AOM Logo December 1999

Apollo Descends Among the Mortals
Blair Roger builds the Welborne Labs Apollo I DHT Amplifiers

It's a human trait - exhibited mostly by males - and that is the imperative to build things. I don't want to start writing a doctoral thesis here so I'll leave the deep thinking on the subject of the urge to build and create to you, the reader. What I do want is to tell you about my experience building Ron Welborne's Apollo I kit amplifiers.

Ron is a smart guy who runs an electronic part supply company out of Colorado. He's been around for more than ten years and he knows what audiophiles crave in the wonderful world of wire, caps, resistors, transformers, and tubes. I would say he is very tuned-in to the fringe crowd he caters to. He understands that they love quality goods and that's what he provides in a no-nonsense, straightforward fashion. He won't waste your time trying to sell you something you don't need. He's too busy for that. On the other hand, he's not one of these moody, smart-mouth vendors who just wants your money or hangs up the phone if you ask more than two questions. Enough said.

The Birth of Apollo
It's interesting how things come together. Welborne Labs started selling triode amplifier kits about five years ago. The first one I remember reading about was the elegant Laurel 300B stereo unit. It was leagues ahead of the ubiquitous Audio Note Kit One for looks (well, anything from a dumpster would be too) with a gorgeous polished brass top plate and an appealingly symmetrical tube layout. At that time, there was a definite lack of good quality 300B power tubes at reasonable prices. There were only two or three choices: hunt for old Western Electric tubes and pay $600 each for them; take a chance on the cheap and cheerful Chinese 300Bs, or follow the middle path with Cetrons. The tube industry was teetering back and forth in a sort of should-we-or-shouldn't-we state of mind until they realized that if they could produce good direct-heated triodes at acceptable prices, people would buy them - lots of them.

As with most endeavors that fuse artistic and technical fields, Europeans took the lead. In 1994, Dr. Riccardo Kron established KR Enterprise in Prague, capital of the Czech Republic. With some Swiss financial backing, he was able to develop a small company based in the building that once housed the Tesla tube research department. The staff was comprised of about twenty former employees of the Tesla tube works, several of whom had over twenty-five years expertise in high-vacuum technology. Within three years, KR Enterprise had developed a successful line of proprietary direct-heated triode tubes that exceeded the performance of NOS types while maintaining exceptional quality and downward compatibility for plug-in replacement use in existing audio equipment.

Not surprisingly, Welborne Labs became the American distributor for these amazing new tubes being sold under the KR brand name. They make an exceptionally good 300B and 2A3 and they also make two 'super' tubes: the KR300BXLS and the KR52BX capable of producing 20 Watts at 4 amps and 30 Watts at 8 amps respectively. Compared with the original Western Electric 300B - 7 Watts at 0.6 amps - there just isn't any contest. Neither was there a purpose built amplifier that could take full advantage of this kind of power, which was a bit of a dilemma, without a doubt most keenly felt by the distributor.

It would seem that the KR300BXLS and KR52BX are part of the solution to the problem faced by most prospective buyers of single-ended triode amps: lack of Watts. Yes, there are a few speakers that will work quite well with 7 or 8 Watts but most of them are benign loads and 90 dB or better in sensitivity. This has not been the trend in high-end speaker design over the last fifteen years. Audiophiles tend to be emotionally attached to their loudspeakers for many personal reasons, some domestic, some aesthetic, and some even financial. And the word on the street is that horn speakers are honky and Lowthers are a big commitment.

So here we are, entering into the Golden Age of the Direct Heated Triode, with some of the finest examples of tube technology available to us and a complete dearth of commercially viable amplifiers to make proper use of them. Harvey 'Gizmo' Rosenberg, the Thermionic prophet in the wilderness of the Solid State, pointed this out to those who would listen more than a year ago.

The answer to the conundrum would be to design a single-ended amplifier that could use the power of the new generation of KR tubes to drive all those non-'SET friendly' speakers out there. And Ron Welborne did it. Enter the Apollo monoblocks.

I Build Therefore I Am
As you may be aware, I own a pair of QUAD ESL-63 Monitors that make me a perfect candidate for evaluating the Apollo amps. I'd never dream of trying to drive them with a 7 Watt amp - even though I wish I could, I know it would never work. However, the Apollo Is produce 18 Class A Watts. Could this be enough to warm the heart of a QUADophile? Ron Welborne thought so and within a few weeks two large white boxes landed (we are talking about UPS here) on my doorstep.

I'd never built a piece of electronics from start to finish before but having refurbished a couple of Dynaco ST-70s and possessing good soldering skills I was confident that I could put these monoblocks together. If you're the sort of person who liked to play with Meccano sets when you were young and put together a battery operated doorbell as a grade school science project, then you are going to love building these amps. You don't even need to be able to read a schematic. So there - no excuses!

The quality of this kit will astound you. This is not a Hammond box with a few holes punched in it and a breadboard on stand-offs. The top plate is ¼" thick aluminum, milled with a beveled edge all around. All the openings are precision cut (probably with a laser) and perfectly smoothed. The aluminum is anodized silver and then bead-blasted so that it shimmers like the finish on a fine piece of photographic equipment. The substantial, custom-wound transformers are enclosed in unique rectangular covers, finished to match the top plate. The only fasteners visible anywhere are the four hex-head bolts that fix the precision milled lids onto the transformer covers. Everything is bolted to the top plate from underneath using pre-tapped holes, including the two Elna Cerafine power supply capacitors. I learned quickly that if something had to be forced, I was making a mistake, using the wrong part or the wrong location.

The layout of the amp is unlike any other I have seen. It is logical and aesthetically pleasing while remaining utterly unconventional. All the hook-ups are hidden at the back and the diminutive power switch has been selected to provide a pleasantly elegant feel every time it is used. The massive frame that forms the body of the amp comes completely assembled and drilled; finished with fine furniture oil. A superb retro-look bias meter that can be mounted into the front corner of the frame is available at extra cost. In fact, there are many worthwhile options that can be specified including Teflon tube sockets, Black Gate capacitors and Jensen Copper Foil coupling caps. I asked Ron to include a set of Jensens so that I could substitute them for the Hovland Musicaps eventually and see what sort of difference they actually make to the sound.

The instruction manual is very well written. I just wish it had been packed on top of all the parts, instead of near the bottom because it was the first thing I wanted to look at when I got the carton open. I wanted to know what dreadful trials I would be facing. I read it through twice before going to bed and slept soundly. I had decided to take my time and work slowly and thoughtfully and I think the manual is written in a way that encourages one to do this.

Everything is logically sorted and packed in little baggies that are labeled by the packer. The major division of parts bags is between the amplifier and the power supply. Mercifully, all the little screws and bolts are specifically labeled as to their ultimate purpose so you don't have to figure out what a 4-40 bolt looks like. Just look for the little baggie marked 'back panel h/w' for example. This saves an awful lot of time and frustration. All the resistors and caps are individually bagged and marked R-1, C-1 and so on. Just match them to the parts list and location on the assembly diagrams.

Ron takes you through the assembly in a very orderly fashion, and the instructions are fully trustworthy, except for a couple of points, which I will mention shortly. A list of required tools is the first order of business and to that, I would like to add that you should buy a good soldering iron. Better still, a solder station with variable temperature will really be a big plus. I found a Weller 921-ZX for about $100 and was very pleased with it. You will also need a set of Allen keys (hex-head wrenches) including some exceptionally small ones. Choose good quality steel Allen keys because the cheap ones will strip easily and round-off the hex-head screws before you can get them securely in place. Wire strippers capable of handling 20 awg. to 14 awg. are a must. You will go out of your mind without them. Needle-nose pliers, small wire cutters, and a hemostat (lockable pliers/tweezers) are necessary as well.

Ron included a package of Welborne Labs solder with my kit so I suppose this is standard issue. It's good stuff to work with. If you've never soldered before, practice with some short pieces of the hook-up wire that comes with the kit. Pick the yellow wire because it is the least used in the building process. A good solder joint starts with a good physical joint. In other words, strip the wire of insulation and wrap the lead around the terminal post like a shepherd's crook. When I solder something, I feel like an Alchemist. I touch the tip of the hot, clean soldering iron to the junction of the wires and wait patiently for both pieces to heat up, testing the temperature by deftly touching the joint with the tip of the solder spool. When the magic moment arrives the solder will flow like quicksilver. Remove the soldering iron almost at once. The joint may be hot enough that solder will continue to flow even after you have removed the iron. The joint should look shiny. Avoid blobby, tubby looking joints. Get the solder-sucker, clean them off and re-do them. Solder joints that look grainy or dull are 'cold' joints and poor conductors. Re-do them as well. Keep your soldering iron clean by wiping it on a damp sponge and above all, keep it hot enough to melt the solder quickly and easily on simple joints that don't 'sink' a lot of heat. You will find the RCA input jacks quite challenging, as they will absorb considerable heat before the solder will flow.

