Once Upon a Tube… 1947-era Williamson Monoblocks Project

Left channel monoblock

Left channel monoblock

Here’s a piece of project history that I felt deserved a mention here, circa 2000-2001.  I had read many times about the ultralinear amplifier topology, and had been doing some reading in the HiFi tube areas, when it dawned on me that I probably shouldn’t be taken seriously unless I, too, had built at least one or two real HiFi projects based on tubes. I had read about the reprints of the Wireless World Williamson amplifier circuits utilizing the ultralinear concept (circa 1947..) and I sent off for a copy of the reprint.  That’s a great set of articles, and I recommend them for any tube library.  I set about collecting parts and settled on some good Canadian iron from the Hammond corporation, and bit by bit the project kind of formed itself.  About the same time I took an overseas assignment in the auto industry, so I made sure I included in my kit of things to ship over to Germany a LOT of tube amp parts, with the idea that I may have the time to do this project while abroad. I almost expected a customs (“Zoll”) problem with all this stuff, but none ever materialized. We labeled them as  hobby electronics workbench supplies and shipped it all.  Since I did document the project in pictures, I present them here for your pleasure.  The first point in any project that might eventually grace your livingroom is getting the input from  your wife or partner. Their buy-in is critical, especially as the bill-of-materials costs start to pile up, but also to prevent building something that will never take it’s rightful place in the livingroom as a useful piece of classic electrical art. The first pic is exactly that.

Nadine's Design Input

We sat down literally at the kitchen table with all the visible bits and played chinese checkers with them until they seemed “right”.  We also decided upon a symmetrical layout between each monoblock at the outset.  The schematic dictated what the wiring would have to be, and suitable mounting strips were located to allow all the wiring to happen.  From that point, graph paper was found the same size as the aluminum project panels I had found at Conrad Electronic GmbH in Cologne, for component mounting.  We transferred all the components in 1:1 scale to the graph paper, at which point I used the “tape it to the window” trick to transfer the centers and holes in a mirror image to a second sheet of graph paper. Copies of the graph papers were then made and actual point-to-point wiring was developed, similar to those useful old Fender layout drawings. The drafting skills I learned at Central Michigan University have served me well… I still prefer paper over a PC for layouts when there is no time pressure. 

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 That done, the sheets were secured to the aluminum panels and all the holes were center punched for location.  Once the holes had been made for the mounting screws, I tore into the panels with Greenlee punches for the tube sockets.

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Greenlee Punching

Having the metal where I wanted it, I took a giant hanging-wardrobe moving box and modified it into a paint booth.  I chose hammertone silver because I like it a lot and it seemed in keeping with the whole 40’s-50’s vibe of the Williamson design.  I de-burred and sanded the panels carefully with Scotchbright sanding pads and applied primer and then the hammertone. This actually took a few iterations to get both panels looking similar.  The first one turned out great, but the second panel had to be stripped and re-primed/re-painted about three times before they both looked good, with a similar grain to the hammertone effect.

Punched, deburred, sanded and ready for paint 

Once they both had dried for several days, I was set to start the wiring.  The build proceeded quickly, starting with getting the transformers mounted, then the sockets and tag strips. I used NOS tag strips for most of the build, and a few extra were included to handle all those power transformer wires – I had ordered the versions with world-wide power taps, and they have a LOT of wires coming out to dress.  The benefit is that one can pretty easily change the mains voltage by re-tapping them, a feature I have used since moving back to the US. 

Hammertone Finished

I tried to keep the signal path as physically short as possible, using only the component leads of the coupling caps between the tube stages.  I handled the build symmetrically – placing the two units side-by-side and doing the same step on each unit in parallel.  This assured that they were as symmetrical as possible.  I must have been dozing at the wheel during some part of the graph paper/hole transfers, because after they were painted I realized I had the 9-pin sockets on the second panel not actually symmetrical in their orientation. this proved to be only a minor quirk and did not change the wiring much, but it does sting a bit when I look a them and notice it.  Most others don’t pick up on this, it’s really just the question of where the tube socket screws appear to be, but annoying all the same.  It was too much a pain in the ass getting the two panels to look similar to go back and re-do the whole bloody thing again for the sake of 4 screw holes. 

Tag strips for components

Tag strips for components

 

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Dan at Bench with Monoblocks 

Component Close-up

 Top Side R & L

Rear Panels, done & ready to install

Rear Panels, done & ready to install

 

Once everything was together and the circuit was tested and set up, I took them to my workplace and threw them on the new Rohde & Schwarz Audio Analyzer we had just bought for some in-car DVD player development work we were doing in Germany, and put the monoblocks to the test.  I have the test results somewhere on file. If I can find them, I will post them up here as well.  All said the numbers were very good, well less than 1% distortion at low power and at full power. They swing about 38-40 watts each, and despite having AC heaters throughout, I have never noted (or measured) any audible hum from them.  One note I would add was that I was not able to get quite as much negative feedback on them as I had expected to before some really slow motorboating would set in. I tried reversing the polarity of the OT, which quickly verified that I did indeed have it right the first time.  That didn’t bother me much, as they measured well in terms of THD and I had a preamp that would drive them just fine as they were. I later paid a gal in the company modeling department to make some black wood frames on a Saturday for the chassis pair, and added an aluminum rear panel for the electrical connections.  I will post some photos of them in their complete form soon.  I guess if you’ve read this far you probably are asking ” Well, dammit… how to they SOUND?!” In a word, stunning. I simply never expected an audio circuit developed in the WWII era to sound so incredible. My first test listening was with a pair of simple Minimus-77’s and a CD player.. I set up Peter Gabriel’s “So” album and from the first few minutes, I stood there, stunned. It was like I had removed the cotton from my ears. The depth of the image, the individual instruments, the detail – it was noticeably better than any other system I have owned, including my project studio monitoring system, which consists of Hafler M5’s and Yamaha NS-10’s powered by a Crown DC150. I love these Williamson amps.  I guess the real reason I did this project, besides just for the joy of it, was to find out the truth behind what all the HiFi nuts were always saying about good tube HiFi circuits.  I was skeptical, but no longer.   Stay tuned, more projects, current and past will be posted soon. 

Left and Right Units tested and ready for housings

Left and Right Units tested and ready for housings

 

Right channel monoblock

Right channel monoblock

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