---
title: How to Build a 30‑Watt All‑Tube Preamp from Scratch – Complete Parts List & Step‑by‑Step Guide
siteUrl: https://logzly.com/valveaudiolab
author: valveaudiolab (Valve Audio Lab)
date: 2026-06-30T21:02:12.838168
tags: [audio, diy, tube]
url: https://logzly.com/valveaudiolab/how-to-build-a-30watt-alltube-preamp-from-scratch-complete-parts-list-stepbystep-guide
---


Ever stared at a blank workbench and wondered how those warm, tube‑kissed sounds actually get made? I’ve been there. In this post I’ll walk you through every part, tool, and trick you need to turn a pile of components into a 30‑watt all‑tube preamp that sings. Grab a coffee, roll up your sleeves, and let’s get building—Valve Audio Lab style.

## What You’ll End Up With  

A solid‑state‑free preamp that delivers 30 watts of clean, vintage‑flavored power. It’ll fit in a 2‑U rack or a small chassis, accept standard RCA inputs, and output to any decent speaker or power amp. The circuit uses classic EL34 output tubes, a 12AX7 for gain, and a simple bias circuit that you can tweak later.

## Parts List – Everything You Need  

Below is the exact bill of materials I used for my first build. Prices are ball‑park; you’ll often find deals on eBay, Mouser, or local surplus stores.

### Tubes  

| Qty | Part | Why it matters |
|-----|------|----------------|
| 2   | EL34 (or 6V6 for lower voltage) | Main power tubes, give you the 30 W |
| 1   | 12AX7 | Low‑noise pre‑amp stage |
| 1   | 5Y3GT (or 5U4) | Heater supply, smooth warm up |

### Transformers  

| Qty | Part | Notes |
|-----|------|-------|
| 1   | 300 V B+ line transformer (≈12 VA) | Supplies high voltage |
| 1   | 12 V/2 A filament transformer | Powers the tube heaters |
| 1   | Output transformer, 8 Ω to 8 Ω, 30 W | Matches tubes to speakers |

### Passive Components  

* Resistors (1 % tolerance, carbon film is fine):  
  - 100 kΩ x2, 10 kΩ x2, 1 kΩ x4, 470 Ω x2, 100 Ω x1, 10 Ω x1  
* Capacitors (film or electrolytic, 100 V rating unless noted):  
  - 0.1 µF (C0G) x2, 1 µF (film) x2, 10 µF (electrolytic) x2, 100 µF x1, 470 µF x1 (for B+ filter)  
* Potentiometers:  
  - 100 kΩ log (volume), 1 MΩ log (bias)  

### Miscellaneous  

* Chassis (steel, 2‑U rack mount) – about 10 × 6 × 2 inches  
* PCB or perf board (2 × 4 in) – I used a pre‑drilled layout from Valve Audio Lab’s archive  
* Tube sockets (EL34, 12AX7, 5Y3GT) – make sure they’re 5‑pin base for EL34 and 9‑pin for the others  
* Wire: 22 AWG for power, 24 AWG for signal, and a few strands of 18 AWG for B+  
* Screws, standoffs, and a short length of heat‑shrink tubing  

You can find the full schematic and PCB layout on the Valve Audio Lab site (https://logzly.com/valveaudiolab). I’ve linked the PDF in the resources section at the bottom.

## Tools You’ll Need  

* Soldering iron (30‑40 W) with a fine tip  
* Desoldering pump or braid  
* Wire cutters/strippers  
* Small screwdriver set (Phillips and flathead)  
* Multimeter (for continuity and bias checks)  
* Tube tester (optional, but nice for checking used tubes)  
* Safety glasses – trust me, the high voltage is no joke  

## Step‑by‑Step Build  

### 1. Prepare the Chassis  

1. **Drill mounting holes** for the PCB, transformers, and tube sockets. Use the template from Valve Audio Lab to keep everything lined up.  
2. **Attach the output transformer** first; it’s the heaviest part, so secure it with at least two screws and a rubber gasket to damp vibration.

