Bumble Tuby Guitar Amplifier

This is a relatively simple, hybrid guitar amplifier, made as a hobby project. To me it's mostly about learning, having fun, and building something tangible.

Features

• Runs off of a single 12V DC supply
• Pre-amp based on the 12AU7 vacuum tube
• FX loop
• Tone control section
• Line in
• Solid state power amplifiers (headphones, speaker)
  • Class D mono speaker amp
  • Class A stereo headphone amp

Bill of materials

Note

This section is yet to be populated.

To-do

Circuit design

• Add headphone output.
• Add effects loop:
  • FX send can be driven directly from the 2nd tube stage.
  • FX return needs a buffer before feeding into the power amp section. Can use the second half of the NE5532 for this.
  • What about operation modes? With/without effects loop. This would require a SPDT switch to either send the pre-amped signal into the effects chain, or directly into the buffer.
• Add bypass capacitors on power supply lines:
  • LM386: 10uF, 100nF
  • opamps: 10uF, 100nF
• Add power switch, LED
• Consider poweramp alternatives
  • TPA3110
• Make buffer bias voltage adjustable
• Fix tube biasing
• Buffer triode outputs (plates)
• Mute speaker PA when headphone jack plugged in
• Add headphone PA
• Add gain adjustment for the power section (LM386 pins 1-8)
• Split design into separate subschematics
  • How to express connections?
  • Hierarchical sheets in KiCad
• Add stereo line-in, signal mixing
• Attenuate FX send 20x, and then on return amplify it by 20x
  • Floor pedals can't handle the 8Vpp the preamp outputs
  • The whole FX section should by itself have unity gain, if the send and return are shorted.
• Add split-rail opamp bias source
• Split design into multiple sheets
• Add a tone control section.
  • Big muff style single-pot tone control seems nice and simple.
  • After tube, before output buffer? Alternatively, right after gain stage, before first triode.
• Add screw terminals for vacuum tube connections
• Does this require ripple rejection?
• Revisit tube input biasing, something seems off. Verify on breadboard.
• Simulate pre-amp in SPICE
• Design perfboard layouts

CI

• Render BOM into README, as a Markdown table.
• Auto update schematic date during artifact update workflow.

Credits

This design was inspired by:

• Ruby Tuby amplifier design.
• Tube Cricket