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My 6c33c OTL project

Posted 21st September 2012 at 03:03 AM by wlowes

I am starting this blog to create a record of my first tube amp project. The project is to build a pair of 110W 6c33c OTL monoblocks based on Bruce Rosenblit's patent. I am starting the blog at the stage that I am well into the project. I started planning and collecting parts well over a year ago and am at the stage where execution is under way.

So how did I end up with this choice? My DIY audio experience got under way with a Peter Daniel chip amp which I enjoy to this day. Along the way I discovered the Lampizator site leading to the creation of a 1541A DAC with 6np2 tube output. When this sprang to life I was hooked on the idea of building a tube amp. I have never actually heard a tube amp. The look is very fundamental. I want to experience the additional nuances in the music reported by tube amp fans.

I wanted to start with something simple but good quality for my first project. I also wanted to keep costs in check. I figured that good quality transformers would be the most expensive part. As I dug further, that led me to the notion of an OTL design. I tripped across the Transcendent site where Bruce Rosenblit offers kits he designed. I picked up his book and realized I could build from scratch. The 6c33c seemed like a good performer and readily available at reasonable cost.

I embarked on building the amp described in his patent. I like to pick up parts at my local surplus store whenever possible. There is the electronics equivalent to a used car wrecking lot close to my home. This guy takes in every form of old electronic equipment and tears it down to sell off old transformers and large can caps for pennies on the pound. I picked up 8 old big can caps for $10 each, and 4 500va torroids which at $20 each. These will be overkill for the B+. One will supply +170, and the other -170. Now my project was officially committed. I started accumulating the 6C33c tubes from Russia.

I kept a spreadsheet to inventory the BOM. It looked like with the deal on transformers and caps I could bring home 2 110 monoblocks for under $1000. Then as I continued to read, I began to think I could make some small alterations to the design for the better. I played around with a simulator for the power supplies. I had gotten really good results with chokes in my CD project so I thought, why not improve on the power supplies a bit. In every supply, I ended up with a dedicated transformer, and a CLC filter added. My salvage guy also sells new parts and is a distributor for the full line of Hammond products. Living in Toronto, we are only 60 miles from the Hammond factory, so the shipping costs are very reasonable. I collected the required chokes and transformers over time. The biggest consumer in these things is the filament supply. I found some used 12V 8amp toroids at $10 each. 2 of these per amp should supply the hungry power tubes and save a small fortune over buying new iron for these supplies.
Next step was layout. I really wanted to keep AC power well away from the signal tubes and the smoothing caps. I came up with what I think was a pretty good design. Sort of a form follows function with the AC at the back and caps and chokes laid out toward the front with the tubes all across the front. I did not want a cramped space. These will sit on the floor by the speakers. So I just laid out the parts and ended up with a 17x 21" footprint. I worked and reworked designs with Front Panel Express. I did not want to commit to milling the aluminum top sheets until I had every part in hand and knew for sure the dimensions and layout. This all worked along as a background project for close to a year as I worked on other projects.

I am also close to PartsConnection. I figured if I am going to do this thing, why not use great resistors. I ended up collecting a nice combination of resistors based on what PartsConnextion stocks. So I have Mills, Holco, Allan Bradley etc. I have all the parts except a last order from digikey that fills in a few bits an pieces.

Another mod that I wanted to make was to substitute the input and driver tubes with Russian. I just think the Russian tubes are very good and very reasonable in price. The design calls for 12ax7 and 12au7 for input and driver. I am substituting 6np2 and 6np6. If it is good enough for BAT, it should work for me.

Now with 7 transformers and 4 chokes per mono, the front panel express milling and some premium resistors the BOM is hovering around $1800 for the pair. Not cheap, but assuming this works out, the result will be something that would cost many times that if I purchased retail. I just better not screw it up!

Now came the question of what the base should look like. I am not a craftsman, and have limited tools. I considered using aluminum structural elements to make the base, but my local suppliers did not seem to stock the stuff that would work well. Also, looking through the gallery of other DIY builders on this site, I was drawn to the basic wood base with aluminium sheet on top.

I contemplated simply going to a cabinet maker to get something done by a pro. I figured this might be my backup strategy, but would kind of take one more part out of the DIY experience and likely cost some serious cash.

I visited a heritage lumber yard and picked out some rough cut black walnut. I made a deal to get is milled to 3/4" x 5 inch boards.

Now came the problem of jointing to get it into a box that is strong and square. Some very careful cutting with a jig saw, and I ended up with some pretty reasonable joints that were tight and square. Got a deal on some clamps and committed to gluing. Once the first bit of glue flows, there is no turning back! I was surprisingly tricky to get the glue on, assemble clamp and end up square. Clearly my hand held jig saw is not a precision instrument and my cuts were slightly off square. The bases came together pretty well and were very close to dead square. Lots of sanding down to 400 grit and the joints and the finish was looking pretty good for a first effort. I can see why people enjoy woodworking. After some web research I decided to go with a tung oil finish. I now have 6 coats on them and the finish is just stunning. The light plays off the grain and it really has some life to it.

I showed off my bases to my wife, showing which of the 18" faces would be in front and how the 21" piece would be on the side. She pointed out that it really looked much better with the wide side in front. I had to agreed and went back to the drawing board with Front Panel Express. I ended up with a layout that has the RCA Input and Driver tubes at the front with the 4 6c33c tubes in a row across the front with the speaker connections at the other end. The left and right channels will be mirror images so the source will sit in the middle and the RCA goes in one side and the signal pretty much goes in a straight line to the other side and out to the speaker. I think it looks really cool and I like the concept. The AC comes in the back and gets progressively cleaner DC by the time it is up front on the Tubes. Everything will be on a star ground. This is likely where my inexperience will be most challenged. I am also going to be very careful with the filament supplies. The design calls for DC filament. I am thinking of trying AC on the filaments with tight twisting of the pairs and carefully keeping the 12V lines well away from signal lines. I figure if I end up with a hum problem, I can always rectify the power tube filaments later.

So this brings me up to the current state. I am reforming my large can caps. Again I don't want to find out after I mill the top sheet that the Caps are beyond repair. So far the 4700u 200v Nippon caps seem perfect. Some older larger Sangamo caps are a little more of a question mark. They are reforming, but taking well over 24 hours each to come up to spec at 150V. I'll see how all 4 fair and then decide if I risk my amp with them or look for replacements.

My target is to have these things playing music by Christmas. Still lots to do and I don't have large blocks of time to devote. Still it seems like it should work out ok as long as the caps and Tubes are solid. I'll post some pics of the current stage in a future post.

As much as I am committed to take the time to avoid big mistakes, I am getting excited to finally hear my new amps. My electric bill will definitely go up when these come online. They dissipate 750W each and take 30 minutes to heat up.
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Total Comments 1


  1. Old Comment
    Alain Dupont's Avatar
    Nice job,
    I wish you a lot of fun with this one
    Posted 3rd October 2012 at 06:52 PM by Alain Dupont Alain Dupont is offline
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