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D.T.N. WILLIAMSON AMPLIFIER
Copyright 2001 Stanley Fay White

 

History

In the late 1940's, component HiFi was a hobby, much like amateur radio. Amateur radio buffs built their own transmitters from parts they bought in electronic distributor stores. A lot of these amateurs had component sound systems as part of their overall short wave radio systems. As industry changed over from wartime products to civilian products, the electronic industry began making various parts for these electronic distributors. Among these parts were audio amplifiers, raw speakers and the like. Because there was a scarcity of parts in the beginning, many small manufacturers found a ready market for their electronic components. For many years, these small manufacturers produced most of the audio electronic parts.
Magazines to serve the nascent component audio hobbyists appeared. These magazines had similar format to the amateur radio magazines and sometimes contained both amateur radio and audio gear. They published numerous articles on various designs relating to electronic components. Almost anyone could get published, as the magazines needed articles to fill up their issues. This is why there are so many different audio tube designs floating around. No one in the media world separated the wheat from the chaff. As few people around today are familiar with the technology of audio tube design, the chaff has puffed along with reputable designs as "antique audio". There are a lot of bad ideas out there along with the good ones. Let the buyer beware.


The Original Williamson Amplifier

Into this free wheeling media milieu, Williamson published an article on audio amplifier design, which impressed everyone with the insight into audio amplifier theory. Williamson gave certain criteria for audio amplifier circuitry that broke new ground. Williamson's concepts were valid, his articulation of his ideas into an amplifier were seriously flawed. A basic component of his amplifier was the Partridge transformer, a creation of Dr. Partridge. The Partridge output transformer had a frequency response from 2 to 200,000 Hz. Its output was 10 watts over most of the range. This quality level for an output transformer was a milestone in the development of Hi Fi. Ten watts of audio in 1949 was about the best that most were doing at the time. It is not known that a Williamson produced amplifier was ever offered as a unit to the American market. Williamson himself, an engineer employed by a tube company never produced the amplifier at all. The Partridge transformer was sold in America as a component and was available through electronic distributors.
As originally configured, the amplifier was constructed on two chassis, the power supply on one chassis, and the audio circuit on the other. The recommended capacitors were huge oil filled, expensive units with low capacitance. No chassis for the amplifier was available, and had to be cobbled together by the constructor. It was all amateur stuff. At the time, the entire industry was amateur stuff. Even so, the quality of the new component audio was so superior to "set" manufacturers like RCA, Philco and Zenith that the Hi Fi buffs lived in the clouds, so to speak. Their gear was a quantum leap ahead of set manufacturers.

The Williamson Theory

In 1950, Hi Fi bugs didn't know what "feedback" was. Any old theory of feedback was an improvement over nothing. Williamson had phase diagrams and curves to show that in order to place a stable feedback loop around an amplifier, the problem was phase margins. In order to assure stability, the output transformer should have a response from 2 to 200,000 Hz. This assured that the amplifier was stable in the audio range of 20 to 20,000 Hz. This criteria had merit. Unfortunately, Williamson did not carry his analysis far enough. Seemingly, he never actually built a model of his amplifier. If he had, he would have realized that there was more to it than an output transformer and feedback loop. The only component in his design that met the 2-200,000 Hz. criteria was the output transformer. As presented, the Williamson amplifier had a number of serious flaws. It was a good start, but it was not thought out properly. When the author built a Williamson with the Partridge transformer, he soon discovered these flaws. The Williamson circuit and a critique will now be discussed.

The Williamson Circuit
We will discuss the circuit, block by block.

The Power Supply:
The Williamson power supply was built on a separate chassis. It had a high voltage cord that connected it to the audio amplifier. Such configurations aren't built any more. They are dangerous. Running a 400-volt DC power line for domestic use is against UL standards. The seeming purpose of this set up was to isolate the power transformer from the audio circuit (and output transformer). It has been shown that there is no purpose in such a setup.
The circuit contained a 5U4 directly heated cathode as a rectifier. The circuit was of the capacitor-input type and contained two chokes, a 5 Henry power choke and a 30 Henry smoothing choke. The filter capacitors were oil filled 4 mf.400 volt units.
This supply had serious deficiencies. The audio circuit had a direct-coupled first stage. On warm-up, the B+ supply initiated high voltage to the bus before the output tubes warmed up. This caused a very positive voltage to appear on the grid of the second stage during warm-up. This caused current to flow from the second stage cathode to the second stage grid. The likelihood of grid damage was high during warm-up as grid wires are very small and won't carry much current.

