ask to Grok: Guitar Tube Amp

[jeffrowland]

yesterday I asked Grok:

DIY 50W guitar tube amp?

A 50W tube amp is a solid step up—great for gigging or just cranking it at home! Building one is more involved than a little solid-state amp, but totally doable with some planning. I’ll outline a classic single-ended or push-pull design (push-pull is more common for 50W). Think along the lines of a Fender Bassman or Marshall Plexi vibe, but simplified for DIY.
Parts You’ll Need:
Tubes:

• Power tubes: 2x EL34, 6L6, or KT66 (push-pull pair for ~50W). EL34s are punchy and Marshall-esque; 6L6s are cleaner and Fender-y.
• Preamp tubes: 2-3x 12AX7 (ECC83) for gain stages and tone shaping.
• Rectifier tube (optional): 5U4 or GZ34 for tube rectification, or use solid-state diodes for simplicity.

Core Components:

• Output Transformer (OT): Critical for 50W. Match it to your tubes and speaker impedance (e.g., 3.5k-4kΩ primary for EL34s, 8Ω secondary). Hammond 1750 series or similar works.
• Power Transformer (PT): Needs to supply ~300-400V at 150-200mA for the plates, plus 6.3V for heaters. Look for something rated around 200VA.
• Chassis: Steel or aluminum box to mount everything. Punch holes for tubes and controls.
• Speaker: 8Ω or 4Ω, 50W+ rating (12” Celestion or Jensen are popular).
• Capacitors:
  • 22µF-47µF @ 450V electrolytics for power supply filtering.
  • 0.022µF-0.1µF @ 600V for coupling.
• Resistors: Various (1W-2W rated) for bias, cathode, and grid circuits (e.g., 1kΩ, 470Ω, 100kΩ).
• Potentiometers: 1MΩ for volume, 250kΩ for tone (bass, mid, treble).
• Input jack: 1/4" mono.
• Switches and fuses: Power switch, standby switch, 1-2A fuse.

Tools:

• Soldering iron, multimeter, drill (for chassis), and wire (18-22 AWG).

Basic Steps:

1. Design the Circuit:
   • For push-pull: Two power tubes (e.g., EL34s) driven by a phase inverter (one 12AX7 triode splits the signal). Two gain stages (another 12AX7) and a tone stack (Bassman-style is straightforward).
   • Check schematics online—Fender 5F6-A or Marshall 1959 circuits are good 50W starting points. Simplify as needed.
2. Power Supply:
   • Wire the PT secondary to the rectifier (tube or diodes) to get ~350-400V DC after filtering with caps.
   • Add a choke (5-10H) or resistor (100Ω) between filter stages for cleaner B+ voltage.
   • Ground everything properly—star grounding avoids hum.
3. Preamp Stage:
   • First 12AX7 stage: 1.5kΩ cathode resistor, 100kΩ plate resistor, 0.022µF coupling cap to the next stage.
   • Second stage: Similar, feeding into the tone stack (search “Fender tone stack calculator” for values).
4. Phase Inverter:
   • Use a long-tailed pair (common in 50W amps). One 12AX7 triode, 10kΩ tail resistor, drives the power tubes.
5. Power Stage:
   • Bias the EL34s (fixed bias with a negative voltage supply, ~ -35V, or cathode bias with a shared 250Ω resistor).
   • Connect grids to the phase inverter, plates to the OT primary, and OT secondary to the speaker.
6. Assembly:
   • Mount tubes, transformers, and controls in the chassis.
   • Solder point-to-point or use turret board (safer and cleaner).
   • Double-check wiring—high voltage means mistakes can fry parts (or you).
7. Test Safely:
   • Use a variac or light bulb limiter to power up slowly. Check voltages: ~350V on plates, ~250V on screens, heaters at 6.3V.
   • Plug in a guitar, start quiet, and listen for hum or distortion. Adjust bias if needed.

Notes:

• Safety: You’re dealing with 400V+. Discharge caps before touching anything—use a resistor or screwdriver with a clip lead to ground.
• Cost: Expect $200-$400 for parts, depending on quality (transformers are the big expense).
• Sound: EL34s give a mid-heavy crunch; 6L6s are smoother. Tweak the tone stack for your style.