Kalamazoo Amp Field Guide:
Adding an Easy Line Out
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!
A small amp like this can make a great overdrive effect for another amp. If it's clean enough, you may wish to record directly from the amp. In live situations with other musicians, the amp may need to go into a mixing console. But it's not a good idea to run the speaker output directly into the input of another amp. (It's really bad for the output transformer.) Fortunately, adding a line out isn't very hard.
You will need:
(Higher wattage resistors are OK as well, but 1/2W is more than enough for this application.)
- a 1/4" phone jack (like your guitar cable plugs into)
- a foot or so of two-conductor speaker cable
- a 100 to 390 ohm, 1/2 watt resistor and a 2.2k, 1/2 watt resistor
a 470 ohm, 1/2 watt resistor and a 47k, 1/2 watt resistor
The sweet spot in terms of resistor values varies from amp to amp. Gerald Weber suggests in one of his books a 2.2k/100 resistor network for an old Fender, and later a 2.2k/400 network for another old Fender. A couple of Gibson amps from the Kalamazoo era use the 47k/470 pair, so I went with those.
Anything in the neighborhood of these resistor values should work, at least somewhat, but you can always start with a pair of pots (perhaps with a minimum resistance in series) to set the sweet spot. Once you find the sweet spot, you can wire in resistors of as close values as you can find.
And keep the speaker plugged in!
Pot valuesJerry used a 5K pot and a 50K pot. For the smaller pot anything from 1K to 10K is probably usable; for the larger pot I'd stick with a 5k or 100K. I'd also put a 50 ohm resistor in series with the smaller pot, and a 1K in series with the larger-- this provides a reasonable minimum so your impedance doesn't drop too far from its nominal value (the speaker's impedance).
Now decide where you want the jack to go. I have done this several ways: I have drilled a hole in one of the back panels, drilled a hole in the chassis, and added a bracket. If you select to drill a hole in the back panel and the panel is too thick for your jack, you may need to countersink it (drill a hole large enough for the jack's nut to fit through) half way through the back panel. Do not drill this larger hole all the way through! Or, find a jack with a longer, threaded barrel! I prefer the chassis hole or L bracket to drilling the cabinet.
Connect the speaker cable in parallel with the speaker. You can run the wires off the speaker or splice them into the speaker cable. I'd suggest using spade clips, which just slip right onto the extra set of ears on the speaker terminals. Connect one wire to one end of the 2.2k resistor. Solder the other end of this resistor, and one end of the 390 ohm resistor, to the phone jack's tip lug. Solder the other end of the 390 ohm resistor and the other wire to the phone jack's ring lug.
I've never seen a Kalamazoo with a grounded speaker lead, but if you do this on an amp with a grounded speaker lead, make sure the grounded lead is the one connected to the jack's outer ring.
It's ready. Mount the jack in your chosen location, and you're set.
Line out in a box
Jerry Donovan (<firstname.lastname@example.org>) has some great ideas on other ways to do this; what follows either came from Jerry or is an offshoot of his ideas.
First, you can put the jack, two pots for the resistors, and a speaker wire with alligator clips in a small box. Hook up the clips to any speaker on any amp, plug in to the "line out" jack, adjust the pots for the sweet spot, and you're all set. A "Universal EZ Line Out"[tm].
Beyond this, to have a permanently set EZ Line Out for a given amp (without modifying the amp) you can use the above as a starter, and once you know the values for a certain amp, build a patch cord for that amp. Use the speaker wires and alligators, but wire the resistors into one of the fat speaker jack housings available. If you can't find one big enough, make one from a used, plastic 35mm film can. Punch a hole in the lid for the jack, and a hole in the cannister base for the speaker wire. Run the wire through the base, tie a knot in it about an inch from the wire ends as a strain relief, and hook up the resistors and jack. Or just use a standard project box (available from any electronics outlet). Just toss the result in the bottom of your amp with its power cord, lunch and whatever else you pack in there!
As Jerry says, ``You can have a cord for every little amp in your collection, all ready to patch into 10,000 watts at a moment's notice. Champs and Ka'zoos for the arenas!''
An alligator free environmentIf your amp has an extra speaker jack, you can avoid the alligator clips and use a 1/4" phone plug to terminate the speaker wire. If your amp only has one speaker jack, and the speaker is plugged into that, this would not work because you would have to unplug the speaker; don't do that! Instead, add a speaker jack in the box housing the jack and resistors, next to the line out jack, or add another speaker cable with a 1/4" phone jack into which you can plug your speaker. Wire this in parallel with the incoming speaker wires.
If you normally plug your amp into a separate speaker cab, you can build this as an enhanced speaker cable. Using the film can (project box, whatever) as above, run two speaker cables out, with a phone plug on each. Solder the second cable in parallel to the first one at the jack.
Most standard phone jack solder terminal lugs are a bit small for 12ga or larger speaker wire, or for two 18ga wires. If you are using two speaker wires in parallel, you should hook everything up separately from the line out jack, and run smaller wires to the jack. 22ga is fine for this.
NOTE: This is not a pre-amp out! It's a quick and dirty hack taken off the speaker outputs. As such it will not be as good an impedance match, or as clean, as a real line out. But it works.
But keep the speaker plugged in! Otherwise, you may well fry your output transformer and/or the 6BQ5!
Thanks!Special thanks to Jerry Donovan <email@example.com> for his ideas, for the discussion, and for graciously allowing me to post them here.
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