S5 Electronics K12G/K12M Amplifier Kit


I built an S5 Electronics K12G amplifier kit, a new version of the K12M kit. The kit came without a chassis. Though S5 does offer a chassis, the style wasn't one I wanted, so I built it on an LMB box as others have done. This page describes some of the choices I made.

The kit uses four 10GV8 tubes, which are triode-tetrode combinations. Two tubes are used for each channel. The triodes are used cascaded as a preamp and the tetrodes are used push-pull.

Current Status

I've finished the kit and it works. There are a few more things to do (see below).

LMB Chassis box


I used a 12x7x2 LMB chassis #7122, from Halted Surplus.

Proposed Layout


I positioned the major parts as shown. Rotating the output transformers to an angle other than 90 degrees might have given less unwanted inductive coupling, but it looks weird and is probably a third-order effect in this device anyway.



I cleaned the chassis and drilled the major holes using a Unibit when necessary. I covered transformer mount points and othe screw holes with painter's masking tape and then painted the chassis with Plastikote black Krinkle-Finish Paint. (I've also used VHT brand in the past.) Since it's cool outside now, I warmed it with a hair drier first, then followed directions. Since I've had trouble with this paint peeling off in the past, I waited two days before attempting to remove the tape, and used a knife to trim the edges safely.

Additional Parts


These are some of the additional parts. The bypass caps and UF4007 diode suggestions came from voltsecond's page.

Not shown: UF4007 diode, a 117VAC neon bulb and housing from Radio Shack (#272-707), Rescue Tape, fish paper, and a variety of grommets.

Board Bottom


In order to put the tubes out the top, I mounted the parts on the bottom. Unfortunately, that also means that the silk screen printing is on the other side of the board, so I failed to read it correctly, and mounted the B+ capacitor backward. It hissed as soon as I applied power (without tubes), and of course staring at the board didn't help. Looking at someone else's photos on the web did help, so I'm publishing mine (warts and all) here so that others might benefit from my mistake. So, if your B+ capacitor is aligned the same way as the other electrolytics, it's wrong!

Board Top


You can't mount the diode bridge on the bottom, as it would reverse the polarity. Since one of the voltsecond modifications is to use a UF4007 from the bridge to the B+ capacitor, I went ahead and cut the trace on the bottom, lifted the pin on the bridge for good measure, and put the UF4007 on the top as well.

Not shown: I also used Rescue Tape (self-fusing silicone tape) to form a grommet around the base of the tube sockets, for insulation, and additionally used pieces of fish paper atop the board, for more insulation from the chassis. The supplied bananna jacks weren't insulated, so I used Radio Shack #274-7258 insulated banana jacks. I used Radio Shack #274-0346 shielded RCA phono jacks, because the supplied ones weren't suitable for chassis mount. I used shielded twisted pair for the input lines, grounded at the chassis end. For the outputs, I fashioned a few inches twisted pair out of some hookup wire; some might suggest I use heavier wire, but it's a very short run.

To mount the board to the chassis, I used the supplied standoff pairs, and 1-inch #4-40 black anodized hex-head screws, with split-ring lockwashers and hex nuts on the bottom. I used Rescue Tape around the HV area. It might be better to use a nylon 4-40 screw for that one corner.

For the transformer mounting I used nickel-plated shoulder washers and pan-head zinc screws. I'll probably be painting them black anyway. I put some heat-shrink tubing on the AC line in to the transformer since it enters at the top, but I didn't put any on the others. For the heater and B+ lines, I ran the grommets directly under the transformers. For the output transformers, I was concerned the wires might not fit through; in retrospect, it would have made it look better to put heat-shrink tubing on the output transformer wires as well.

Inside Chassis

inside chassis

The electrical tape on the board edges is holding rectangular pieces of fish paper to the board, for insulation.

Parts List


S-5 Electronics K-12M Tube Amplifier Kit
The best build walk-through and notes.
S5 Electronics K-12M Tube Amp
voltsecond describes each modification and shows before/after test data. I decided to do a few of his suggestions.
S-5 Electronics Tube Amplifier Kits - DIY Audio Projects Photo Gallery
Lots of photos of completed kits; good for ideas.
S-5 Electronics K-12M Tube Amplifier Kit
Another build similar to mine.
DIY: Tube Amplifier Kit Build (In Recycled Box), Part 1
Another build similar to mine.
S5 Electronics Tube Amplifier
A wooden case.
Hammond Ventilated Steel Cage for 4" x 8" Chassis, Black Finish 1451-10BK3
A tube cage that might work, though it's slightly larger than necessary.
Perforated Aluminum Sheet
This sheet is 12x24". The 12x12" one is too small.

Some relevant discussion:

S-5 Push Pull Power Supply Upgrade! - diyAudio
10GV8 Bias point for the K-12G
A discussion of how the bias is set.


Here's my todo list, in decreasing order of likelihood: