupled from the heater wires into everything else. Sometimes this doesn't help much, but it often helps a lot. I run all the AC power wires up high like this, and as much as possible, keep the related wires together, and twisted. Keep all other wires as close to the chassis as possible; this provides a ground plane for additional shielding.

Ground loops (more wiring)

The Kalamazoo grounding is OK. Not great, but I've seen better (and worse). If you still have hum problems after doing the other things recommended here, you almost certainly have ground loop problems. (You'll definitely want to work on this in a Reverb 12; in particular run a wire from such common ground as there is to the power tube cathode resistor ground terminal.) Ground loops are caused by having multiple paths (with different impedances, even though you may not be able to measure the difference) to ground or common returns. You can require with a star ground or buss ground (I prefer the star). An excellent starting point is at Randall Aiken's web site: http://www.aikenamps.com/ -> Tech Info -> Technical Papers - Advanced (Aiken) -> Star grounding I think of the better star grounding as "constellation grounding". Each section (each preamp stage, power stage, whatever) has a local star ground (which is not connected to the chassis at this point!), each connected via a wire to a central ground point, which is then actually grounded to the chassis. I'll try to get some photos up, soon.

Last updated: 23 March 2006

Copyright Y2K, 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