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Default A Strawman, Constructed and Destroyed-Williamson's Folly?

D.T.N. WILLIAMSON AMPLIFIER

In the late 1940's, component Hi Fi 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.

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wrote:
D.T.N. WILLIAMSON AMPLIFIER

In the late 1940's, component Hi Fi 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.


So far so good, although the hard core buildasauruses used headphones
and disdained open speakers for serious "DXing".


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.


There were schlock publications galore, most notably anything
connected to Hugo Gernsback. A long-lived bunkum artist whose name
still resonates with the Hugo Award for science fiction writing, his
publications-numerous magazines and the paperback Gernsback Library
which became G/L-TAB Books- were often pure horse poop.Nonetheless,
C.G.McProud and i Audio Engineering /i magazine set a high standard
with contributors like Sarser and Sprinkle, Hafler, Keroes, Klipsch,
Curtis Schafer and others.


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.


This is pure bunkum and horse poop, Stan.

DTN Williamson did indeed publish, over several issues of the British
magazine i Wireless World /i the Williamson Amplifier: he built a
prototype complete with a complete "Wickelschema" or wind sheet for the
nine-layer OPT for the hobbyist to tediously wind himself. And he wrote
it up so well that audiophiles all over the world, even behind the Iron
Curtain, in Africa, and everywhere else, built Williamsons. The article
has been reprinted in pamphlet form in numerous reprint editions even
to the present day.

http://www.audioxpress.com/bksprods/books/cdaa2.htm

American builders usually bought transformers as opposed to winding
them, but usually from American wind houses such as Triad, UTC,
Peerless, Freed, or the like. Partridge trasformers were never marketed
in the US to any substantial degree.

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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.


So far so good, except back then everyone had basic sheetmetal skills,
or at least enough sense to find someone with shears and a metal brake.
Bending up a chassis was seen, rightly, as easy and simple. Moderns
apparently have lost this knowledge.


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.


Williamson built _several_ prototypes of his amplifier and understood
its practical limits quite well. This is writing by an Old Guy who
didn't RTFM back then and hasn't since. Stan is like the old uncle who
loudly insists P-38 Mustangs were built by Vortiskorky and American
mores were perverted when "that J** ******* Henry Miller" corrupted
literature with that book they made into a movie where the guy openly
grabs Jayne Mansfield's ass while playing Ping Pong.

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.

Many commercial amplifiers were built this way including several
McIntoshes, the Gotham Audio cutting head amp, and others plus several
Amateur Radio sets ("Amateur" means "used in homes") and who knows what
else. It was particularly in vogue among homebuilt projects because it
DID offer huge advantages in noise as well as being able to power
multiple widgets with the one supply. If you wanted to build another
amp and reuse the power supply you could. Or you could use it with any
homebuilt whatever. I believe this is STILL good practice even todayand
encourage it. When Ed Dell built his "Super Dynaco" amp and wrote it up
in Stereopile in 1967 or thereabouts he did exactly this.

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Phil Allison
 
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** Is there a Part Three yet to come ?

The false allegations re phase inverter imbalance and inductive coupling
caps has got off scot-free so far.




........... Phil


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Margaret von B.
 
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ignore




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Patrick Turner
 
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Snip a bit,


The Williamson Theory

In 1950, Hi Fi bugs didn't know what "feedback" was.


But many ppl did know what it was, and how to use it in various ways.

Today, many ppl still don't know what NFB is, or how to use it.

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.


This isn't correct. The original 1947 design had a 6SN7 3 stage voltage amp
which
did have a bw exceeding 2 - 200 kHz. I know, I have built williamsons.

The other thing about feedback is that many folks used it with OPTs
and tube stages with maybe only 20 kHz of bw.
The simply used slightly less FB and used phase tweaker networks.

Hardly anyone in Oz purchased partridge transformers,
and nearly everything made in 1950 was crap with poor bw, but you could buy
it,
use it, and it was cheap.

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.


Yes they are.

They are dangerous.


Not if its done properly.

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.


But its a good idea to have a separate power supply.
It makes the amps able to be lifted around easily.
A remote PS is good for noise.

