DIY Audio Projects Forum

First amp build: K-12G tube amp kit
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Author:  DisasterArea [ 02 Dec 2011, 10:12 ]
Post subject:  Re: First amp build: K-12G tube amp kit

OK, so it's been about a year, time for a post-mortem on the amp.

First off, it just keeps sounding better and better, I managed to find a full set of four GE brand 10GV8's and they improved the sound even further... though not by leaps and bounds. Soundstage got a bit bigger and they're slightly more detailed than the stock tubes. Originally I had it powering 2 Mordant-Shorts, since then I've added an Acoustic Research Sub to the system which really helped fill out the sound. Volume wise, I'm not shaking the rafters, but it's plenty loud enough to be enjoyed two rooms away in the kitchen while I'm cooking. During testing, I also had it hooked up to my two Minimus 7 test speakers... I was really surprised how good they sounded, within a hair of the Mordant-Shorts... to the point where if the Mordant's blew, I could take my time finding replacements while using the Minimus 7's in the meantime.

Second, the kit write up mentions in passing: "Be sure to always have a load on the OPT's when powered up"... DO THIS... I accidentally pulled a speaker wire out while it was playing and blew my left channel OPT... that sucked. I only replaced the blown OPT, there was only 4months of off and on usage on them, so I figured a new and a slightly used one wouldn't be too mismatched.

Third, as I stated in my first post, the build quality of the VT4C chassis was kinda crap-tastic... yeah, when I had to replace the left OPT, I ended up stripping a couple of the bolts holding the transformer casing trying to get them out and ended up having to drill them out. If you buy from VT4C, DO NOT use the supplied bolts, buy your own, you'll be happy you did. The chassis has also picked up more scuffs and scratches than I would have expected... maybe they use a particularly soft aluminum, I don't know... I'm thinking maybe I should have put some kind of clear coat or something... no biggy, with brushed aluminum they're hard enough to see, but *I* know they're there :)

Another small pet peeve, the chassis said it was compatible with Alps volume pots... well, it kinda is... the little hole next to the main hole, where the little post on the pot goes to keep it from turning wasn't QUITE in the right place, so I had to drill a new one, and the post on the 100K alps is too long for the supplied volume knob. Little things, to be sure, but worth mentioning.

Over all I REALLY like this kit, I'm sorely tempted to pick up another one just to shelve it for a later project. I'd like to try their 16watt kit, but I haven't seen many reviews of it, I think people may have been scared off by the terrible review of their other 8 watt tube amp kit (K-8LS) ... too bad...

Author:  DisasterArea [ 06 Dec 2011, 14:04 ]
Post subject:  Re: First amp build: K-12G tube amp kit

I'd also like to note... one of the mods I was planning on doing was an LED power light as there's a hole for one in the chassis. I didn't end up doing it because I just didn't have the parts on hand and I was impatient to get it built. In the end I'm glad I didn't... the glowing tubes are indication enough that the amp is powered up and it's one less thing in in the current path.

The chassis also has a second knob option, the middle is being used for volume, the right knob is just cosmetic right now, but I may add a signal selector knob there in the future. Don't need it at the moment as my line level is coming from a Tube Magic D1 which servers that function right now.

I've also been toying with adding a headphone jack option, I've seen various write ups on how to do this (8ohm resistor in series with a 100Kohm resistor in parallel) I'd like to find a way to have plugging the headphones in kill the speakers I'm just worried that any kind of switch would mean that (for however small a fraction of a second) there would be no load on the OPT's... is there any method to make sure there's always a load on the OPTs?

Author:  gofar99 [ 06 Dec 2011, 18:12 ]
Post subject:  Re: First amp build: K-12G tube amp kit

Hi, Nice build. My original K-12 has the11MS8s. Considering that the tubes are nearly identical in spec, I doubt that there is much difference in sound. Unless S-5 has perfected the K-16 I would pass on it. The increase in power output will not be great (loudness is not a linear factor of power input). I personally was quite unhappy with the K-16 I got (an early one) and another diyer was so unhappy with his that he mailed it to me for parts. IMHO the K-12 is a far better product and even without mods pretty good sounding. With mods it can be even better. To be fair to S-5 they may have fixed the problems with the K-16 since then. Just my two cents. For what it is worth...they were a big chunk of the incentive for me to design my own stuff. I felt I could come up with something that sounded better and was better suited to my needs. A few folks seem to think I did :)