While on the subject of challenges - I found stripping the varnish off the output transformer leads very difficult. The instructions say that the varnish will melt away with the heat of your soldering iron. It doesn't melt and I had to use a little hobby file to scrape it off. Soldering the transformer leads to the terminals was difficult too because there's a lot of wire that's taking heat away from the joint. Ron promises that the next batch of amps will have pink varnish/insulation that will be easy to burn off. Mounting the transformers to the top plate with the bolts provided seemed impossible because the bolt holding the transformer laminations together was in the way. I phoned Ron and he calmly suggested that I merely tilt the transformer slightly to get the bolt aligned with the hole. It worked! When in doubt, just ask. It will save you a lot of grief.

Now, the subject of grief. The final instruction in the manual requires one to unscrew a bolt that holds down one corner of a transformer cover and to slip the bolt through an eyelet attached to a ground wire previously soldered to the IEC outlet plug. The problem here is that unless you want to separate the wooden frame body of the amplifier from the completely assembled top plate (and believe me, you won't want to do this) you will have to undo the hex-head bolt with an Allen key - 1/8 of a turn, over and over and over. There is simply not enough space under the massive choke, which obscures the bolt to turn the Allen key. I got all hot and bothered the first time I did this but it did give me time to think of better ways to install the final ground connection. Here's a hint: solder the eyelet to a five inch length of black hook-up wire and remove the hex-head ground bolt before installing the frame onto the top plate. Slip the bolt through the eyelet and secure with the Allen key leaving a flying lead. When it comes time for that last instruction, locate the wire soldered to the center pin of the IEC plug, match it up with the flying lead, strip the insulation from both ends and slip a half inch length of heat-shrink tubing over one wire. Solder the two pieces of wire together and insulate the joint with the heat-shrink tubing.

This is the reason why I liked building both chassis at the same time rather than going completely through the assembly before starting the second one. If I made a mistake the first time or found a good way to do something, it made the second chassis much faster to assemble. Net result is that you save time and have both channels ready simultaneously. I honestly found that I could only put in about two hours before my concentration began to flag or I was ready for a break. It seems that the manual is written in modules that take about that much time to complete.

Inside the completed Apollos

Apollo - Power UP!
I was initially disappointed that there was no tube rectification in this design - just two tubes! Ron explained to me that because the KR tubes in the Apollo I and II are capable of putting out 8 or 10 amps, tube rectification was out of the question as it would just choke off the potential power. Once I realized this, I didn't mind stuffing the power supply boards with four big soft-recovery Hexfred diodes. Actually, switching over from point-to-point wiring to PC board stuffing was a pleasant interlude. Another hint: I found it easier to install the 'L' brackets that support the power supply board to the top plate first. That way, I could attach the board in its vertical position with loosely fixed bolts so that I could tilt it back to install some of the final wiring along the lower edge of the board. You'll understand what I mean when you get there.

At last, the really exciting part: the first power test or 'the smoke test' as Ron calls it. The instructions at this point are very clear. Follow them precisely. I bought a digital multi-meter from Le Shaque to do the voltage checks and I particularly liked the auto-ranging feature. It was incredibly satisfying to plug in those big KR tubes, flip the power switch and see the voltmeter come up to the proper level on the first try! I only had one glitch - no sound from one channel. Ron suggested checking the input and output terminals first to ensure that I hadn't missed a connection, which is a common, non-critical error. In my case, I quickly found that the coaxial braiding on the input wire was just barely touching the terminal, causing a short. After warming up the soldering iron, I was quickly and easily able to correct the fault by shifting the connection slightly.

Ron recommends 40 to 50 hours of break-in time before listening critically to these amps so that's what I am doing right now. I'll have a full review on the sound of the Apollo Is in the next issue.

I highly recommend this kit to anyone who has the time, patience and desire to build something that will give them lasting satisfaction and pride. The quality of the parts and finish is astoundingly good. It does not look like a kit on the outside and if you are really patient, it will look like a piece of art on the inside.

The Apollo I Monoblock Amplifiers
Manufactured by Welborne Labs
P.O. Box 260198, Littleton, CO, 80126, USA

phone:(303) 470-6585, fax: (303) 791-5783

Price: kit US$2650.00
Numerous options or assembly available at extra cost - visit the website for complete listing of options and upgrades.
Source of review sample: Manufacturer loan
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