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
We will discuss the circuit, 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
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
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
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
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
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.
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
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