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S-5 Electronics K-12M Tube Amplifier Kit

 Gio Militano    Canadian Flag     To email Gio, type out the email address.
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Page 1: K-12M Tube Amp Kit, Kit Review, Enclosure Construction
Page 2: Upgrades and Modifications

S-5 Electronics K-12M Tube Amplifier Kit

See the S-5 Electronics Tube Amplifier Kits page for additional information on these tube amplifier kits and where these easy to build kits can be purchased.

The K-12M kit is a push-pull stereo power amplifier rated at 8 watts per channel. Included with the kit is a kit assembly and operating manual, drilling template and schematic. The assembly instructions are fairly brief and do assume that the builder has soldering and basic electronics skills. The amplifier does not come with an enclosure, but rather a 7.25" 9.5" pine board. The photographs below show the typical contents of a K-12 tube amplifier kit as well as an assembled tube amp kit on a pine board. Provided you have some soldering skills, the tube amplifier kit can be easily put together on the pine board in a couple of hours.

S5 Electronics K-12G Tube Amp Kit Contents
Photograph 1: Typical Contents of an S5 Electronics K-12 Tube Amp Kit (K-12G shown in photo)

S5 Electronics K-12M Tube Amp Kit
Photograph 2: K-12M Tube Amplifier Kit on Supplied Pine Board

The amplifier kit uses two 11MS8 tubes per channel. The two DC blocking capacitors and the four coupling capacitors are metallized polyester, while Samsung aluminum electrolytic capacitors are used to filter the solid-state power supply.

Stock Tube Amplifier Kit Review

A fairly in-depth review of the S-5 Electronics K-12M kit (stock) was published in the November 2002 issue of audioXpress. However, since the audioXpress review, it is important to note that a few changes have been made to the kit. Most importantly, the tube sockets have been upgraded to ceramic sockets and have completely addressed the melting socket problem outlined in that review. In addition, the circuit has been modified with the addition of two small ceramic capacitors and better grounding to reduce potential noise and hum.

Here are my impressions of the stock kit:

  • The amp is very quiet and the transformers do not hum, but the power transformer does get hot.
  • The tubes run very hot, so do take that into consideration when fitting the kit into an enclosure.
  • This little tube amp has much better overall sound when compared to a typical mass produced solid state amp (Sony, Kenwood, Pioneer ...) and can easily compete wih amps in the $800 price range.

Specifically, I compared the stock K-12M Tube Amp Kit to a Sony STR-DE585. The bass response of the K-12M was tighter, deeper, and much better defined than that of the Sony. The midrange response of the K-12M is superb and makes the Sony sound sick!. The high frequency response of the K-12M is also better than that of the Sony.

Tube Amp Enclosure Construction

While the stock kit as provided is completely functional, the lack of an enclosure does not make it desirable in most environments. For that reason, the K-12M will most likely need to be fit into some sort of enclosure. For those who do not want to do any metal work, you can purchase a metal enclosure for the K-12M from S-5 Electronics.

For my kit, I purchased an aluminum chassis from Hammond Manufacturing. The 1444-24 (12" X 8" X 3") Hammond chassis was available locally from Tip Top Electronic Supply for about $26CDN and is shown in Photograph 3 below.

Hammond 1444-24 Aluminum Tube Enclosure Chassis
Photograph 3: Hammond 1444-24 Aluminum Chassis

To fit the amplifier into the chassis, I mounted all the components on the rear side of the printed circuit board (PCB), with the exception of the ceramic tube sockets. Drilling the enclosure with proper alignment turned out to be much more difficult than I imagined and required a lot of planning and double-checking. However, I did somehow manage to get everything to line up. Photograph 4 below shows a front view of the amplifier.

Front View - K-12M Tube Amp
Photograph 4: Front View - K-12M Tube Amp

The power switch and knobs are from Radio Shack. I used insulated wire from a salvaged amplifier for the input and pot leads. The PCB is suspended from the top of the chassis using screws and the nylon spacers provided with the kit. I had read that the tubes run hot, so I tried to get as much of the tubes and ceramic sockets sticking out of the enclosure as possible to provide better cooling. The handles are from some old cupboards. A picture looking at the rear of the K-12M amplifier is shown below in Photograph 5.

Rear View - K-12M Tube Amplifier
Photograph 5: Rear View - K-12M Tube Amplifier

The power cord and IEC socket were salvaged from an old computer power supply. I drilled a large hole and used a small sharp file to make the rectangular hole for the IEC power socket. The fuse holder was salvaged from an old amplifier. The RCA chassis mounts and the banana jacks are plain and were purchased from a local electronics shop.

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Upgrades and Modifications