Kalamazoo Amp Field Guide: Model 2

Photos

Early Model

To my horror, I find I have no good photos of the early Model Two. There are far more of these than of the later model (below), so this is a bit strange. I've sold all of mine, or used them for prototypes of my own amps, and even these last don't have original cabs. I didn't think to photograph the others. They are identical to the original Model One, with a tremolo control added between the volume and tone controls.

Last Model

A late 60s Kalamazoo fashion statement. The Tolex isn't really blue; that's an artifact of the flash. This amp is very good condition. Corner scuffing is evident. The grill cloth is darker than on earlier models. CMI left the handle, jacks and fuse holder alone.	The controls haven't really changed, but the faceplate and knobs have. Control layout and spacing are identical to older models. Internally, CMI switched to only using two chassis bolts from four, good enough but not as good. On the other hand, the new cab design protects the controls somewhat.	The back is similar to the older version, except that five screws now hold each panel instead of four. Other than minor rust on the screws and mild scuffing, it still looks good. Note the poor bottom panel fit, though.	Every open-backed Kalamazoo (all but the bass amps) has the serial number sticker on the speaker. The speaker, transformers and wiring are all holding up well. This amp has been kept somewhere with reasonably controlled temperature and (relatively low) humidity or there would be more rust evident.

Close up after surgery. This is from a completely rebuilt Model Two, but who can tell? The only obvious change to the front is the power switch where the fuse used to be (it's now in the back, underneath the chassis). The tone control switch no longer does anything, though it could be wired up to disconnect the tone control. The chassis is laying on a clone Model Two cab I got off ebay. An amp's cabinet is often one of the best places to put the chassis while working on it. Since the customer only sent the chassis, I used this one with the funky cover material. What would you call this? Mauve snakeskin?	The larger output transformer (OT) is the new one. The smaller one has been set beside it for comparison. The customer provided a Hammond 125DSE, an 8 watt transformer with 5 output taps, suitable for use for 2.5K, 5K or 10K nominal primary impedance. The Kalamazoo Model One and Model Two need to see a primary load in the vicinity of 4K to 5K. This allows possible secondaries of 16, 8, 4, 2, or 1 ohm[s] (although Hammond does not list 1 ohm, and it may be too much current for the tranny; I haven't tested that). As you can see, the Hammond is over twice the size of the original. A bigger OT really opens these amps up.	The customer wanted the OT leads intact in case he decided to reuse the Hammond at a later date, or just to have other impedances available. I ran the leads across the chassis, tied them with a cable tie (and stick-on tie point), ran them back across, shrink-wrapped the loose ends, and cable tied the loose ends down. I added a hole with a grommet to run the secondary leads back inside the amp next to the new, shorting jack for the speaker (between the black/yellow wires and the OT). I also installed a grommet where the primary leads (blue/brown) enter the chassis. You can see the fuse holder at the lower right, near the power cord.
Yes, from here it looks like a bit of a snake pit. Actually, the power section looks like one from most angles. I left the power transformer (PT) wiring more or less in place, though I did add a cable tie to keep it up in the air. I also ran the heater wiring up in the air. There are a couple of ways to run the wires to get the least noise; I prefer to keep signal, ground and B+ wires next to the chassis (it's a big ground plane), and run all AC wires in the air. B+ is a crapshoot; I ran the 6BQ5 screen grid wire in the air, too. After these photos were taken, I put cable ties on the power cord on either side of the grommet, and on the B+/ground wire bundle. The power supply caps are all Sprague Atoms[tm].	It's not pretty. It's certainly not a Hi-Watt. But it's rock solid, it has every bit of tone you can get out of a Model Two, and it's very, very quiet. About the only way to make it quieter is bigger filter caps. It won't win any beauty contests for looks, but I'll put the sound up against anything in its class. The black cap and resistor across the yellow and blue Atoms is the low voltage section of the heater bias circuit. The black cap is a 100uF/100V Atom. The heater bias is tapped off the screen supply, so it also adds more capacitance to the screen, which means less hum there. Two quieteners in one! I added more terminal strips, and replaced one with a larger strip. While this adds to the visual clutter, it allows me to take stress off the socket terminal lugs. That's a good thing, because they tend to break easily. At least half the Kalamazoos I see have a broken socket lug, or one so weak that it breaks from a light touch on a wire or component lead. That long, yellow wire running across the middle is the tremolo drive lead.	The customer sent all new resistors and caps to match the bill or materials on my web page. The green resistors are Kiwame carbon film resistors. Most of the signal and trem caps are Orange Drop[tm] 715Ps. The yellow cap on the tremolo terminal strip is a RelCap. The cathode bypass cap at lower left is an REC. The non-green resistors are metal oxides I provided for things that weren't on the original BOM. None of these are in the signal path, or even close. For instance, the two large, gray resistors at the lower left are part of the DC bias on the heater wires for hum reduction. I added shielded wire where possible for signal runs. The amp now has a "constellation ground"- a layered, star ground scheme. The only violation is that the original input jacks were used, which doesn't really cause a problem since they have a proper star ground connection. The output jack is also grounded, but since there is no negative feedback loop, this is also not a problem. I have no idea what the penciled writing is about!
Andy Hardy kindly sent this annotated photo of a nearly untouched Model Two chassis. All the components are labeled except C10, the oscillation killer cap, which is either behind the terminal strip TS1 (lower right) or simply missing on this amp. Two versions available: 3456x2304, 2.3MB and 1728x1152, 1MB.

Thanks!

There are no "before" photos of the rebuild, because our 35mm camera had died, and I didn't get the digital camera until I was almost done with the rebuild.