Articles for Audiophiles by Steve Deckert
AUDIO PAPER #003
A SINGLE ENDED TUBE AMP FOR THE MASSES
Part III - The Chassis
Well I have pondered the chassis design for some time now. Not only from the perspective of cost and appearance, but from the resulting topology of all the parts. Things like heat and shielding become large considerations not unlike where the thing is intended to be placed in the room. Do you make it like a conventional tube amp and plop it down on the rug, or try to cram it into a more standard chassis like a 19" rack mount unit.
A unique opportunity with this little amp lies in the simple fact that it is so small. The 6BQ5 output tubes are not much larger than a normal preamp tube, and have nowhere near the heat of a larger output tube. Because of this and the size of the transformers I could build this into a nice rack mount chassis the size of an average preamp (3.5" high).
Okay, how to give the owner of this amplifier the satisfaction of hypnotically staring into the golden glow of the tubes... you know that's a must - too many people talk about it. I happen to be one of them. In fact I'm not sure sometimes if it's a vice or a gift, but I have a real hang-up with how things look. What followed was the all too familiar, slightly out of phase, molting period of several days when I walk around in circles a lot. Two days ago while I was meditating to music, (I was really enjoying some "FRESH AIRE" on American Gramophone being played on this amp) it hit me like a Zen Lightning Bolt in the forehead! An image of exactly what it should look like, how to lay out the entire parts topology, and flashes of it sitting in many different listening rooms blew into my head. I love it when that happens!
From that image I did this conceptual drawing of it exactly as it appeared. The picture can be seen in figure 1. The ventilation will be through the top and bottom center of the chassis. The tube topology will be horizontal opposing channels with the rectifier centered between them in the back of a shielded "room" inside the chassis. I'm not sure how cost effective this chassis would be, but I will definitely be building some.
NOTE: Now 2023, I can see the only reason this chassis didn't happen was money and resources. As it would turn out a small 10 x 6 x 2 inch steel box would half to work and the resulting amp would look absolutely nothing like concept sketch.
I hoped it would be ready by now, but you know how that is! Currently I am waiting for the output & power transformers I requested that all transformers be hand wound by one particular man, (one of the few older fellows still alive doing it) and am still waiting. I believe it will be worth the wait.
Last night (this is a classic example) a gentleman stopped by with his ADCOM amplifier to hear what it would sound like matched with a tube preamp. He had the most expensive and largest amplifier they make. It looked very nice. We listened to it for a half hour or so and he was impressed with the idea of a tube preamp. Between you and me, it sounded fair with the a few shortcomings of being a little dry and a little thin sounding unless it was really cranked. I don't remember how much power it is, 500 watts per side or something.
As we finished, he asked what that preamp on the floor was like. He was pointing to the SE-6BQ5a. I told him that was an amplifier, currently running pure class A triode with a 1.8 watt output per channel. I went on to explain the whole thing to him. Well of course he wanted to hear it so I hooked it up where his Adcom had just been. The bass was full & rich, and everything sounded better.
It was having no problems maintaining the 90 dB playback level we had been listening to and in his own words, besides saying he couldn't believe it 42 times, he said "At this playback level my Adcom's got NOTHING on this!" He was right.
I am very excited about this project! With the perfectly flat hand wound output transformers, silver wire, and 100% polypropylene power supply there are very few amplifiers that will sound as good. The only ones that would stand a chance are other single ended triode amps costing as much as a new car.
While waiting for my transformers, I have had some time to re-analyze my thinking on all aspects of this design. The only thing that is certain is the circuit itself. Parts and layout are still open to review. Certainly for the purists and seasoned audiophiles I am on track with my 100% poly caps in the power supply and signal path. Make no compromise on parts quality, including wire and solder and lets see how good- good can get.
Perhaps if I'm going to make references to "Zen" in the design of this amp I should pay some attention to it. An interesting thing has happened this past month while waiting for the transformers. I was commissioned by a studio in New York to build a tube equalizer that would warm up the recordings. It would be the last thing in the signal path just before the master deck. They wanted that enchanting "tube sound" in their final product, the recordings.
I took on the project with the approach that I would give them some basic tone shaping in the bass and treble area nothing else. I figured if they need to EQ it more than that there is something wrong elsewhere in the signal path. This being the case, the front end of the SE6BQ5 circuit would work perfectly. So would the basic chassis. This would give me an opportunity to explore some variations of the project, and let me really analyze the sound of the front end as a separate pre amp. This also gives me an opportunity to try some different twists on the power supply.
Thinking about marketing and other non Zen thoughts I did a really stupid thing. I built the full blown power supply that goes in the Zen amp for this pre-amp project. I also made some changes to the solid state rectification using diodes and very large filter and B+ caps. Here is a picture of that power supply. A little overkill ha?
