Kalamazoo 1

Background

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 from different manufacturers 🙂 . The idea is to pack everything into small head, only 22(w) x 14(d) x 15(h) cm.

Mods
  • 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 🙂

Pictorial

Click on a photo to see more details

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
B+1
250V
B+2
238V
B+3
142V
EL95 plate
245V
EL95 cathode
8.3V
EL95 plate current
20.5mA
6N2P plates
95V
6N2P cathodes
1V
Heaters
6.3V

*All voltages measured in respect to ground

One more interesting story related to this project. While testing this amp I got zapped really bad. Still can’t explain why (See warning for an explanation). One hand was on guitar strings (stupid) and with other I tried to press standby switch. It 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

Comments
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.

    Cheers,
    Bane

  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?

    Thanks.

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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.