Too bad about the problems with the chassis. Still it looks sharp from here. :thumbsup:

Author:  DisasterArea [ 07 Dec 2011, 09:18 ]
Post subject:  Re: First amp build: K-12G tube amp kit

Yeah, it kind of boggles my mind why they would be phasing out one of the most successful, cheapest and best sounding tube amp kits and continue to sell what all accounts say are just terrible kits. The only thing I can think of is a dwindling supply of 10GV8/11MS8 tubes. It's really too bad... if I have the cash after Xmas I'm going to grab another k-12g tube amp kit to shelf, just in case I want to build another in the future

Author:  gofar99 [ 07 Dec 2011, 11:25 ]
Post subject:  Re: First amp build: K-12G tube amp kit

Hi You can always use a different power trannie and use the ECL85 (AKA 6GV8). If I were to build from scratch that is the route I would go. Add in a pair of Edcor output trannies...most likely the G series 10 watters (far better than they spec them) and end up with a really fine sounding amp.

Author:  Gio [ 07 Dec 2011, 13:40 ]
Post subject:  Re: First amp build: K-12G tube amp kit

DisasterArea wrote:
dwindling supply of 10GV8/11MS8 tubes.

Yes, it was getting harder to find 11MS8 tubes, so they switched to 10GV8. The only changes between the amps is that R8 is 300R/3W for 11MS8 and 150R/3W for 10GV8 tubes. So it is fairly easy to swtich the amps over.


Author:  hatrack71 [ 14 Feb 2013, 10:25 ]
Post subject:  Re: First amp build: K-12G tube amp kit

Just starting one of these myself. Love the look of these chassis and trannie covers. Gonna go this route except I'm going to opt for the slightly smaller chassis with only power and volume cutout in the front. A couple questions. How did you orient the transformers inside the cases? Any chance you have any pics of the guts too? I'm trying to decide on whether I want to use "stilts", mount all the components on the bottom of the board except tube sockets, or just use 4 socket savers (like those used for tube testers to raise the tubes up an inch. Any ideas on what is the easiest/ best method or if using socket savers degrades the sound at all? This will be my first kit. I also have the S-5 Electronics K-12g. After seeing how your's looks, I'm excited. :idea:

Author:  gofar99 [ 15 Feb 2013, 10:03 ]
Post subject:  Re: First amp build: K-12G tube amp kit

Hi, The output transformers ought to be mounted at right angles to the power transformer. I find it best to keep at least 2 inches between any of them. More is better. That way you will avoid interaction between them. Additionally be sure the transformer cases if you get ones with them are grounded to the chassis. You may need to scratch off some paint on one foot and use a toothed washer to be sure of good contact. I highly recommend again using small chassis. The amplifier will not only be much easier to build, but less likely to have interactions between components and and heat related problems if you use a bigger chassis. 8X12 inches is as small as I would go on a K-12 type amp.

Good listening

Author:  hatrack71 [ 17 Feb 2013, 06:43 ]
Post subject:  Re: First amp build: K-12G tube amp kit

Thanks Bruce. The chassis I had in mind is only an inch or two smaller in length and width, same height. Thanks for pointing out the grounding on the tranny covers- was not even remotely aware of that type of issue. I think I'm gonna go with the AL-316s chassis instead of the AL-421S the OP used. See any issue with it being too small or is that OK? I have the K-12g kit. Was still wondering which option was best too- under board component mounting, stilts or socket savers. Any advice there? Here is the link to the page for the chassis- scroll down to find the chassis mentioned above: ... 2&hit_cat=

Author:  gofar99 [ 17 Feb 2013, 14:25 ]
Post subject:  Re: First amp build: K-12G tube amp kit

Hi, I believe I would mount the components on top of the board. If you lay down the filter caps they will only stick up about 1 inch. The tube sockets stick up about 1/3 inch as is. You could use taller ones, many come with up to 1 inch leads. The problem with them is they are not as sturdy as the shorter ones. Either way use 1 to 1.25 inch spacers between the board and the top of the chassis. That way the only thing sticking up would be the tubes and trannies. Be sure to add a few extra support spacers as you don't want to stress the board. If you use the short tube sockets make the chassis holes about 1/2 inch larger than needed for the tube to allow for heat escape. If you are handy with drilling holes in the right place (not one of my strong points) you could fasten the board to the bottom of the chassis and just let the stuff poke though the top holes.

Good listening

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