Kalamazoo Amp Field Guide: Bass Amp Schematics

Technical Overview ] [ Schematic ] [ Photos ]

Click on either image to get a large (~1600x1120) GIF version. The larger images are both around 32Kb in size. Notes on the circuits follow the schematics.

Bass 30 Bass 50
[Bass 30 schematic here] [Bass 50 schematic here]

The two most interesting features of the circuits to my mind are the tone circuit (a modified James (simplified Baxandall) turned sideways) and (on the Bass 30) the fact that the final gain stage comes after the cathodyne inverter, with one half of a 6EU7 amplifying each half of the inverter output.

The two circuits are similar, but not identical. Besides the output tube differences (7591s vs EL34s) the main differences are:

  1. The Bass 30 has an RC network off the Loudness (Volume) pot wiper to drain high frequencies to ground.
  2. The Bass 50 adds a choke to the power supply filter network.
  3. While each power power supply provides five voltages (A/B/C/D/bias), they are not the same between the amps.
  4. The Bass 50 output derives its output tube screen bias via resistor from the output tube B+, whereas the Bass 30 provides a PS voltage specifically for this.
  5. The Bass 50 includes a classic, negative feedback loop.
  6. The Bass 30 uses a cathodyne PI followed by a driver stage on PI output, whereas the Bass 50 uses a standard, class A gain stage followed by a long-tailed pair phase inverter. Thanks to Dave Moylan for pointing this out.

These schematics are reproduced from a book of schematics provided by Gibson. The actual book was produced by a third party, not Gibson, and is fairly rare.

Thanks to Chris Terry for the scans. He gave permission to copy them here. All the Kalamazoo schematics I have ever seen were uncopyrighted. Check out Chris's Bass 30 page at http://www.mindspring.com/~christerry/kayzoo.html . And thanks to Bill Kahle for making sure I knew about the schematics (both Chris and Bill emailed me within a day of each other). He was going to scan his in for me, but found this first.

Technical Overview ] [ Schematic ] [ Photos ]

Last updated: 21 October 2007

Copyright 2001 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