6N2+6P1 DIY single ended class A tube amp with headphone and triode mods

This is a popular DIY kit available from a number of Chinese distributors. There are several variations, with various size tubes and with either vacuum tube or solid state rectifiers. They range in price from about $130 to $250 depending on configuration. I went with the smaller 4 watt/channel 6P1 tubes and a solid state rectifier for simplicity and reliability.

Why bother with a system like this instead of buying a cheap Class D amp with USB, bluetooth etc?

• This is a high quality class A amp with simple all-tube signal path
• Tubes are much more linear than transistors and require less corrective circuitry: an equivalent solid state class A amp would be much more complex and expensive.
• The design has a clean, vintage look without taking up as much space as a vintage amp
• It's a fun DIY project, and easy to customize

I'm publishing this page because the kit came with minimal instructions and it's easy to make mistakes in spite of the simple circuit. Moreover I'll provide instructions on adding two modifications:

• Headphone jack. A rear-panel switch selects the headphone output along with high-power resistors to protect the output transformers from an excessive high impedance load.
• Switchable Triode/Tetrode mode. The 6P1 beam tetrodes (similar in operation to pentodes) can be wired for triode operation, reducing the power to about 2.5w/channel and enabling the reduced distortion characteristic of triode tubes.

WHAT TO BUY

I ordered the 6N2+6P1 kit via AliExpress, from Breeze Audio Store, which specializes in DIY audio. The price was very reasonable and it only took a week for delivery. The circuit included relatively large value capacitors for power supply filtering and good quality components for the price. The kit came with an accurate printed circuit diagram, but unfortunately no wiring diagram.

I also ordered a Neutrik NYS212 insulated headphone jack (more on this below), two each of high-power 8-ohm and 4-ohm resistors, and two extra DPDT switches.

HARDWARE ASSEMBLY

Before getting started I drilled an additional hole in the front panel for the headphone jack -- next time I'll invest in a carbide bit! Rather than drilling holes for the Headphone switch and Triode/Tetrode switch, I repurposed two of the speaker binding post holes on the rear of the chassis. This required eliminating the taps for 4 ohm speakers.

Also this would be a good time to drill and screw on the wood side panels, since this is hard to do after the wiring is done.

I bolted the transformers in place and also mounted the switches and connectors on the chassis. Be careful not to overtighten the RCA jacks because it is easy to strip the threads. I ended up doing so and had to use glue to keep them from twisting.

Also be sure to use the plastic washers where provided to prevent grounding components to the chassis and introducing ground loops. When I first assembled the project I used a metal-grounded headphone jack which resulted in hum. Swapping it for a plastic insulated jack eliminated the connection to the chassis and reduced hum to inaudible levels.

CIRCUIT DIAGRAM

Here is the circuit diagram (for a single channel only) supplied by Breeze Audio, with my added triode/tetrode switch and headphone mod.

WIRING

Here is a wiring diagram and photo, following a star grounding scheme to minimize the chance of ground loop-induced hum. For North American 120v power, combine the like-colored primary wires of the power transformer as shown. Also be careful to use the output transformer wiring as illustrated -- using the wrong windings can result in positive feedback instead of negative feedback, producing loud oscillation.

RESULTS

I checked the frequency response at the headphone output with a frequency analyzer and confirmed it to be flat within the audio band. Powering high impedance Sennheiser HD-580 headphones the amp is dead quiet, clear, and powerful. Driving passive desktop speakers (KEF K-120) the amp sounds natural and produces convincing soundstage width and depth. Of course for high volume listening, the limited 4w/channel would necessitate high efficiency horn-loaded speakers.