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DIY Single-Ended (SE) 6L6 / 5881 Tube Amplifier

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DIY Single-Ended (SE) 6L6 / 5881 Tube Amplifier

Single-Ended (SE) 6L6 / 5881 Vacuum Tube Amplifier

Photograph 1: DIY Single-Ended 6L6 / 5881 Tube Amp

This past summer I saw Transformers: Revenge of the Fallen and I was inspired to build an enclosure for a vacuum tube amplifier that incorporated the Decepticon logo. The tube amplifier replaces a solid-state amplifier which was driven by a tube preamplifier. For the tube amplifier circuit I used a cutoff pentode (6SJ7 or 6SD7) driving a beam power pentode (6L6 or 5881) in a single-ended circuit (class-A operation). The amplifier schematic is very similar to the one I used for my very first tube amplifier.

Single-Ended (SE) 6L6 / 5881 Vacuum Tube Amplifier Schematic

Figure 1: Single-Ended (SE) 6L6 / 5881 Vacuum Tube Amplifier Schematic

The amplifier circuit is very straight forward following the basic application notes. The quality of the audio output transformer (OPT) will have the largest effect on the sound quality. You can consider an OPT with a 43% screen grid tap to allow ultra-linear mode. Use a good quality capacitor (film, pio ...) for C3.

Power Supply Schematic for 6L6 / 5881 Single-Ended (SE) Tube Amp

Figure 2: Power Supply Schematic - 6L6 / 5881 Single-Ended (SE) Tube Amp

This tube amplifier project uses high voltage and contact with high voltages can cause serious injury or death. If you are not experienced with high voltage safety and working with high voltage, this is not the project for you. Please take electrical safety seriously. Complete the mains, ground, fuse and switch connections in accordance with your local electrical code.

The high-tension (HT) power supply is solid state using inexpensive UF4005 "ultra-fast" silicon rectifier diodes. Because I did not use a choke, several capacitors were used to create a quiet HT supply. For the heater supply I used direct current (DC) which is regulated to 6.3VDC by a LM338K adjustable voltage regulator. The LM338 regulator gets rather hot so be sure to use a suitable heat-sink. Consult the data sheet to determine the heat-sink required. Of course you may use alternating current (AC) for the tube heaters, but it may result in a higher noise level.


6L6 / 5881 Tube (Valve) Amplifier Construction

The tube amplifier enclosure is constructed using aluminum as it is easy to cut and work with. For the sides a C-channel section was cut down and held together at the corners using cut down pieces of angle (L) sections. Aluminum sheets were used for the top and bottom of the chassis. If you take your time it is possible to build a very attractive enclosure with minimal tools - a hand drill, hacksaw, file and sandpaper. Here are some photos of the chassis construction.

Tools used for construction of Aluminum Tube Amp Chassis

Photograph 2: Tools for Construction of Aluminum Tube Amp Chassis

Tube Amp Chassis Parts

Photograph 3: Tube Amp Chassis Parts

I had previously mentioned the Transformers movie and I wanted to put the logo on the audio output transformer (OPT) covers. For the OPT covers I used copper clad board. The logo was drawn onto the board and I used small hand saw to cut the logo out which was glued to OPT covers.

Decepticon Artwork for Audio Output Transformer (OPT) Cover

Photograph 4: Decepticon Artwork for Audio Output Transformer (OPT) Cover

The OPT cover box is also built using the copper clad board and held together using solder and glue. By soldering small copper legs to the OPT cover the copper cladding is "grounded" and used to shield the output transformers. The finish on the OPT covers is a metalic-silver spray paint and then baked in hot sunlight.

Audio Output Transformer (OPT) Covers

Photograph 5: Audio Output Transformer (OPT) Covers

6L6 / 5881 Valve Amp Chassis Dry Fit

Photograph 6: Dry Fitting Tube Sockets and Capacitor Bank

For the circuit I used home made (hand etched) circuit boards, but the tube amplifier is so simple it can be easily connected with point-to-point wiring, especially if you choose to heat the tubes using AC. The LM338K regulator is mounted (with electrical isolation) to an aluminum angle which is in turn attached to the aluminum chassis which makes for an adequate heat-sink.

6L6 / 5881 Tube Amp PCB and Aluminum Chassis

Photograph 7: 6L6 / 5881 Tube Amp PCB and Aluminum Chassis Construction

You may have noticed on the chassis that there is room for three driver tubes. I've included another socket (9-pin miniature socket shown and not connected) so I can later experiment with an additional voltage gain stage or driving the amp with a double triode tube (like 12AX7 or 5751).

Finished Tube Amplifier Enclosure / Chassis

Photograph 8: Finished Tube Amplifier Enclosure / Chassis


Sound - Single-Ended 6L6 / 5881 Valve Amplifier

After giving a little time for the tubes to burn in I really warmed up to the sound from this tube amp. I am using new old stock (NOS) Tungsram 6SJ7GT driver tubes with new production Sovtek 5881WXT power tubes. The Sovtek 5881WXT work well with guitar rock music but for more refined music like jazz the JJ / Tesla 6L6GC power tubes sound very nice. This single-ended tube amplifier is a welcome replacement over the solid-state amp I had been using.

5881 / 6L6 Single-Ended (SE) Tube Amp

Photograph 9: Finished 5881 / 6L6 Single-Ended (SE) Tube Amp

Rear View: 5881 / 6L6 Single-Ended Valve Amplifier

Photograph 10: Rear View - 5881 / 6L6 Single-Ended Valve Amplifier

A high-resolution photograph of the amplifier is available in the Photo Gallery - Single-Ended (SE) 5881 / 6L6 vacuum tube amplifier photograph. Questions and comments are welcome in the DIY Audio Projects discussion Forums.