Kalamazoo 1


The idea for this amp came from a friend along with most parts for it. I got both power and output transformers, terminal board and two EL95 tubes, all recycled from old radios. Not long before I got some Russian 6N2P tubes (12AX7 near equivalents), so this was a good opportunity to try them. I don’t like recycling resistors, caps and pots so I got all new. As always, I used metal film resistors in signal path, but this time I wanted to try carbon film resistors on preamp tube plates. Coupling caps are polyester, all made by different manufacturers. The idea is to pack everything into small head, only 22(w) x 14(d) x 15(h) cm.

  • EL95 tube in output section for about 3W of power. Data sheet says for single ended operation it needs 320ohm resistor at cathode, 10K load and 250V at plate. The closest resistor value in local shop was 330ohm so I got that one.
  • 6N2P preamp tube
  • Solid state bridge rectifier instead of 6X4 tube used in original amp.
  • Added one more filter stage in power supply with 33uF electrolyte and 100ohm resistor for better filtering and to simulate voltage drop caused by tube rectifier in original circuit.
  • Added 1M resistor from input to ground.
  • Added virtual center tap for referencing heaters. Both heaters are connected to ground through 100ohm resistors. At first I mounted those resistors on output tube socket between heaters and cathode but it was very messy so I switched to this solution.
  • Added 22uF electrolyte in parallel with output tube cathode resistor.
  • Removed infamous 22nF "cap of death" between mains and ground.
  • Unlike original amp I used three prong cable for safety. Ground lead is connected to chassis.
  • Changed input grid resistor from 47K to 10K
  • Changed second coupling cap from 10nF to 22nF. Those 10nF caps I had didn’t have long enough leads so I just used 22nF. I don’t expect any noticeable difference in tone.
  • Changed indicator light to two-color green/red LED. I found this LED on flea market and it gave me an idea: use one color when amp is on standby and other one when amps is fully on. It’s basically two LEDs packed inside one. They share common cathode and have separate anodes. I used one pole of standby switch to select which of two anodes will be connected to V+. Next schematic shows how it’s done. V+ is DC voltage rectified from heaters and filtered with 47uF electrolyte, 330ohm and 100ohm resistors are added in series with LED(s) to reduce current.
    NOTE: Unlike regular LEDs, longer lead means cathode, not anode. It would save me some nerves if I knew this before 🙂


In the spirit of recycling, chassis is made from steel sheet I had lying around which came from scrap yard. It\’s about 1mm thick and powder coated in cream. Steel is not easy to work with like aluminum, but it\’s not that bad. The only problem is making large holes for tube sockets because the largest drill bit I have is 10mm, so everything wider than that means endless filing with a dull file 🙂 .

I marked positions of transformers, circuit board and controls on piece of paper and then transferred it to metal sheet. Luckily, I had some scraps of plywood left from Princeton project that was large enough so I cut it, made joints and glued it together. Three clamps are there to ensure that everything is tight. Piece of plywood sitting inside is there to ensure that sides are glued perpendicularly. Note that circular part of plywood I inserted to prevent damage from clamps. It\’s a leftover from Princeton’s speaker baffle.

After glue has dried I used very small pieces of soft wood and glue to fill small voids in joints. When I made finger joints, middle “finger” was always little shorter so there was about 1mm of space between fingers. Then all edges are beveled with router and hand sanded later just for small corrections. Inner edges are only slightly beveled by hand.

Next step is tolexing. This is my second time to deal with tolex and I think I became pretty good at it 🙂 . Technique I use for edges it very effective so next time I do some tolexing I’ll make a pictured tutorial.

And finally electronics 🙂 . Circuit board is very interesting; it has screws below each lug which can be used to make connections between components. It looks very neat this way. Shielded wire is used from input jack to first triode, from volume pot to second triode and from OT to speaker jack. Jack is mounted near tubes so I wanted to make sure there isn’t any interference.

This photo shows how switches are wired. Also you can see small board that carries two bridge rectifiers (one for amp and one for LED). All grounds are joined to one point and screwed to chassis. Speaking of chassis, as you can see I sprayed it with black acrylic paint. Looks much better this way.

For heaters I used solid core telephone wire which turned out to be much better for this job than #18 stranded wire from PC PSU units. First try was with that PSU wire and it was very hard to work with. Also, result was much less clean than this.

And finished amp sitting on the top of my 1×12″ cab loaded with Celestion Greenback G12M. Front panel is laser engraved from my design. It’s not actually metal, although it looks like it. I plan to make something to put on that front cover. Right now it looks little empty to me.

It’s not shown on photos, but I used aluminum foil to shield bottom side of the chassis. It’s glued with double sided tape and I made sure that it has contact with chassis.

I’m very happy with sound of this little fellow. It has more gain than my Princeton 5F2A and it’s only a bit quieter with Greenback compared to 5F2A though Weber Sig8. At higher volumes even my Squier Strat can push it into overdrive. It’s nothing brutal but can do the job for classic rock stuff. Also, it responses very well to pick attack. I’m curious to test it with new EL95 tube. This one is used, apparently not much because it sounds great, but still…

Update: I did get a new EL95 and it sounds even better 🙂

Voltage reference
EL95 plate
EL95 cathode
EL95 plate current
6N2P plates
6N2P cathodes

*All voltages measured in respect to ground

One more interesting story related to this project. While testing this amp I got zapped really bad. One hand was on guitar strings (stupid) and with other I tried to flick the standby switch. The jolt went though both arms and it hurt pretty bad but luckily without any consequences. I measured resistance and switch and strings are on the same (ground) potential. The nearest non-isolated lead at that moment was 220V mains going to PT, but I don’t think I touched it. I tested it two times before the incident and it worked great. The only difference is that this time I used other EL95 I had, but I don’t think if bad tube can cause that. Anyway, it’s experience I wouldn’t like to repeat.

WARNING: disconnecting circuit ground from ground for standby switch can be very bad in cases when input jack is isolated from the chassis. When amp is on standby guitar strings are not grounded so there is difference in ground and string potential. Touching anything grounded (like standby switch) with one hand while having other on strings will zap you. Solution is to either ground input all time or just use standard standby switch.

Useful links

Kalamazoo Amp Field Guide

2 Responses to “Kalamazoo 1”
  1. Bancika says:

    Next time I have a chance I’ll try to record some clips and post them here.

    As far as schem goes, I don’t have it, but it’s nothing special. Just take original schematic from this link (http://www.netads.com/~meo/Guitar/Amps/Kalamazoo/) and apply my minor mods as explained above.


  2. kespers says:

    Hi, i like very much your diy amp, I’m trying to do a power amp to practice at home and test some preamps like slo or jcm800.

    How clean is the amp? I think that EL95 crunch smooth at middle volume, its true?

    Can you send me the schematic o publish here?


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    The idea behind this site is to share my experience with Do It Yourself approach to guitars, amplifiers and pedals. Whether you want to save a couple of bucks by performing a mod or upgrade yourself instead of paying a tech, or want to build your own piece of gear from scratch, I'm sure you will find something interesting here. Also, this is the home of DIY Layout Creator, a free piece of software for drawing circuit layouts and schematics, written with DIY enthusiasts in mind.