K-12G Tube Amplifier Build

Recently I was perusing the classified section on the diyAudio forum (https://www.diyaudio.com/) and a fellow was offering a small, four tube, amplifier kit for a very reasonable amount. I contacted him and he assured me that all the parts and instructions were there. Seems he had bought this kit years ago and simply never built it. He then made me an offer I simply could not refuse. The K-12G amplifier kit and two “Alpair 10 / Gen 2 loudspeaker drivers for $200…Shipped! Like hitting the lottery! Thanks Robert!

This kit was offered by s5Electronics http://www.s5electronics.com/

(But sad news, a few days after I had assembled this kit, and realized I’d need new tubes (more on this later) , I contacted S5 and was told the owner, Larry Stafford, had passed away just a few days ago of heart failure. Kathy in the s5 office told me they were liquidating the inventory and shutting the company down. Larry was a VERY divers person. He sold tube amplifiers, hearing enhancement devices and antique truck parts……May he rest in peace…)

So the kit arrives and it looks quite easy to assemble. The parts were laid out and the soldering iron heated up. Away we go!

A couple hours later and the amplifier is finished. It went so fast I didn’t take any pictures of the assembly process.

But I did do a video of the “First Turn On”…

https://images46.fotki.com/v1671/photos/2/36012/16435085/P1050125-video.mp4

First listening impression was not too bad. Much more on this later.

Now that the amp was done, it was time to make it look pretty.

Last year I built a much larger and more expensive tube amplifier, VTA (Bob Latino) ST120 Dynaco Clone. You can read all about that adventure here:

http://rvbprecision.com/stereo/vta-st120-dynaco-clone-tube-amplifier-build-in-pictures.html

I wanted this “Baby” amp to look somewhat like the VTA. I went to Home Depot to buy some walnut to build a case for the amplifier. They didn’t have any, but they did have some kind of African Mahogany decking boards at a very reasonable price. This stuff is harder than Oak with a very straight grain and $11 for a 1x4x8′ piece. At todays lumber prices, this is a real bargain!

Once home and in the woodshop, the saws commenced cutting and shaping.

The results:

VU meters were sourced from China via Ebay and installed. I decided to run them off a remote wall wart as they required 12V “AC” and I was concerned about the little AC transformer causing unwanted noise if it was inside such a small, wooden cabinet. A 3.5mm power jack was added to the rear panel.

A toggle switch was added to turn the VU meters on and off .

A few mods were still needed.

I had painted the transformers green, to match the circuit board, but I really didn’t like the look. So off they came again and Black Wrinkle Paint was applied and the wires covered with split loom.

I also didn’t like the cheap volume knob the amplifier came with. So a few minutes on the metal lathe with a piece of 6061 aluminum bar stock…

At this point I’m pleased with the project. But “Houston, we have a problem”…

The amplifier has a VERY faint hum in both speakers and one channel has a very low buzz.

Attacking the buzz first, I switched the two left tubes for the two right tubes and the buzz switched channels. Ah, easy peasy….Needs new tubes….Not so fast Batman! This amplifier uses 10GV8 tubes which have been out of production for years. A quick search of Ebay found a matched set of two and three other singles. All NOS (New Old Stock)…I bought them all. Then I contacted s5Electronics, the vendor for the amp (See the note above about Larry) and they had only four in stock and sold them to me. When the new tubes were installed the buzzing disappeared.

But the very low hum was still there. OK, I get it, many tube amplifiers of this caliber hum. And this hum was , ear one foot from the speaker, hum. It didn’t influence the music…to my hearing. But I knew it was there and it had to go.

I was told on various web sites of others that built this amp that the volume control was junk and had to go. So at Parts Express I ordered an ALPS control and installed it. It dropped the hum down about half!

Next, the output transformers were moved to be 90 degrees to the power transformer.

This knocked the hum down another 30%. Now the hum was “Ear ON the speaker” hum. I could live with this.

The hum does not increase with volume. It is constant regardless of input, volume level, etc. I even built a small cage to go over the power transformer and grounded it…No improvement. And tried all kinds of different grounding topographies. No Joy.

But another issue surfaced. That really nice volume knob I made was aluminum. Every time I touched it the hum would increase to the point where I was concerned for my speakers.

Another knob was turned out of Plastic Delrin material. No more issues!

Finally, an indicator light was added to show the unit was powered up.

So, let’s cut to the chase…How does it sound and what is its performance.

The November 2002 magazine, “Audio Express” did an article on this little amp. A Google search can find a PDF of the article. (I don’t know how to post a PDF on this web site)

They ran it through their test equipment:

Output Impedence: 6.6 ohms @ 1kHz

Power Output: 8w/ch at 1kHz /4 ohms

Freq Response: 14Hz-58kHz +/-3dB

THD: 1.3% 1W 1kHz 8 ohms

SN: -66dB

As you can see, the specs are nothing to write home about…..but I’ll quote the reviewer in the article:

“What can you get for $140 nowadays. If you are willing to play by the rules as defined by this amplifier the answer is quite a bit. The K-12M produces a smooth musical sound that rarely offends. When coupled with appropriate components, it is a joy to hear. We have not heard a better sounding amplifier in this price range.”

A lot to digest here. First they were reviewing an “M” version. that uses 11MS8 tubes. I have no idea how sonically this differs from my amp with 10GV8 tubes.

“Appropriate Components”…IE, efficient speakers

“In this Price Range”……

So, what are my REAL WORLD listening comments?

I’ve been listening to this amplifier for near 50 hours so far. Using a Zero Zone (Conrad Johnson Chinese clone) tube preamplifier, Node2i streamer into a Gustard X-16 DAC and KEF LS50 Metas, B&W DM601, and JAMO 801 speakers.

If a piece of HiFi sounds good to me, I can listen to it for hours. If it just isn’t right, 30 minutes into listening session and I’m off doing something else and the equipment is shut off. Well, this little amp kept me up well after midnight a number of times! I simply loved the way it sounds. Sure, you are not going to blow the windows out of your house. And organ music would suffer. But for jazz, vocals, early soft rock (my kind of music) and at 75-80 dB listening levels with any of the three speakers, it is really very, very good!

Other than bass response, it is surprising how loud 8w can really play!

Another option is to use it just for the mids and highs and let a subwoofer handle the lows….Now we are are talking!!!

I used inline, 80hZ high pass filters between the preamp and the K-12G tube amplifier and then to the KEF LS50 Metas and an SVS 1000 subwoofer handling all below 80hZ….Oh my Lord! This was magic!

80hZ High Pass Inline Filters

Steely Dan, Blood Sweat and Tears, Paul Brown, etc just sounds marvelous! Great imaging….Lots of air around instruments and that “tube” high end smoothness…… Yup as the reviewer said, nothing like it for the price!

And for you “Wire Wizards”, here is the Schematic:

Hope you enjoyed this little tube amplifier adventure…..be sure to check out many other “adventures” on my web site

http://rvbprecision.com/

and my YouTube channel:

https://www.youtube.com/channel/UCHnX8-AC4ylVovILzLro8Vg/videos

UPDATE!

This amplifier is now the heart of my office system….

KEF 103.2, K-12G Tube Amplifier, WiiM Mini streamer, Schiit Loki EQ,¬†Topping DX3 Pro+ DAC…Sony DVD/CD Player…DIY input selector……Amazing sounding system!.

Mini Ipad Dock…..For accessing streaming services…

THANKS FOR LOOKING!

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