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Modifications for the K-12M Tube Amplifier Kit

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Page 1: Introduction, Modifications
Page 2: Oscilloscope Measurements
Page 3: Summary
Page 4: Follow Up


S-5 Electronics K-12M Tube Amplifier Kit - Introduction

Since you are reading this article, chances are you either own or are thinking of buying a S-5 Electronics K-12 Tube Amplifier Kit. As a bit of background, the K-12 is an inexpensive tube audio amplifier available as a kit from several sources. I don't know the complete history of the amp but it is a product of S-5 Electronics in Mesa Arizona. I make no claims regarding its design and have no desire to infringe on any of their rights. I only want to share some of my thoughts and experiences on making modifications to the basic amp.

First off, this is a good product. Right from the start it has an alluring sound. But if you are like me, you feel obligated to tinker and improve things. In that case, I suggest that you do a search on the amp and see what others have done and see what fits your budget, skills and desires. The first two of these are relatively easy to figure out, how much do you want to spend to modify a $139(US) amp and do you know which end of a soldering iron gets hot? For beginners I additionally caution that this amp uses lethal levels of voltage and if you aren't careful it has the potential to kill. If high voltage is a concern to you, I would stick to solid state amp projects like the gainclone / chip amp variety. The third area regarding sound quality preference is a lot more personalized. What sounds good to one person can easily sound horrible to another. So please accept my assessments on the changes (as well as those of others that have modified these amps) as fulfilling personal desires on how they should sound. That being said, some things make good sense in any amplifier and I'll point them out as I go along.


Modification - K-12M Tube Amplifier Kit

I give great credit to S-5 for a quality product and their straight forward design of the K-12. There are slight production changes during its history but my comments should be applicable to all models. Mine is a fairly new "M" model. In order to get a decent starting point I assembled the amp with relatively few modifications. I employed snubbers on the heaters (0.47 uF fit nicely as you can see in the photo), an ultra fast rectifier in the power supply (see VoltSecond's description), noise filters on the secondary side of the power transformer and a twisted pair of wires to reduce the heater inrush current. These are all pretty standard type modifications and I would do them regardless of all else. At this stage the amp was still on the supplied wooden board. I painted mine flat black and put "feet" on the bottom. Personally I thought it was kind of cute, but having 225 volts DC exposed to casual contact, just doesn't seem to be a great idea and is unlikely to meet with spouse approval. If you are good with metal (I'm so-so) you can build a case. I have seen several nice efforts (check out the diyAudioProjects.com for some). If you aren't real talented, spend the money and order the case from S-5. It is well made and goes together easily. I bought the case (about $85US) and I'm glad I did. It looks good and allows you to show off the amp. The only problem I had with the case was a tendency for the output binding terminals to come loose. I finally used Locktite on mine. This stopped the problem. Since then, I have switched to using banana plugs on everything. As often as I switch things around this has been a great time and effort saver. I highly recommend the change if you run into similar issues. I let the amp burn in for a few days. I only did non-critical listening and basically let it rip whenever I was home. I didn't notice any dramatic changes but generally the bass seemed a bit better (more about that later). I would characterize the sound at this point as nice. It was warmer than my other amps (Marantz 2250 and Sonic Impact Super T). but it didn't seem to have the detail I expected and the bass seemed a bit muddy. All this must be kept in context with the rest of the system. I use Dayton BR-1s with dual 15 inch subwoofers (with separate amps and variable electronic crossovers). The BR-1s are fed directly from the K-12. The Daytons are not efficient and would seem to be a bad match for the K-12. But my listening area is small (150 square feet) and thus the power requirements are greatly reduced. The rest of the system usually consists of a Velleman 8021 solid state preamp, CD player by Yamaha (vintage natural sound model), Sony DVD, Onkyo FM, vintage Dual 701 turntable with Grado cartridge, vintage Harmon Kardon ultralinear tape deck and an XM satellite tuner. Amps switch around from Marantz 2255B, 2265, 1060, Sonic Impact Super Ts and Kenwood. Not to knock Kenwood, but it was the handiest amp around to substitute for the K-12 during one set of modifications. Oh yuck! It is probably good for high power demands, but in my listening room it was way off the mark for serious listening. The closest match and at least decent was a Super T.

Bottom View of K-12M PCB
Photograph 01: Bottom View of K-12M Printed Circuit Board

Notice in the above photo that I hadn't yet cleaned up the board after the modifications. The fast diode, 0.01 uF capacitor and 710 ohm resistor are on the bottom of the board as well as the 0.47 uF snubber capacitors. The 0.1 uF capacitor in series with the resistor is on top of the board.

After about two weeks of listening, I decided to "scope" the amp. There was a bit of hum evident on the B+ so I increased C5 to 47 uF. This reduced the ripple a small but noticeable amount.

The next modification was replacement of the output transformers. There are a number of transformers on the market that will work with this tube set. The ones I settled on were the PT-1609s from Hammond. Another alternative is the PT-1608 which has a lower primary impedance. There are various reasons for picking one over the other. My motivation was to keep the secondary impedance close to the original value so as to limit the chance of introducing unpredictable changes. The PT-1609s, just fit inside the S-5 case. They can not be rotated for least hum pickup (from the nearby power transformer) though. Something I might try later is to rotate the power transformer to reduce hum transfer (more on hum later). The transformers are wired in ultra linear (UL) mode. This will usually reduce the output slightly, but really didn't seem to matter in my case. The installation involves cutting the circuit traces that supply power to the screens near the tube sockets and rerouting the power to the driver tube sections. I used a cordless Dremel tool with a small round burr bit to cut the traces. It makes a clean cut and is easy to control. Attaching the PT-1609s is not particularly difficult and the leads easily reached the tube sockets (see Photograph 02). The transformers do require some sort of standoffs as the leads are on their bottoms. Silicon or rubber spacers that are not too soft work well here. I used rubber grommets. This can reduce the transfer of vibrations to and from the transformers. It is essential that the transformer cases be grounded. I scraped the paint off one mounting hole on each transformer and attached ground wires that ran to the main circuit board ground (conveniently near the back of the board). Failure to suitably ground the cases can result in oscillation (more on that later). Back to the listening room.

The bass was a bit fuller and the sound stage seemed to be a bit wider with more detail. My opinion is that this change is worth the cost about $80US for a pair from Antique Electronic Supply.

The next step was to change the coupling capacitors. Many "DIYers" would do this first, I chose to wait and see what the other changes would do. I replaced all six 0.22 uF caps with Auricaps of the same capacity. To fit the physically bigger caps I had to drill holes in the circuit board then run the leads through and solder them on the bottom side. BE CAREFUL doing this. It is easy to create problems. How do I know? Well, I believe you can fill in that part of the story. I succeeded in finding the traces on the bottom of the board when I drilled through from the top. I ended up doing repairs to two traces. This isn't a crisis for any serious DIYer, but certainly is annoying and slows things down. There is some theory and thoughts on how to orient inter-stage coupling capacitors. The guidance generally seems to be to have the outside foil side go to the side with the least impedance to ground. This is thought to reduce noise and hum pick up. I did that and made sure all caps had the same orientation in the various circuits. Incidentally, the least impedance to ground can be through the B+ supply. Back to the listening room.

Hammond PT-1609 Audio Output Transformers and Auricaps
Photograph 02: Hammond PT-1609s and Auricaps

Oh, Wow! Lot's of change here. It sounded like a different (better) amp. There was a dramatic improvement in the mid to upper ranges. The sound was smoother, and more detailed. Well worth the trouble and cost (about $60US). My opinion is that this is probably the single most "bang for buck" change that can be made.


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Oscilloscope Measurements