When the Williamson was turned off, it fed a large very low frequency power pulse to the speaker. Many Hi Fi speakers couldn't take this very low frequency pulse and blew out. This turn off power pulse showed that the power supply was unstable, poorly decoupled, poorly regulated and prone to motor boating, a very nasty instability problem. The capacitor-input system rendered marginal the use of the first choke as a filter element. Capacitor input systems defeat the advantage of using chokes in a power supply.
When the Williamson circuit was published, inexpensive electrolytic capacitors were already available in the post war market. Oil capacitors are very expensive per mf. and as obsolete as copper oxide rectifiers. It was a poor choice for home use. Oil capacitors make very poor power supply filters, for they lack enough capacity to do the job. Electrolytic capacitors are much preferred in good designs, being available in sizes to 500 mf. or more.

Continuing our critique of the Williamson amplifier, we turn now to the amplifier circuit. The amplifier is composed of two sections; the "front end" or voltage amplification and phase inversion section and the power output section

The voltage amplifier and phase inverter:

This section or block is composed of a voltage amplifier, phase inverter and driver. The amplifier is a double triode, a 6SN7, one section direct coupled to the phase inverter. Most of the front ends at the time used pentode tubes like the 6SJ7as amplifiers. These tubes had good amplification, but high distortion. Williamson's all triode amplifier set a new standard for the industry (unfortunately not followed by the Dyna). There are really no problems with Williamson's triode input. It was a clean amplifier.
The phase inverter is another matter. Because good design demands a "push-pull" power stage, the output tubes must be fed by phase inversion of the driver. Good design mandates that the driver has certain characteristics. The drive should be balanced amplitude wise and phase wise. The careful phase inversion is the most difficult to achieve. The Williamson phase inverter was a split load phase inverter. The plate and cathode resisters of the second section of the 6SN7 were matched at 47 Kohm resisters. This balances the amplitude of the inverted signal (as long as the load resisters don't drift with time), but there is a hidden serious flaw, not dealt with by producers of the Williamson amplifier.
The plate impedance and the cathode impedance are not of the same value even though the load resisters are the same. This means that at high frequency, the output of the phase inverter is no longer balanced. A scope sampling the signal between the two driver signals shows the discrepancy. This unbalance causes distortion. This distortion is amplified by a negative feedback loop around the amplifier. If the driver signals are tested with the feedback loop in place, the unbalance is seen to be objectionably high, particularly at high frequency. The poor phase inverter was the Achilles heel of the Williamson circuit. Transient response, due to this defective phase inverter is also poor. This type of distortion was not tested for at the time the Williamson appeared. It is one of the reasons some claim that negative feedback is "bad". Negative feedback is not bad if it is around a clean amplifier. Negative feedback around a Williamson is a mixed bag because of the flawed phase inverter.
There is another flaw in the front end. Examination shows that there are two sets of coupling capacitors in the front end. This means that when negative feedback is applied, the amplifier becomes unstable at very low frequency because of the time constants of the capacitors. At very low frequency a phase shift occurs of over 180 degrees around the loop and oscillation can occur. This is aggravated because of the poorly regulated power supply. At the time Williamson wrote his article, capacitors had inductance. This limited the high frequency response of the front end. 6SN7's have poor high frequency response further limiting the high frequency response of the front end. The front end began to roll above 30 Hz. The point is that Williamson's Partridge transformer was not of much use in this kind of amplifier. 200,000 Hz is well beyond the response of the rest of the system

The output stage:

