Tubes Asylum: REVIEW: Electronic Tonalities ParaS.E.X. Amplifier (Tube) by bp

I'd like to share my experience with the paraSEX amp. The paraSEX amp is a 5WPC, single ended, true triode, dual monoblock parallel feed Single Ended eXperimenters amp. I had read about how great single ended amps sounded but my single ended experience really started when I was simply blown away after listening to a pair of $4500 CARY 300B monoblocks for a couple of hours. I realized that the 12 huge watts of power contained much more music than any number of SS or even push-pull watts. Of course, the price tag was a bit on the high side, but I knew I wanted amps like these. What to do? Well, I had heard about this strange group of people who actually took the time to built their own amps to their own personal taste and with better than commercial quality. Hmmm, this sounded interesting. I looked around on the web and found out that there are a lot of people involved in this obsession, I mean hobby, and that there were a ton of schematics out there including 300Bs. My mind was made up, I wanted to build one myself.

However, there were some problems, I knew some basic electronics (V=IR!) and nothing about tube electronics let alone about how these amps actually work. I mean, I didn't know my cathode from my anode. There were other problems as well, such as how do you make all those big holes in a metal box to put those transformers, I mean Iron, the tubes and all the other parts. So a kit seemed like a good place to start. Well CARY has a kit division, Audio Electronic Supply, that I've read good things about and that sells a 300B kit. Also, Sonic Frontiers was coming out with a stereo 300B kit with a printed circuit board. Both these were about $900 and without the tubes. I really didn't want to commit that much at one time and risk not liking it. I then found out that Bottlehead had a popular $400 kit that could be easily modified and upgraded to either a 2A3 or 300B. Not only that, but they are monoblocks! So I ordered a pair.

After some anxious waiting, the kit came in a nice big box that UPS had done a nice job of denting in one corner. Everything inside was nicely packed and undamaged, however. I briefly looked at the directions, warmed up my soldering iron and realized that I couldn't get the nuts off the power transformer. OK, all I needed was some lacquer remover or acetone (nail polish remover). Also, I needed some wire (I used the 20 gauge solid wire from Radio Shack). After a quick trip to the hardware store and applying the lacquer remover, the nuts slid off easily. Also, the instructions were intended for the non parafeed SEX kit so I had to cross off the items pertaining to the original SEX kit and cross reference them to included modifications to the paraSEX kit. From there, it was simply a matter of following the instructions point by point. In a couple of nights I had a working pair of amps!

The sound was really nice when I plugged them in. I can't honestly say that the performance was as good as the $4500 CARY amp but it was definately very good. After extensive listening, compared to my Sonic Frontiers Anthem Int-1 the mid-range sounded much fuller with more harmonic richness. On the other hand, the 5Watts of power through 92db sensitive speakers didn't quite have the visceral impact in the bass and the highs were a bit less detailed. But, then again there seemed to really be something to the triodes. Also, I had no problem with clipping. The SEX amps were able to play to very loud levels and the bass was there, but without the drive. At that point, I decided that triodes were the way to go and modded my Anthem Int-1 to triode mode. That was a good change, the midrange of the Int-1 totally changed character with much of the richness of the SEX amps while retaining the bass drive.

With the replacement TFA204 output transformers, the music really started to come out of the SEX amps. Most noticable was that the highs became much more detailed but remained very smooth. The midrange really began to sing as well. Vocals and guitars really had a richness that can't be found in any SS amp. The bass was better, still a bit soft, but I believe that is due more to my relatively insensitive 92db speakers than a characteristic of the amp itself. For smaller scale music 5 Watts is just fine, but with large scale orchestral and music with very low bass the dynamics are just a bit compressed. Overall, I can say the sound surpassed the Anthem Int-1. I then C4S'd the SEX amps and that seemed to improve the bass dynamics. This involved changing the topology of the input stage from a mu follower to parallel operation. One other point here, the technical support is unsurpassed by any company I've encountered. I ran into a couple minor problems along the way where I was stumped and also had a few other questions that DocB was able solve and provide excellent suggestions for in a most friendly and helpful manner. By this point I had learned a great deal more about tube amp design and had been able to implement it directly with the paraSEX amp.

Now, I was ready to build a 300B from a schematic. I chose the JELabs 300B from Angela, which is based on the Herb Reichert 'Flesh and Blood'. I did have to modify the cathode bypass resistor to 1250 Ohms in order to reduce anode current to the 60mA maximum rating of the TFA 204s. Also, I retained the original SEX power supply, the TFA 204s and one of the octal sockets. All I needed to implement the new design were the 6SN7 driver tubes (Phillips JAN), the 300B output tubes (Valve Art Nickel Plated), 4 pin tube sockets, a SCR tin film/polypropylene coupling cap, hum balance pots, teflon coated wire (kimber TCSS) and few assorted electrolytic caps and resistors that I got locally. With the previous SEX experience under my belt, I was able to put these together in a few hours. I was completely taken aback when I heard these amps. The sound was incredible, the immediacy of the 300Bs is amazing. The bass drive and depth far surpassed the Anthem Int-1 and most solid state designs. The highs had a clarity and delicacy that shimmered. I am immensely pleased with this amp and would take these over the CARY monoblocks (Well, OK if you want to give me the CARYs I'll take them too).

To further refine the sound I upgraded the power supply caps to 2x47uF/500V Cerafines and a 47uF/630V Solen metalized polyprop. I also put in a standard Black Gate 100uF/100V 300B coupling cap, and a cerafine for the 1st 6SN7 stage. I'm using the original 47uF cap as the bypass cap for the 2nd 6SN7 stage. These changes seemed to clean up the sound a bit and provide some better dynamics, but really seem to be more of a subtle refinement. However, I didn't have the patience to allow the power supply caps to break in for evaluation before I changed the coupling caps. For the coupling cap, I'm using a 0.22uF NOS Vitamin Q oil filled cap. The end result imparted a beautiful midrange bloom to the sound and seemed to give the highs a silky smoothness.

At this point, I'm still tweaking the original SEX amps beyond all recognition and will probably try other designed including some in VALVE (which by the way is an excellent resource), but in heart and spirit these are still SEX amps. The real bargain of the SEX amps is the learning experience gained from building and modding these amps. The SEX amps don't seem to be intended as an end in themselves, but are really a starting point where you can learn by experimenting with various changes. There are of course other routes as well, including sticking the the original paraSEX, but the SEX amps are an excellent and affordable way to get into tube amp design without the hassle of making major mistakes along the way. The best part about the kit is that you can costume tailor the amp to suit your own taste. This is Fun Stuff!!!

REVIEW: Electronic Tonalities ParaS.E.X. Amplifier (Tube) Review by bp at Audio Asylum