Kalamazoo Amp Field Guide: Revamp

Kalamazoo Amp Field Guide:

Revamping Kalamazoo Guitar Amps

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!

If your amp is sounding tired, or you just want it to sound better, there are several, standard procedures to carry out.

Replace the tubes
Tubes eventually wear out. And not all tubes are created equal. If you have access to a large selection, try several preamp tubes, even if they are all the same brand. Justin Belshe likes the Ruby 7025 (12AX7A replacement) and the Tesla EL84 in this amp in this amp. The tubes in our first Model 1 are still better than almost anything else I have tried, but the Sovtek 12AX7LPS, Ruby 12AX7A-C Selected, and Ei ECC83 all do a good job, as do the Sovtek EL84 and the JJ/Tesla, both of which I like in my Model 2. The Electro-Harmonix (EH) is fine, too. My favorite 6BQ5 in the Model One and Model Two is a NOS Raytheon (USA or Japanese). The Reverb 12 doesn't care for the Raytheons, but the EH 6BQ5 really brings this amp alive. Read some tube reviews, then get as many of the tubes that sound promising as you can afford, and try them all.

Replace the caps
The power supply capacitor can certainly use replacement. People disagree on how long these can last, but these amps are nearly 40 years old, and everyone agrees that's more than enough time for the power supply caps to age beyond their useful life. Unless you have a cheap source of multi-section capacitors like the 10/10/20 used in this amp, just get three, separate, electrolytic caps. If you want to use slightly larger values (say, up to 40uF or so), this is fine, and it will beef up the amp's capability to keep the volume up when played hard. I put a 20/30/40 in my Model 1 that I got out of a relatively new, defunct stereo or something. For my Model 2, I used Sprague axial electrolytics, (two 22/450s and a 33/450). If you convert to a solid state rectifier, you can use caps as large as you like; I like to go 47/100/100. Just realize that larger caps take longer to charge.
If you aren't sure how to replace a multi-section cap with separate caps, help is available at pscaps.html .
You may want to go ahead and replace the other capacitors as well. You can get a direct replacement for the Orange Drops. For the others, most vintage guitar amp techs recommend polyester or polypropylene foil tubular caps (I prefer the polyprop, myself) as having the best "vintage" sound, since the original, paper caps are no longer available at reasonable prices (although occasionally someone will have some spare NOS parts cheap). Don't bother to replace the cap hooked to the power switch and ground; it goes away when you replace the line cord.
If you get any amp from the 60s or early 70s with tremolo, and the tremolo doesn't work (or even just doesn't sound that great), go ahead and replace all the caps in the trem circuit. About 80% of the Kalamazoos with original caps have no tremolo effect at all.

Replace the output transformer
A larger output transformer is an excellent idea. Inexpensive routes include something from another, old amp, or a Hammond. You can get a small OT very like the stock Kalamazoo Model One or Two OT very cheaply from Antique Electronic Supply. The Hammond will cost more than most mass market transformers, but is made the same way as the older transformers, which yielded a very different sound than today's transformers, which are built as cheaply as possible. Anything that will work with a single 6BQ5 (about 4K to 5K to 8 ohms at 6 watts minimum (8-10 is better) should work fine. Don't use a Champ transformer unless you switch to a 4 ohm speaker!
One of the best replacement OTs I have heard was made by Simcha Delft. Unfortunately for most of us, he lives down under, which means it's not always easy to get one of these. An even better option (IMO) is the Heyboer that Chris Hurley of http://www.ax84.com/ fame had made up on the 18 watt size frame. Both the latter OTs feature multiple output taps. A Cerniak or Mercury Magnetics transformer should be in the same class as the Heyboer.
Justin Belshe points out that a multi-tap transformer gives you more options, especially for use with speaker cabinets (yes, even a 5 watt amp will sound better (and usually louder) through a big cabinet!) If you do this, I would recommend either using several speaker jacks, one per impedance tap (common grounds back to transformer on the jacks, label them, and only use one jack at a time!) or a jack and a switch.
In the photo, the output transformers are lined up across the front of a prototype amp chassis. From left to right, they are a standard Model One or Two OT, a Delft AX84 P1 OT, and the Heyboer P1 OT.

Replace the power transformer
You should not have to do this unless the original is dead. If only the filament winding is dead, you might choose to simply add a 6.3V transformer to supply the filament voltage; this is usually cheaper than replacing the entire power transformer.

Replace resistors
You should only need to do this if they are defective. If any of them appear burnt (the 1K or 2.2K in the power supply filter often does) you should consider replacing it with a higher wattage version, even if it shows the correct resistance on a meter. If you have random, static type noise, and your tubes are good, you may have a defective resistor. Start with the plate resistors.

Replace tube sockets
If any of the tube sockets have dark streaks between pins, or between a pin and the chassis, you have an arcing problem, and that socket should be replaced. Drill out the rivets or remove the nut and bolt after cutting or desoldering the leads to the socket. I like to cut a grommet in half and use each half as a shock absorber between the chassis and socket. You will need to use a slightly longer bolt to do this; I like 4/40 nuts, bolts and lock washers for small sockets.
After replacing a socket, check voltages on that socket (with tube in place) to make sure they are reasonable. The terminals on the component side can break easily; these are low end sockets. In a pinch, you may be able to solder directly to the broken tang of the socket, but this is not recommended.

Thanks!
Special thanks to Justin Belshe <JBelshe@AOL.com> who first pointed out what a huge change the OT can make.

Last updated: 14 September 2005
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