The Williamson 807
THE WILLIAMSON 807
- Category: Tube Amplifiers
- Published: Monday, 07 September 2009 19:39
- Written by Super User
In this article Brendon Lee Leads us through his build of a classic amplifier design, the Williamson 807. Using anodised blue metalwork for the amplifier, Brendon certainly demonstrates an eye for detail. Read on for all the juicy details on what is a visually stunning project.
This article is re-published with kind permission of Brendon Lee. Originally hosted on Brendon’s Geocities site prior to the closure of Geocities hosting.
In general the high end audiophile community may not be aware of the 807 tube and instead, favour more towards other tubes used in SET designs such as 300B, 2A3, etc. During the past 10 or 20 or so years, classic designs such as the Williamson have simply been forgotten. Is there a reason why? It certainly isn't the cost factor. Perhaps it's due to the lost art of building high quality transformers? The variables can keep going on. I will note that I can't understand why such a marvellous amplifier has simply been neglected. There can't be any real serious flaw in this design as it's published in the RCA Radiotron's Designers Handbook" aka Red Book Bible.
General Parts Description & Construction :
While many debate over the choice of resistors, capacitors, and even the hook-up wiring, I felt that the most critical component had to be in the output transformer. I must stress that the high performance could not be obtained without having a truly awesome transformer. With credit to Joe Bogatek (from Australia), he hooked me up with all the iron (chokes, toroidal power transformer, and he himself wound the output transformer).
Power supply capacitors were Vishay ATOMs as they appear to be the only ones with a high voltage rating and came in axial form. I used PIO capacitors "Sequa" brand made in China from Angela.com - unfortunately 1 was defective as it leaked voltage. Other caps I used were the 'orange drops' which you can't go wrong with using.
Majority of resistors were old carbon compositions. Apparently metal films are nasty for valve amplifiers. I've use the odd Riken carbons which seems to be the only carbons that come in high wattage rating (5 watts).
Choice of tubes was quite time consuming. In fact I don't think any particular 807 tube sounded better than the other. I've had 3 sets (Radiotron, G.E.C., & Russian 807s). At the end I settled with the Russians the other brands were way out of match - i.e. the heater voltage could be more than 0.3 volts more or less than the spec 6.3V. Choice of 6SN7 tubes were also NOS Russians.
You can almost forget about tube rolling as each tube swapped could cause the tubes to be out of balance and the bias going off the scale. Cathode of 1 tube could 'run away' with too much current and kill it at power on.
The only drawback would be the series heaters I implemented. The 2ndary tap was 0-9VAC so I had to use a series way of heating the filaments in each tube. I even had to use Schottky diodes as the voltage was too low using regular bridge diodes.
Both amps came out pretty much identical. Even the voltages was within 1 or 2 volts on the B+ (420VDC). Actual hum measured from the speaker post was pretty low (0.4mV) which is not bad considering the Klipschorns are 104dB efficient. Virtually inaudible on modern speakers.
Dual fuses - 1 for mains and 1 for the B+.
INCREDIBLE FREQUENCY RESPONSE!! Unlike most tube amps, I'm able to play full rock music without certain freq that will rip your ears off. I still can't get over the amount of detail there is between the low & high freq sound. - like sitting up in front stage. The only negative I could say is the amp doesn't have the 'romance' / 'exaggerated' sound of SET tubes like 300B.
The greatest strength definitely would be in the bass area. It has far more authority in the low end than any of my solid state amps I've built - forget the sub woofer. Another thing I'm not so use to is the amount of clarity in the way upper frequency area. The drum cymbals come out with real character instead of a typical 'splish splash' sound you get from most Hi-Fi systems. I'M ENJOYING IT!
The Rest of the Build Pictures