If you love EL84 tubes then this is the perfect Pre Amplifier for you! This circuit is highly inspired by the Salas' 6V6 Pre Amplifier. I have reworked the circuit from the ground up to work with EL84 tubes. You can use this PCB as a pre-amplifier. You can also use it to drive a Single Ended EL84 output stage, an example for this can be seen in the SPICE simulation.

This PCB is a Mono amplifier. You will have to order two PCBs for a Stereo Amplifier. I have left out most parts of the power supply so you can chose your own rectifier and regulator. I will likely draw an extra PCB soon for a high quality power supply which will integrate the Capacitance Multiplier as used by Salas.

Power transformer requirements

The circuit is designed to operate at a B+ voltage of 300V. Each channel will consume roughly 25mA. The heaters are in parallel with each other. The input voltage for the heaters is 6.3V and the current requirement is 1.6A. Due to the nature of the design it is sufficient to power the heaters from a 6.3V AC winding.

Features

• Safety first! Both power supply inputs are fused. High Voltage traces have plenty of spacing to prevent over-arcing.
• SRPP Gain Stage for extreme low distortion
• Large Footprint for your favorite Coupling Capacitor. I have added extra holes around the Coupling Capacitor so you can use tie wraps to mount even the biggest caps.
• Integrated RFI/EMI shield
• Output relay to short the output to ground during start up.
• "Twisted" heater trunks, which are shielded by a ground plane, to eliminate noise from heater traces.
• SMA connectors are used for optimal shielding.
• Tube Heaters are elevated to +70V. Vhk for an EL84 is 100V, at +70 Volts both heaters are within their operating limits.
• Plate current is set to 20.78mA and the Plate dissipation is under 3.4 Watt to ensure a long lifetime for the EL84 tubes.
• VIAs are stitched for VIAs where the current exceeds 1A.

Feedback

• Local Feedback is used for V2 to increase linearity and reduce distortion.
• Global Feedback (Post Capacitor Feedback) is applied to reduce distortion even further. PCFB is optional, you don't have to use it!

PCB

This PCB is specifically designed to be manufactured by PCB-Way. You must ensure that you configure these settings when you place your order:

• Layers: 4 Layers
• Material: FR-4 (default)
• FR4-TG: S1000-2M TG170
• Thickness: 2.4mm
• Min track/spacing: 6/6mil (default)
• Min hole size: 0.3mm (default)
• Solder mask: Chose your own color, I prefer Matte black
• Silkscreen: Chose your own color, I prefer White
• UV printing Multi-color: None (default)
• Edge connector: No (default)
• Surface finish: Immersion gold (ENIG)
• Thickness of Immersion Gold: 1U" (default)
• Via process: Tenting vias (default)
• Finished copper: 2 oz Cu
• Remove product No: Yes (extra+$ 1.5)

Layers

This PCB uses 4 layers. The layer names are defined as:

• L1: Top
• L2: Route2
• L3: Route15
• L4: Bottom

Licensing

This product was designed under the CC BY-NC 4.0 License.

Components

All components used for this project, except for the Tube Sockets, can be ordered with Reichelt.de.

Support

I have opened a thread for this Amplifier where you can discuss it and receive support from myself, and perhaps also from the community.

You can find this thread here: https://www.diyaudio.com/community/threads/el84-srpp-pre-amplifier.442771/

Mounting

I have added 3.2MM holes for mounting. The tube sockets are inverted and should be mounted on the rear side of the PCB. This allows you to mount the PCB within an enclosure and have the tubes stick out the other side, on the top of your chassis.

Performance

Amplification factor: 20dB / 10x (You can adjust this by adjusting the feedback resistor R9)

THD: 0.000528% at full power