Guitar Preamplifier Design
0:30
Considering schematic tools.
Analyzing the schematic.
There isn’t a built schematic in this project yet — the current project schematic is empty with 0 components and 0 nets.
Based on your requirements so far, the intended preamp schematic is this:
Current schematic blocks
| Block | Function | Main parts |
|---|---|---|
| Input network | Guitar input, grid leak, stopper, coupling into first tube stage | 12AU7 first triode section |
| First gain stage | Initial voltage amplification | 12AU7 |
| Vox tone stack | Classic AC30-style EQ after stage 1 | treble/bass cut network using resistors and capacitors |
| Second gain / recovery stage | Recovers loss from tone stack and adds drive | 12AT7 |
| Output stage / driver | Sends preamp signal onward to the power amp or next section | remaining 12AT7 or remaining 12AU7, depending on final topology |
| B+ supply feed | High-voltage rail feeding tube plates through dropping/filter nodes | AC30-style preamp B+ ladder |
| Heater supply | Tube filament power | 6.3 V heater rail for 12AU7 and 12AT7 |
Main power connectionsDiagram
Diagram
Practical connection summary
| From | To | Purpose |
|---|---|---|
| Guitar input jack | 12AU7 first grid through input network | First amplification |
| 12AU7 first plate output | Vox tone stack input through coupling capacitor | EQ shaping |
| Tone stack output | 12AT7 grid | Gain recovery after insertion loss |
| 12AT7 plate output | Output coupling capacitor | Send signal to next stage |
| B+ rail | Plate resistors of each triode stage | Tube high-voltage supply |
| Heater rail | Filament pins of 12AU7 and 12AT7 | Tube heating |
If you want, I can next give you the actual stage-by-stage resistor and capacitor values for this Vox-style preamp as a clean build sheet.
List power section
Trace MCU connections
Group by function
Find missing decoupling
9 days
Refine this doc
Ask about this doc
Requirements Capture: Vox AC30-Style Update for Tube Guitar Preamplifier
Status
High-level requirements capture only. No schematic or layout changes are to be made in this step.
Inspection Result
At the time of capture, no existing project documents, schematic components, BOM entries, or nets were detected in the current project context. This requirements capture therefore records the requested target direction and preserves the stated topology intent without implementing or re-topologizing the design.
Goal
Update the existing tube guitar preamplifier concept so that it:
- keeps the requested topology,
- uses Vox AC30-style resistor and capacitor values where practical,
- places a classic Vox-style tone stack after the first gain stage,
- uses typical AC30-style preamp supply expectations for B+ and heaters,
- reports the selected resistor and capacitor values back to the user before any implementation work.
Non-Goals
- No topology changes beyond inserting/confirming the requested Vox-style tone stack location.
- No schematic rewiring in this phase.
- No PCB/layout work in this phase.
- No final bias optimization or simulation in this phase.
- No parts procurement or footprint changes in this phase.
Preserved Topology Intent
Assume the topology remains:
- First gain stage
- Vox-style tone stack immediately after the first gain stage
- Remaining gain / recovery stages
- Existing overall preamp signal path preserved
If the existing schematic later differs, implementation should adapt values to the existing stage order rather than re-architecting the preamp.
Captured Vox-Like RC Targets
These are the target starting values to apply around the 12AU7 and 12AT7 stages while preserving the requested topology.
First gain stage target (Vox-like starting point)
Use this around the first triode stage feeding the tone stack:
- Plate resistor: 100k
- Cathode resistor: 1.5k
- Cathode bypass capacitor: 22uF to 25uF
- Grid leak resistor: 1M
- Grid stopper resistor: 68k
- Coupling capacitor into tone stack: 10nF to 22nF
Subsequent gain / recovery stage target
Use similar Vox-family values for the next voltage-amplifier stage unless the existing topology clearly requires otherwise:
- Plate resistor: 100k
- Cathode resistor: 1.5k
- Cathode bypass capacitor: 22uF to 25uF when a fuller Vox-style response is desired, or omit bypass if the existing topology already depends on lower gain / tighter voicing
- Grid leak resistor: 1M
- Grid stopper resistor: 68k
- Interstage coupling capacitor: 10nF to 22nF
12AU7 / 12AT7 adaptation intent
Because classic AC30 preamps are more commonly associated with ECC83/12AX7-family gain stages, these values are captured as Vox-like starting values, not final bias-verified values, for the user-requested 12AU7 and 12AT7 stages. Final implementation should preserve the selected RC family first, then confirm operating points in a later validation step.
Captured Vox-Style Tone Stack Target
Default requirements-capture choice: later classic Top Boost-style family.
Default tone stack values to carry forward
- Treble pot: 500k log
- Bass pot: 1M log
- Treble capacitor: 500pF
- Additional small treble capacitor / bright shaping capacitor: 100pF
- Tone network capacitors: 10nF, 10nF
- Series / slope-style resistor: 10k
- Additional shaping resistor: 56k
Acceptable historical alternate family
If the user later prefers an earlier JMI-style variant, the alternate historical family is:
- Treble pot: 1M log
- Bass pot: 1M log
- Capacitors: 50pF, 22nF, 22nF
- Tone-network resistors: 100k, 10k
For this requirements capture, the default selected family is the later Top Boost-style set unless the user requests the earlier variant explicitly.
Captured Supply Targets
Use typical AC30-style preamp supply expectations as the requirements baseline:
- Preamp heater supply: 6.3 VAC nominal
- Raw B+ reference: approximately 320 VDC after rectification from a classic AC30-style HT supply
- Preamp-node B+ target: 200+ VDC, typically in the low-200 VDC range at preamp stages after dropping/filtering
High-Level Execution Plan
- Inspect the existing schematic once actual components are present and identify the first gain stage, tone-stack insertion point, and all 12AU7 / 12AT7 triode stages.
- Preserve the existing requested signal-path topology.
- Assign the captured Vox-like RC starting values around each stage without changing stage order.
- Insert or confirm the default classic Vox Top Boost-style tone stack after the first gain stage.
- Set preamp supply expectations to 6.3 VAC heaters and AC30-style preamp B+ nodes in the low-200 VDC range, referenced to a raw B+ around 320 VDC.
- Produce a clean resistor/capacitor summary for user review before any implementation or tuning.
Deliverable Back to User
Before any schematic edits, report:
- each stage's target plate resistor,
- cathode resistor and bypass capacitor,
- grid leak and grid stopper values,
- coupling capacitor values,
- tone stack resistor/capacitor/pot choices,
- preamp B+ and heater targets,
- any assumptions caused by missing current schematic detail.
Welcome 👋
Flux helps you build PCBs faster with an AI teammate!
Create your account to collaborate, stay updated, fork your own version, and get instant answers from our AI agent.