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123rmp 21st April 2008 08:28 PM

K-16LS kit Assembled and Working
As I indicated on earlier threads where I asked a few questions, I intended to put the S5 Electronics K-16LS kit together. I have it assembled 100% stock with the optional tone control module also together and working. It only took about 2 hours to assemble.

So here's the completely qualitative and subjective review:
The device works quite well for me and produces an abundance of sound out of my old Genesis II speakers, enough to make it unpleasantly loud if I crank the volume. The tone controls are definitely worthwhile. I must state that this is my first tube amp so I can't compare it to others. My "normal" daily use amplifier was a Harmon Kardon 3470 receiver. Compared to that, the tube amp definitely provides a "different" sound overall (warmer and more rounded I would say) and it is on par quality-wise (to my ear) with the HK 3470. I am slightly disappointed by what I detect as a slight lack of bass but that is again purely a subjective comment. There is a slight 60Hz hum which you can hear when there is no input signal provided but it's not detectable to my ear when input is provided.

I plan to do some of the usual simple changes and upgrades to components etc, such as the coupling capacitors, output transformers, filter caps, etc. A couple of you have provided suggestions already and I would welcome any suggestions or guidance as to how to improve this.

One of the first things I do plan to get is the Edcor CXPP25-8-5k based on an earlier suggestion. I am also considering upgrading the coupling caps with the Auricap type, which Bruce Hearn in his upgrade of the K-12M ( said produced the biggest improvement in performance.

Any other suggestions or perhaps alternate replacement parts, anyone?

gni 24th April 2008 02:19 PM

Congrats. . . sounds like a success. . . pics?

bayermar 24th April 2008 03:16 PM

Hi 123rmp,

I finished a K-502 amp for a friend half a year ago.
Although the tubes are different I think the same compromises have been made in this amp.
My upgrades included other coupling caps, a higher quality potentiometer and changes to the power supply.
From all these, the power supply made the most audible change in my ears but the caps are also worth changing.
First of all, since the K-502 transformer runs boiling hot, I spend an additional filament tranny to ease the burden. This will IMHO not be necessary for the K-16LS.
I could get rid of the hum entirely by including a choke in the power supply (IIRC this was a small one ~1H I had lying around).
I still used capacitor input topology: The rectifier was changed to 4 UF4007 diodes paralleled with 10nF/1000V film caps. From there to a 100uF electrolytic, then the choke, then to a 220uF electrolytic.
I hope I recall that correctly since I do not have the amp anymore (this was a gift).
This topology eliminated most of the hum.
I do not know which single change made the difference since I did them all at once. Maybe the separate filament tranny had also something to do with that?!?

I paralleled the last HV electrolytic with a 22uF Solen film cap that I had lying around and was surprised how different the amp sounded afterwards. The response for low frequencies was much more "direct" and there was more punch in the bass. Keep in mind I do not claim to have golden ears (maybe not even brass ones :xeye: ).

I put the whole thing in a hammond chassis and spend higher quality connectors. IMHO this was a mostly cosmetic upgrade.
I liked the sound very much after the upgrades and it was not easy to part with it. I hope the K-16LS sounds similar or better and does have a beefier power tranny. The K-502 runs too hot IMO.

I hope this info helps.

123rmp 3rd May 2008 04:04 PM

I am holding on pics until I get it further along, right now it's just a naked PCB on a board, which you can see on the S5 Electronics web page. I have some interesting form factor plans and will definitely give pics then.

This amp is warm but never really hot. The tubes are the only source of heat really, so I think they solved the undersized power transformer problem in the new design.

I have got some Auricap capacitors to upgrade the coupling capacitors.

As I said in the initial thread, at the advice of on of the forum members I have ordered two Edcor CXPP25-8-5K output transformers and hope to use them.

I do need to ask a stupid question though and I ask it simply to assure myself that I won't cook the amp. The question is: Given the specs of the Edcor transformer, which are:

Pri. Watts: 25W
Plate (Z): 5,000 ohms
Sec. (Z): 8 ohms
Screen Voltage: 40%

Is it possible to just swap out the transformers or do I need some mods to adapt the amp to receive them? Likely I did not give enough data here; so let me know what other data I need to give you. I can provide the kit schematic if that helps. My engineering skills just are not up to the level that I can figure this out.