### 2. Assemble the Power Supply  

1. **Mount the line and filament transformers** on opposite sides of the chassis for good airflow.  
2. **Solder the filament winding** of the 12 V transformer to the 5Y3GT heater pins (pin 4 and 5). Keep the leads short and use 22 AWG wire.  
3. **Build the B+ filter**: chain the 470 µF electrolytic, then the 100 µF, and finish with a 0.1 µF film capacitor across the filter node and ground. This smooths the high voltage.  
4. **Add the bias resistor** (1 MΩ) from B+ to the bias pot. This is where you’ll set the idle current later.

### 3. Lay Out the Pre‑amp Stage  

1. **Place the 12AX7 socket** near the input jack. The first stage is a simple voltage‑gain circuit: 100 kΩ plate resistor, 1 kΩ cathode resistor (bypassed by 0.1 µF).  
2. **Wire the coupling caps** (10 µF electrolytic) from the plate of the 12AX7 to the grid of the first EL34. Keep polarity straight—negative side to the grid side.  
3. **Insert the volume pot** (100 kΩ log) in the signal path after the first coupling cap. This gives you a clean level control before the power stage.

### 4. Build the Power Stage  

1. **Seat the EL34 sockets** opposite each other, leaving enough room for the output transformer leads.  
2. **Plate resistors**: each EL34 gets a 100 kΩ plate resistor. Connect the other end of each resistor to the B+ rail.  
3. **Cathode bias**: solder a 470 Ω resistor from each EL34 cathode to ground, bypassed by a 10 µF electrolytic (for smoother bias).  
4. **Screen grid**: tie each EL34 screen grid to B+ through a 10 kΩ resistor.  

### 5. Connect the Output Transformer  

1. **Primary side**: the two EL34 plates feed the primary winding. Wire them in a push‑pull configuration (plate of tube 1 to one end of the primary, plate of tube 2 to the other).  
2. **Secondary side**: connect the 8 Ω speaker leads to the secondary terminals. If you plan to use a power amp downstream, you can add a line‑level tap (check the transformer datasheet).  

### 6. Final Wiring and Safety  

1. **Ground everything** to a single point (star ground) near the power supply. This helps keep hum down.  
2. **Add a fuse** (1 A slow‑blow) on the mains input.  
3. **Label the high‑voltage nodes** on the chassis with a permanent marker.  

### 7. Bias Adjustment  

1. Power the unit on with a **dummy load** (e.g., a 8 Ω resistor rated for 30 W).  
2. Measure the voltage across each cathode resistor. For EL34s, you’re aiming for about **70 mA** idle current.  
3. Adjust the **bias pot** until you hit the target. If the voltage is too high, increase bias (turn clockwise).  

### 8. First Sound Test  

1. Connect a modest speaker or a power amp, turn the volume knob up slowly, and listen. You should get a smooth, warm break‑up as the tubes approach clipping.  
2. If you hear hiss, double‑check the grounding and the 0.1 µF coupling caps.  
3. If the tone is too bright, try swapping the 100 kΩ plate resistors for 120 kΩ; this reduces plate voltage and mellows the sound.

## Testing & Tuning Tips  

* **Check for microphonics**: tap the tubes gently while the unit is on. Any howl means a cracked tube—replace it.  
* **Measure B+**: you want around 300 V on the EL34 plates. If it’s lower, verify transformer connections and filter caps.  
* **Heat management**: after a few minutes of operation, feel the chassis. If it’s scorching hot, add a small fan or improve the airflow path.  

## Wrapping Up  

Building a 30‑watt all‑tube preamp is a fantastic way to get hands‑on with classic audio electronics. The parts list isn’t overwhelming, and the steps are straightforward enough for anyone with basic soldering chops. At Valve Audio Lab we love seeing these projects come to life, so feel free to drop a comment or share a photo of your finished board. The joy of hearing that first tube‑saturated chord is worth every minute of wiring.

Happy building, and may your tubes stay warm and your sound stay rich.