In the American version of the Williamson, two 807's, triode connected were connected in push pull and fed into the primary of the Partridge output transformer. The pair of tubes were cathode biased (together) with an unbypassed common cathode resister. This arrangement cost output power and high frequency response. It is somewhat strange that an engineer working for a tube manufacturer would recommend such an output circuit. His company made KT66's, a beam power tube, ill suited to be used as a triode. The 2A3, a power triode available at the time was not used in the Williamson. Brook began making an amplifier with the 2A3 shortly after the advent of the Williamson. The Miller effect (grid to cathode capacitance) reduced the high frequency response of the Williamson amplifier as the 6SN7's had fairly high plate impedance for a triode.
The 807's were fed with 400 volts on the plates, which allowed them to put out 10 watts RMS. This was in line with what some others were doing in Hi Fi amplifiers, but less than the 20 watts output generally available with 6L6's pentode connected. The distortion in the 6L6's was higher, but with feedback, the distortion was acceptable. The real advance made by Williamson was the design of an all triode amplifier. It inspired others to meet its distortion performance, (with a resistive load) poor though the Williamson was.
The input to the output stage had 1000-ohm suppressor resister in the grid circuit. It also had a resister in the screen circuit, but the screens were not regulated, and tied to t he plates.
This brings us to the operating characteristics of the 807's, triode connected. Power triodes are voltage amplification devices. They try to amplify voltage. With an output resistive load, this presents no problem to the load line. By contract, power pentodes or beam power tubes try to present a constant current to an output load. With a resistive load, this also presents no problem to the load line.
The problem is that loudspeakers (the intended load of the output transformer) are not a resistive load at most of the used frequencies of a loudspeaker. When a loudspeaker is attached to an output transformer instead of a resistive load, the load line of the output tubes goes crazy, whether the tubes are triode or pentode connected. Neither triode or pentode mode operate well with loudspeakers. This is why all performance tests are carried out with resistive loads.
Keroes and Hafler invented the tapped screen mode of operation of output tubes. By connecting the output tube screens to a tap at an appropriate winding location, the output tubes put out constant power into a load, rather than either constant voltage or constant current.
Distributed inductances and capacitances in the speaker circuit cause the varying impedance of a loudspeaker over the used range (see: Acoustical Engineering--Harry Olson, Chief Engineer, Audio, RCA. Harry also taught acoustics at Columbia University when his book was written.) Olson's book is the bible of the audio industry to this day). As is easily shown, inductances and capacitances are reactive in nature. They generate what is known as reactive power. You cannot hear reactive power. What you hear with reactive power is phase shift, which in stereo blurs the stereo effect.
By operating in a constant power mode, the output REAL power from a loudspeaker is more constant. The frequency response is more linear. It is obvious that "ultra-linear" (constant power out) is a better mode of tube operation than either constant voltage or constant current. When a passive crossover network is used in conjunction with a loudspeaker system, the quality of sound degrades more with constant voltage or current than with constant power ("ultra-linear") mode.
There are those presently practicing the art of audio tube design who do not understand the nature of output tubes or circuits. This results in a lot of false statements made around this subject. Given everything else held constant, no triode or pentode tube operation equals "ultra linear", (constant power) operation. The physics is against it.

Some Observations

As mentioned elsewhere, Williamson was a tube engineer who worked for a tube company. Williamson never made the amplifier bearing his name. If he had, he would have made some modifications. Using chokes for instance, and then negating their advantages by using a capacitance input was rather silly. Williamson used a 5U4 power rectifier, which was a directly heated cathode rectifier tube. This meant that the amplifier tubes saw B+ before they were warmed up; bad for cathodes and capacitors. The B+ (without load) was higher by far than normal operation. Williamson's capacitors weren't rated for the voltage surge.
Others subsequently used a mechanical switch to keep the high voltage from the amplifier until the tubes warmed up. A far better circuit uses a 5V4, an indirectly heated cathode rectifier, which does not draw current before the rest of the circuit is ready as it takes it cathode time to warm up too.
The Williamson feedback loop did not respond properly with crossover networks in the output. Then too, the only component in the amplifier that went to 200,000 Hz. was the output transformer. The Williamson circuit did not meet Williamson's own criteria for open circuit bandwidth. (Operation without feedback)
In 1947, speakers were mainly high efficiency types. This meant that the bass resonance was high by modern standards, and the high efficiency created a more ragged audio response curve. The electrical impedance curve was more ragged also. However, a ten-watt amplifier drove the speakers to acceptable levels. In today's world, ten watts doesn't make it, as the speaker systems are no longer high efficiency.
Williamson used a two chassis system for his amplifier, believing that magnetic coupling between transformers caused hum. Poor power supply filtering caused Williamson's hum problems. No one produces two chassis audio amplifiers today. There is no purpose.
It is curious that Williamson did not have his company design a good triode equivalent to his triode connected tubes (KT66 or 807--U.S.) The 2A3 was a better triode than triode connected KT66's. The Brook amplifier that used 2A3's was a better amplifier than the Williamson.
There are those who will be talked into building this "antique". I would suggest that if they build one, put it on the shelf and just look at it. Williamsons and buggy whips don't have a use in the 21st. Century. Also, as far as is known, no one is making the Partridge. In today's world, it would cost too much for 10 watts out.


©2001-2004 Stanley F. White