And the two chassis can mount in a frame, PS on the bottom,
so its all one unit when set up.



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.


But the grid current initially flowing in the CPI stage was limited by the
cathode R of the stage. And it wasn't for a long period.
I have used SS rectifiers, the voltage comes up high within 3 seconds,
and sure that direct coupled grid goes high, bit it comes down
to about 100v ok after 15 seconds.



When the Williamson was turned off, it fed a large very low frequency
power pulse to the speaker.


This slow pulse from the collapsing B+ voltage didn't last long.

Many Hi Fi speakers couldn't take this
very low frequency pulse and blew out.


Its no worse than running the amp to clipping with a signal at LF.

Any speaker that blew after one pulse from the amp
wasn't a hi-fi speaker; maybe an ex radio speaker.
I have had a real hi-fi 12" woofer get +65 v from the rail of an SS amp
when a mosfet shorted without a protection circuit.
I heard a bang, and a hum, and instinctively reached across and turned off
the amp.
The 100,000 uF rail cap discharged through the speaker.
No damage was done.

The qualification that a speaker is hi-fi includes the ability to take the
full power
of the amp for some specified time.
The williamson could only make 10 watts, and with a 37:1 OPT ratio, the
full 450 v B+ rail voltage translates to a max 12 peak volts that is
possible at the
speaker. It cannot be a long lasting voltage, as it can be with an errant
direct coupled SS amp.

Its a very low quality speaker than can be fused by a williamson turning
off.

The ones I built did not display the problems you speak of.


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 williamson was only barely stable as originally presented.
And with a preamp with bass boost, and a phono stage
and with all the power taken from the power amp,
indeed the amp could oscillate at LF.

But where the preamp had its own supply, this didn't occur.


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.


Rubbish.

Cap inputs with chokes, then more caps make brilliantly good PS.


When the Williamson circuit was published, inexpensive electrolytic
capacitors were already available in the post war market.


They were regarded as unreliable.

The oiler caps were bullet proof.
High values were not needed, because the output stage stayed in class A.

Quad even used only 16 uF to anchor down the CT of their
class A output stage.

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.


Today that all is the case and I routinely use 470 uF caps where once the
same space
was taken up by a 4 uF paper in oil cap.
I still use chokes in my designs, but they don't need to be such large
values as used
in 1950.



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.


Not utterly clean, some 2H was made by the SE input tube.
Pentodes usually make at least as much thd, but have a broader spectra.

It didn't matter because 90% of the thd produced by most tube power amps
is produced by the output tubes.
There is 20 dB of NFB.
This reduces **ALL** of the thd including the 1/10 of it from the input
stages
by a factor of about 0.1.
And because the output stage was a triode satge thd at 10 watts
was 1% with no loop FB and 0.1% with FB.
Dyancos and many other amps with UL, pentode, tetrode stages,
or class AB1 operation had thd at up to 5% without loop FB
so with FB they could only get down to around 0.5%.




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 Williamson CPI works fine.


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.


But it only becomes unbalanced at above 20 kHz.

If you wish to extend the balanced bw, a 15 pF across the Rk of the CPI is
about
all you'll need.

The anode output of the CPI sags as F rises before the low Ro of the
CPI cathode sags.
A little compensation to the Rk thus boosts the output from the CPI
cathode.

Ppl didn't need to know this because W amps work without the compensation
at this point.


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.


And the distortion gets reduced by a factor of about 0.1.

Its only important to reduce distortions below 20 kHz.

Above 20 kHz, it matters less, since the first possible harmonic
of say a 20 khz tone is 40 kHz, and we cannot hear that.

Some would say distortion is OK of signals above 10 khz.

If you have 5% thd of a 10 khz tone, and switch it on and off while
listening to a 10 khz squeal, you will hear no change.


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.


I dispute this as well.
The open loop bw of the W amp was about 80 kHz.
With NFB applied, the bw could be 100 kHz.
But many W amps are now trimmed to go to 65 kHz,
and at full power into the rated load, and without any stage saturating
with
grid current.

The resulting HF response is fabulous, and a reason why tube amps are so
detailed and
fine sounding; they have no problems with musical transients.
It does depend on clipping never occuring, but ppl with hi-fi systems
never go near clipping.



