The current as-built schematic.

The holes on the front panel reflect an earlier build that included a tonestack and volume control.

Note I should have redone this sheilded connection on the Cliff jack above, because I melted the inner insulation sleeve. I didn't bother because this was a rough little prototype and not intended for anything but experimental use.

The milliAmp — a single 12DW7 tube amp

The milliAmp started out life as a request from Chris Hurley for ideas of a simple beginner's tube amp project. Though the design wandered a bit, I have kept the original documents for those interested in how designs evolve.

In it's current form, the milliAmp is really a test bed for ideas about parafeed output stages. It will drive a reasonably efficient cabinet, but not by much. It is possible to talk comfortably above it at full volume. Additionally, the 12AX7 doesn't afford enough gain to overdrive the 12AU7 half of the tube very well.

In all it was fun to build, but I would hesitate to recommend it to someone looking for a versatile, inexpensive guitar amp. As a Garnet-style Herzog driver it works fairly well, though as built, you must run it it through an appropriate Herzog box to keep the output load reasonable.

Besides the as-built schematic above, there are number of ideas I intend to try someday, time permitting:

  1. A single-tube 6CU8 pentode/triode design.
  2. An two-tube Garnet BTO design.
  3. The original as-built milliAmp with a simple tonestack and volume control. The tonestack proved to drop the signal too much and never turned the volume down from max, leading me to remove those features.

There are also available some photos of how to take plate voltage measurements using this amp as the demonstration unit.