Saturday, January 10, 2015

1/10/15 Kalamazoo Reverb 12

It's been a minute since I've written here. I ran into some folks in Maryland at a wedding that read this blog and wanted more so I figure, well, give the people what they want!

A lot has happened in my world since the last blog post, lotta projects. I've been getting some good little jobs that require modifications. Amp behaves this way, can you make it behave another way. Here is one of them:

The Kalamazoo Reverb 12 or as I like to call it: the poor mans Princeton Reverb. The last cool amp I had on the bench was a Vox Cambridge 12 (I may write about that one soon) which, well, they aren't cheap any longer. They aren't as good as a Princeton but with some mods, those amps really open up.

This Kalamazoo was produced by Gibson in 1966. It's go a pair of EL-84 tubes producing about 15 watts going into a 10" CTS alnico speaker. I like these amps. They aren't built nearly as well as a Fender but who cares, they can be had for less than half a G and they make great studio amps or they are perfect for your singer songwriter gig at Pete's Candy.

The complaint with this one is the reverb kicks on way too strong and way too early. It also cuts in and out. Gibson reverb is like no other reverb. It's the wettest and wildest to begin with and this amp has a new MOD tank which makes it even wetter, very long decay. I dig it but I can see how it is hard to control. Quite literally, the mix is terrible, like 2 different amplifiers in one box competing for space. What I do like about these, and in a perfect world where I have my own studio, is you can turn the volume all the way down and turn the reverb up for pure reverb, so in that perfect world I would use it just for reverb and mic my other amp for the dry signal. Amazing.

So the objective is to get it balanced without losing its Gibsonee qualitees.

First thing: The reverb pot is 2 Megs! Get that out of there! I replaced it with a 250K audio pot for a nice smooth taper. I also added a .001 cap in series with the 150K resistor that feeds the reverb circuit after the first gain stage. Took some experimenting but this did the trick. Reverb is "wet enough", still nicer than a Fender reverb, but way more in control.

These amps with a Fender guitar can be very harsh too. I so want to open it up but the customer wants to keep it as is. Some simple tone stack mods would work coupled with a bit larger coupling caps in the phase inverter. I've done this before. He has a humbucker so it will be full and fine.

I also added a line out off the speaker. Very simple mod that requires a jack and 2 resistors: 220 ohm and 2.2k.

This amp runs exceptionally quiet. The Kalamzoos can be quite hummy. The filter caps were already replaced but even with that one still needs to address the filament. I've had success by lifting one side of the filament from ground and adding a hum balance. In this case another tech simply made the filaments DC. The job was sloppy so I cleaned it up but, the objective was more than achieved. This amp is quiet as a dead fish. I'll consider doing the same on the next one I get. it only requires a bridge rectifier and a 1000-4700 capacitor.

Anyway, that's it! Happy soldering in 2015. Feel free to ask any questions about this job.



  1. I like the way this reverb sounds better than a Fender. Come to think of it, If you want a more lush reverb on your Fender, might consider this: replace the 500pf 'feed' cap with a 1000 or .001. Then replace the 100k pot with a 250k and surf away. Worth of a try! You can always go back.

  2. Thanks for this post. Do you have a schematic for the filament fix? Thanks

    1. Missed this one! I'll draw one up and post it. For now I can tell you how it's done. Find which side of the filament is grounded. Lift that side. Take 2x100 ohm resistors (1/2 watt or better) and install one end on each to each side of the heaters (pins 4 and 5). The other ends should be twisted together. Solder those to the cathode bias capacitor and you're done. You can also sub a 100 ohm potentiometer for the 2 resistors to make a 'hum balance" pot. You can copy the cathode biased Ampeg B15 schematic for reference.

  3. Hey,

    What model of reverb tank works in these things?

  4. Hi! Great question. It's been a long time since I've had to replace a Gibson Reverb tank so I may be incorrect with this:


    Trouble is, I don't know where to get one! A 4AB3C1B like you see in a Fender won't work but in a pinch I have replaced the reverb driver transformer with a Fender one (did this when the Gibson driver transformer went bad only, tanks were plentiful back then!) then used a Fender tank.

    Thanks for your question and sorry about the delay in getting it answered! Hope this helps! JB

  5. Here is one: