Well, I'm not really that good at reviewing things but I'll give it a try.  Here goes...

I have a pretty strange system, somewhat mis-matched.  I built a pair of Sachikos about four years ago with the help of a cabinet maker friend of mine.  He loved them so much he kept them at his shop using them instead of his Altec A7's and it was only the fact that I bought a new house that allowed me to guilt him into giving them back.  These use the limited edition Fostex FE206ES-R drivers that came out about six years ago and are about 98db efficient.  I built them because I'd wanted to hear what the single ended triode phenomenon was all about.  Well, shortly after they were built I met my wife to be and spent the next three years getting her over to the states from her native England so audio pursuits took a back burner for a while in favor of getting married, buying a house, renovations, etc.

Initially I was using the Sachikos with a 25 year old boom box but that got old really fast.  It wasn't a very powerful one and although I could rock the entire house with it I needed a better solution as its limitations were very apparent.  I had a bunch of transformers and tubes from projects I'd been planning to build and never finished but I really wanted to find something that I knew would see completion.  That, and the wife really wanted good tunes.  One of the (many) reasons I married her is that she's a total music head and she was very excited and encouraging about my audio hobby.  She's a fantastic gal but she was getting impatient with the boombox/horn system.

I stumbled across the ZKIT1 and it seemed tailor made as a project that was a proven design that a beginner could finish in a short period of time.  I ordered the power transformer from Edcor and all the parts on the components list.  I had some output transformers from an SE EL84 amp that I'd planned on building so I didn't think I'd need output trannies.  Well, all the parts arrived and unfortunately it took a long time for the Edcor power transformer to get here but in the meantime I also sourced a couple of Svetlana SV83 tubes along with a JJ E88CC driver tube plus fuse holder, power cable, etc.  

Finally the power transformer arrived and it was time to solder.  I went over the instructions and much to my dismay the primary impedance on the output trannies needed to be 9.6K rather than the 5K of the Edcors I had on hand!  So much for RTFM!  Somewhat dismayed I rummaged through my audio geek stash and came across a pair of One Electron transformers with 3.5K primaries but 4, 8 and 16 ohm secondary taps.  I decided that since the FE206ES-R's impedance never dropped below eight ohms I could use the four ohm taps and the reflected impedance to the output tubes would be 7K - not the 9.6K of the recommended transformers but since the speakers weren't a difficult or reactive load I figured it would be just fine and I might get a little more power output to boot.  I decided to proceed.

Now, I do HVAC tech support for a living which mostly involves reading manuals to people too lazy to do it themselves but I have to say that it was a bit frustrating to have the info on the website not match the board.  I was a bit worried but after reading many of the posts on the forum I was able to suss out that the CCE mod was now incorporated into the board and how to wire the transformers properly.  The board is well made and I was able to stuff it and get it all wired on a slab of plywood in the space of an afternoon.  

Then the moment of truth.  I connected the boom box speakers (with their 3 ohm load) to the terminals of the board and powered it up.  After a few seconds, no smoke, no fire but I could hear the hum through the little loudspeakers.  I never thought I'd be so happy to hear hum!  Next I powered it down and connected my Wavelength Proton to it, fired it up and my wife and I were surprised to hear very nice sounding music!  I was so happy!  I decided to let it play for a little while to make sure that an early component or tube failure wouldn't take out my irreplaceable loudspeaker drivers.  As it warmed up I could hear the sound improving even on those old boombox speakers.  

Finally I couldn't take it any longer and I decided it was time to bite the bullet and wire it into the Sachikos and my other source - a Wavelength Crimson.  Yes, I said earlier that my system is a bit lopsided.  After a few tense minutes wiring it all up with zip cord speaker cables and a pair of 20+ year old freebie interconnects I got with some component long ago I turned the volume up and the whole house was filled with incredible sound.  I don't know who was happier, me or my wife!  She absolutely loves music of all sorts and had been really pushing me to get a good system going.  She was thrilled with the sound quality as was I.  I've heard my Crimson DAC on some very high end systems and I know very well what it's capabilities are.  Immediately I could tell that these six hundred dollar baltic birch single driver speakers and three hundred dollar two watt triode amp connected to a DAC costing ten times that weren't the mismatch I thought they'd be.  Detail.  Slam.  Depth.  Imaging.  Being curious, I turned up the wick more and it just got louder and louder with no strain until I was well above the level I'd be able to listen for any real length of time.  These things ROCKED.  We stayed up very late that night listening to track after track marveling at the little circuit board, rats nest of wires and lumps of iron and the sound they were making.  As the night wore on the sound got better and better as the components broke in.  Finally we reluctantly called it a night and I headed to bed a very happy guy.

More to follow.

Back when the parts came with the Zkit1, there were 2 output transformer options:  9800 ohm and 3300 ohm.  The 9800 ohm was the default and works fine with 4 ohm speakers, but the winding ratio is simple too large to get full power to an 8 ohm speaker load.  So for 8 or 16 ohm speakers, the 3300 ohm transformers worked better.  Either your 5k or your 7k transformers should work ok with your 8 ohm speakers, I'd guess your 5k's might be the best fit.

Dan

Wow, I can't believe that it's been since last August that I built this.  Well, a few more impressions and an update:

The amp has stood the test of time and remains in my system.  It sounds great but I have done some mods.  

First, the Cornell Dubilier coupling caps on the recommended parts list are a waste of time and I wouldn't bother with them.  After about a week I spent the princely sum of around four dollars and replaced the recommended caps with Sprague 716P film and foil caps.  The results were spectacular - for the money I can't even imagine buying the CD caps unless you want to see for yourself how much a difference upgrading to the 716P caps makes.  716P capacitors are one of the best bangs for the buck in all of DIY audio in my opinion.

Second, the 5W bias resistor for the output tubes has left a nice scorch mark on the board.  I have it mounted WELL proud of the board but I do have the board built so I have the tubes up top and all the other stuff underneath.  I have the amp sitting on standoffs and the resistor has also made a dark mark on the wood underneath the amp.  Now I've been running this amp constantly since last August but I think an uprated part might be in order.  I may get myself two 5w 300 ohm resistors and wire them in parallel at the same spot split apart sort of like a "Y".  I'll do a temp reading of the resistors pre and post mod and let you all know how it works out.

I really hadn't done much beyond that, I hooked up some of Nordost's low end speaker wire, used a pair of Nordost Blue Heaven interconnects and that was it.  I sat back and enjoyed the music.  The hum was a pain, though.  My Sachikos use the 98db 1W/1M Fostex FE206ES-R drivers and there was a lot of hum, even after the resistor mod.  I also wondered what the amp would sound like with a tube rectifier and choke input.  My goal was to make a minimum of changes to the board itself, merely adding a level of filtering between the power transformer and the existing power supply circuit.  

What I ended up with was a 360-0-360 secondary power transformer going into a 22uF capacitor, then into an 8H choke that has 149 ohms of series resistance.  The rectifiers I used were a pair of 6AX4-GT damper diodes.  I also used a proper center tapped 6.3V supply for the signal tube heaters.  Modeled on PSUD2 it didn't look bad at all and when assembled the voltage at A on the ZKIT1 schematic is about ten volts higher than the schematic says it should be and I can live with that.

The result?  There is no hum.  Nada.  Zilch.  None.  I put my ear an inch from the cones of the Fostex drivers when I hooked the amp up to the system and at first I thought I'd disconnected something when I moved the breadboarded amp over to the speakers.  I was wrong.  I pressed play and the music just came out of nowhere - I was actually startled by it.  This addition to the power supply has unmasked more detail and helped a great deal with microdynamics.  I'd recommend this mod to anyone who is buying a new board or has a board lying around waiting to be used.  6AX4-GT damper diodes are ultra cheap and even cheaper if you buy the 12 volt versions (12AX4-GT) and use an outboard 12V filament transformer for your rectifier tubes which is a good idea anyway.  The beauty of this mod is that the only real change to a fully stuffed board is the removal of the two solid state rectifiers.

So what's next?  Well, I'm going to experiment with resistors and possibly replace the 22uF cap before the choke with a 20uF polypropylene in oil motor run cap just to see what difference it would make.

I'll be sure to let everyone know how it goes.

Jora,
  Can you post some pictures of the mods you have done. A parts list and a schematic. I have a hum problem with my ZKIT1 - see "first time build". The hum is strange as it sometimes a buzz - 120HZ? or more of a grunting sound that starts off low and gets louder and stops over a period of about 1 second. The both are heard when the volume is set high and gets louder as you turn it up.
 I look foward to the other mods you are doing, I know we can do better.
John

JPV, normally hum is pretty constant in level and continuous, what you're describing doesn't really sound like hum except that it gets louder when you turn up the volume.  I really don't know what it would be but I'd be happy if my mod corrected the issue.

I was wondering today if there was a way to use tube rectifiers and a choke without using a different power transformer and I think I've found out how to do it.  It will require different value power supply capacitors but they're not very expensive.  I'm ordering the parts and once I know it works I'll do a write up with parts lists.  It will also solve the center tap issue because it uses the six volt tap on the recommended Edcor power transformer as a filament supply for the 6AX4-GT rectifiers and you then use a separate 6.3VAC transformer with a center tap for the filament supply for the signal tubes.