My thought was build a solid state rectified duplicate of the tube rectified power supply I would be using in the Zen amp and see which I liked better. The problem was that I had a B+ of 435 volts and only needed around 270v. Not wanting to introduce solid state regulation (the natural solution) I tried to wrestle it down and work with it. The result was a poor signal to noise ratio that was audible.
I solved this problem by coming back down to earth and just building the proper supply to do the job. I chose two 300ma transformers, one to run each channel. I ran a full wave bridge off each one through a one ohm resistor, followed by three sections of filter caps. This gave me what I needed to accomplish a true dual mono unit. Why not right? I did the same thing with the heaters, using one large 100,000uf 20v cap on each side. Now I had a dual mono supply with dual mono DC filament supply and no solid state regulation. This is an obvious improvement over the first circuit.
The next interesting development was the way I immediately dumped the very best poly caps I had (combinations of Multicap and Rel caps) for everything in the signal path. I let the unit burn in for a week and was surprised to hear it sounding as good and actually a little better in some ways than my pre amp. This is my own interpretation of the "Last Pass" for the popular Dynaco tube preamp.
The result was that the pre amp was "too good". In other words it was very very very fast, extremely revealing. On my high end system it sounds wonderful. Substitute a normal CD player, or mid-fi solid state power amp into the equation and the sound (for me anyway) was unlistenable. One of the secrets I know to be true in all musical sounding systems is balance. All pieces of electronic gear in the system must be of equal speed and quality regardless of the value.
Since I wanted to accomplish a "tube sound" in a solid state signal path of a recording studio, this wasn't going to work. I needed something that warms, soothes, which translates into subtle filtering. Something that wasn't so fast and so transparent. To do this I removed all the high end caps and installed normal mylar caps very similar in sound to original caps used in the 60's. I also changed the resistors back to 5% metal films. Part of me thought I was really crazy at this point.
With great anxiety I took the reworked unit back into the listening room and plugged it in. Get this... I was actually shocked when music came out, and then had to quickly remind myself that of course music came out. Just shows how brainwashed we get by listening to everybody else. I expected the sound to be so pathetic that it would justify my original implementation of the good caps. And it was... for the first excruciating 30 minutes... while the caps seated. But then guess what?
The stupid thing sounded warmer, richer, and the highs were nice and sparkly and most important, the sound stage was still there! Hmmm. Okay, we can fix that, bring in the Sony power amp... and that 200.00 CD player over there. Now we have a mid fi system on high end speakers, a cripple from the start. Well, that was the best I had ever heard that gear sound. Gee, it's working, this thing is making flat dry nasty stereo gear jump back into the 3D high end sound stage. The difference is rather amazing.
This means that the secret to a musical Zen amp for the masses is not going to be found in the high end caps. (A rather hard pill for me to swallow btw.) That means I am going to have to build it exactly like I just did this pre amp and that means I will have to make some "Silver Edition" units with the originally intended parts for those audiophiles who have the gear to back it. Consider the conceptual drawing I did of the chassis to be what the Silver or Special Edition model will look like if and or when I decide to sell some.
Meanwhile, staying focused on the original intent of this design I have elected to go with a more straight forward look. And because of the success I just had with the studio's pre amp, I am going to offer two products. The Zen amp as intended, and a pre-amp. This way those of you who have the larger amplifiers/less efficient speaker thing happening can benefit from the corrective qualities of this pre amp. In other words we can help more people create a musical sounding system. See how things work out?
The iron samples have finally come in and I am very pleased with them. Now all that remains is to build the little guy and see how it compares to the prototype.
I've made all the decisions needed to build the first pre-production unit and have completed it. I kept the layout symmetrical so the amp looks good.
Well, what a rush! This is the first time I've gone back to this page since August of 97. The Zen was optimized for Triode use only after the first 25 units were sold. Shortly there after the tubes were upgraded to SV83's and 6N1P's and it has remained basically unchanged since then. We have hand built over 600 of these amplifiers. In fact we have developed an entire line of Zen Triode Products along the way. Many Thanks to everyone who is supporting us by enjoying these magic little amps!
January of 2008
Serial #1000 of the little gray amplifier (Model SE84C) was built and shipped. The chassis was retired but the amplifier lives on in a slightly larger and significantly heavier black steel chassis with a front mounted gain control and 2 sets of inputs. The new model is called the SE84C+. A history detailing the revisions can be found in the articles section of the web site. At the time this article was written I never dreamed we would have hand built and shipped a thousand of these, not to mention a few thousand more amps that we designed based on this concept. Just goes to show I was on the right track, and it remains our top selling amplifier today.