Samber 11th July 2008 08:05 PM

I, too am in this same dilemma. It seems as though no published primary impedance for 4) 6AQ5's can be found. I've also considered a Hammond 1615 (5K) other than the Edcor iron. Can anyone enlighten us as to a correct transformer impedance?

I tried an old pair of OPT's pulled from a 7591 pp amp that provided MUCH better bass to the 16LS but alas, lost almost all of the top end.

I've upgraded my kit with Wima Polypropylene Film Capacitors 250V .22uF 10% from Mouser. A 'Po' boy can't afford $12 each for "audio grade" caps. I also had to buy some 68k resistors as my kit came short 2 (just my luck).

Doug Kim 11th July 2008 08:48 PM

I cannot quite remember the website right now, but I saw somewhere that 6AQ5's in 16LS kit is good for 5K loads. Thus, para-PP gives 5K p-p impedance for OPT. (correct me if I remembered wrong).

I also recently finished 16LS kit, and yes the power tranny does not get boiling hot like their old kits. And the hum level is also much lower. Simply many things seem to be improved.

I don't want to spend too much money on this kit, but I do think about upgrading coupling caps and OPTs. Please post follow-ups whenever done.

Anyone tried bypass caps on the kathode resistors?


gofar99 11th July 2008 11:04 PM

Hi All, I'm glad to hear about some success with the K-16. Replacing the stock coupling caps with Auris was the single most effective thing (cost wise) that I did to the K-12. The second was replacing the output transformers. I used Hammond at the time, but would now switch to Edcors. I have the parts for an upgraded amp, but haven't built it. Too many other projects have gotten into the way. I would suggest if your budget and chassis space permits the Edcor CXPP25-8-5K. I use the CXPP25 series in other amps with excellent results. (the 60 series is equally good). If space or budget are issues the the next choice would be the GXPP15-8-5K. They are listed for guitar use, but are better in quality than the generic XPP15 series. Those are good and really quite cheap ($20), but IMHO the quality of a tube amp is limited largely by the transformers. Using good low ESR caps in the PS helps as well.

Good listening

jrebman 12th July 2008 01:24 AM


Some cheaper and equal or better coupling cap choices would be Sonicaps (original and Gen II), Xicon polypro, and a lot of people find the Dayton caps from parts express to be surprisingly good -- and they're dirt cheap.

Bruce, watch those Edcor guitar OPTs -- they typically low frequency -3 dB spec somewhere in the 120-150 Hz range -- not what you'd want in an audio amp unless you were using it for mids and up in a bi-amping arrangement.

These kits look interesting -- I've been looking for some low cost tube amps for a few friends who want to get their feet wet with the tube amp thing.

-- Jim

gofar99 12th July 2008 03:52 PM

Hi Everyone, I have had good results with the Dayton caps as well. Very cheap. The low cost Jantzens work pretty good too. A good friend of mine uses the Sonicaps and likes them. On a tight budget, I would use the Daytons. On the Edcors, The Guitar trannies (according to the folks at Edcor) have a wider response than the "budget" outputs and can handle more power. The reason is largely a matter of price. No free lunch. The guitar ones are about 50% more costly than the standard outputs. The CPXX are about 50% more. I have heard from other diyers that both are pretty good. I personally only use the CPXX series and hesitate to comment much further. If you do change transformers, be aware of the NFB loop phase. It is easy to get the phase backwards (50-50 odds) and the result can confuse you. If you do you will get positive feedback not negative. An easy way to tell is leave the ground and feedback connections off temporarily and try one way then the other. The way with the least output is correct.

Jeb-D. 12th July 2008 09:45 PM

I put Wima MKP4's in my K-502 (which is an older S5 kit). The spacing fit the PCB perfect and the sonics are excellent. They are fairly inexpensive too.

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