Here's to hoping it works.

Okay, so I'm getting started on this thing.  I'm not going to post about how my amp is currently wired as even though the voltage checks out okay on the board it looks like I was running the simulations incorrectly in PSUD2.  It seems that you're supposed to put the unloaded secondary voltage along with the secondary impedance of the transformer, not the rated voltage and secondary winding resistance of your power transformer into the simulation.  

PSUD2 actually has an impedance calculator built into the program and by measuring your wall voltage, power transformer winding resistance and measured unloaded secondary voltage you can come up with the impedance of your transformer.  It turns out my earlier calculations were way off because of this but let's see what happens in reality.  

I'm going to bring the amp back to stock form with silicon rectifiers and the recommended transformer and use a separate proper 6.3VCT filament transformer and do some measurements of voltages with the stock supply and see how they compare with the PSUD2 simulations.  In my haste to change to tube rectifiers I neglected to take voltage readings of the stock supply and write them down.  

I'm such a tool sometimes.  



So, there is good news and bad news.  I'd hoped to be able to replace the first voltage dropping resistor (1K 6W) with a choke with high resistance.  Well, that ain't gonna happen because the supply rings like mad.  What appears to work is adding a choke input filter (Triad C-24X) and a 33uF capacitor filter to the existing silicon rectified power supply.  This doesn't drop the voltage much but it drops the power supply ripple a LOT.  So it appears that with around ten bucks expenditure you can get a lot less power supply noise.  

So...  first bring it back to stock form and see how that performs (with a proper filament transformer) and then see how it sounds with the little choke.  I'll post the PSUD2 sims as well so you can see the circuit.  

Here is the stock power supply with a 33uF cap and C-24X choke:

Here is the PS ripple with the choke:

Here is the supply with the Triad C-7X choke in the supply.  The specs say the choke has 270 ohms of DC resistance but I measured 235.  Ripple on point A is now down to 2mV.  Actual voltage at point A drops to 343.6 in the simulation.

OOPS!

I didn't put the correct secondary power transformer winding resistance in the impedance calculator in PSUD so the correct impedance for the recommended transformer is actually 46.91 ohms instead of what I have in the previous posts.  It makes very little difference in the simulation but I figured I'd let people know if they're trying it on their own.

Okay, I've got the amp wired with solid state rectifiers, Triad C-24X choke and the separate properly center tapped filament transformer.  

Upon fire up there were no flashes or puffs of smoke so that's good.  There is very little audible hum unless you put your ear right up against the speaker but it is not completely silent.  Before the choke was added you could easily hear the hum from the listening position.  I have the choke wired so that I don't have to de-solder anything to change it out.  I'm currently getting 352 volts at point A on the circuit as configured so even with the choke I'm still getting a bit more voltage than I should be.  That's not really surprising since the secondary winding of the transformer is rated for 150mA and the circuit only pulls 80.  

I would rate the sound definitely superior to the way it was originally when I had it with the un-center tapped filament supply and no choke.  The hum was really loud back then and it really distracted you from the music, especially in quiet passages.  Is it better than when I had it with the tube rectifiers?  I don't think so.  I'll have to try it with chokes with higher henry ratings and see if they improve things.  I can say that adding a choke that costs less than eight dollars and a 33uF cap should be a no brainer for anyone building this amp.  It's not the ultimate but it will go a long way towards making the amp more fun to listen to.

Currently I'm running One Electron output transformers on their four ohm taps to an eight ohm speaker.  This is presenting a 7K impedance to the output tubes.  My single driver speakers never drop below eight ohms so I'm going to hook up my Edcor 5K/8 ohm transformers as well, this should give me a bit more power at the expense of a little bit more distortion.

The ohm rating of the transformer in the program is not the resistance but the impedance.  There is an impedance calculator in the program that uses measured input (wall) voltage, resistance of the primary and secondary winding of the transformer and the unloaded voltage output.  

When you right click on the tranformer in the program to edit, both parameters have a button after the data entry box with "..." in them.  Click on the resistance box and it will help you calculate impedance.

The current tap ratings were a guess based on tube ratings.  At some point I will measure the current draw of the small signal tube and subtract that from the total 80mA of current draw.

You are of course right about the capacitors in series.    

Is the R core transformer 250 center tapped, i.e. 250-0-250?

You can use the zoom in and zoom out buttons on the display graph, I zoom out then click and hold on a point to the upper left of the graph then drag to create a box around the area you want to look at.  Doing this repeatedly allows you to zoom in on a very small part of the graph so you can see the actual ripple which can get very small.

You got my attention!  

Can't wait to see those speakers!  

Mike