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.


Wrong, the W amp has a fast drive amp including the CPI.
Its bandwidth with 6SN7 was over 200 kHz.

Many other designs were slower, such as a mullard 520, which has a
****ant EF86 driving a 1/2 of a 12AX7 of an LTP,
and the response at the output of the 520 12AX7 anodes was much
reduced below what W did.
The CPI acts as a buffer between the SE input stage and the balanced drive
amp.
the CPI has its own large local amount of current FB.
To make a maximum of 30 vrms at the balanced amp output to drive the output
triode grids,
only about 3 vrms needs to be applied to each grid of the balanced amp.
This is really easy for the CPI to do, even at HF, since the current needed
to
charge and discharge stray and miller C is low, since the signal voltages
are low.


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.


Many W amps were made and worked as W said they would.

6SN7's have poor high frequency response
further limiting the high frequency response of the front end.


Wrong, a 6SN7 has a fairly decent HF capability; typical of many medium µ
triodes.
You are confused; its the high gain high Ra types that have the limited bw.



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 Partridge was an item better had, than not had;
the OPT market was riddled with crap OPTs.

the OPT market is still riddled with crap OPTs, and nearly every major
maker
between 1950 and 1970 tried to cheapen and ruin OPT construction down to
crap, eg, Leak, QuadII, etc....



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.


Wrong.

The signal voltage across the common Rk of the W amp wastes almost
no power at all because this common cathode signal is so small.
Ever measured the signal power generated in the Rk?

While in class A the common Rk does not spoil the bw.

One can even use a CCS a s a common Rk, and the amp will work
as well as ever; but only in class A, as it it was meant to.


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.


But KT66, and 6L6 and 6V6 were very common tubes; they still are.

Brook began making an amplifier with
the 2A3 shortly after the advent of the Williamson. The Miller effect
(grid to cathode capacitance)


The Miller effect is the gain x capacitance between grid and anode, not
including to the cathode.
Triode gain for KT66 was low, so Miller C was low

reduced the high frequency response of
the Williamson amplifier as the 6SN7's had fairly high plate impedance
for a triode.


No, the 6SN7 was and is regarded as having a low Ra.
The data suggests 7.7k at 10mA of Ia, but at 3 mA,
its more like 10k, but that's low enough to allow the W amp to work ok.

12AX7 with Ra = 65 k was regarded as high Ra.


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.


So what?

Triodes seem to work well into reactive 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.


So what?

The result of UL is that Ro of the output stage about equals RL.
With triode RL is a lot more than Ra, and with pentode
Ra is a lot more than RL.
UL was used to maintain pentode power but allow
enough plate signal fed back into the tube to linearize the
current flow.
Its because to get a wide plate voltage swing from a triode,
you have to operate class AB2, which was a pita and cost another 6SN7
used as a CF driver.



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.


The reactive nature of speakers cause currents to flow through the amp
which are not producing audio power, but which rob the
amp of its maximum current ability.
But despite all you say about a W triode amp, used at normal levels into
almost any speaker, they manage to sound and measure well.

This BS about reactives being so evil is just a myth.
They are an inevitable part of converting electric power into sound.
If you have a speaker with an ill concieved Xover network with a
series resonance at say 500 Hz, and a 2 ohm impedance, then
when a 500Hz note plays loud, the amp can be overloaded, or clip at that F,

thus intermodulating all the other musical notes.
But while the music contains no 500Hz, all is well.

Speakers need to be designed well.


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 too many variables I have not got time to discuss here.
There are no general rules that favour UL exclusively.




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.


Perhaps you are a leading light in this trend.....


Given
everything else held constant, no triode or pentode tube operation
equals "ultra linear", (constant power) operation. The physics is
against it.


But the power in music is ever changing, and the RL ever changing.....
Power levels vary regardless of triode, UL, ot pentode/beam.




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.


The PV rating of many of the capacitors I have seen in W and other
tube amps and countless radios is quite adequate.
What killed caps in PS was a saturated output tube, and the ripple current
took out the cap.





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.


In a recent upgrade of a Quad II with 5AR4, the B+ soars up to about 440V
before being pulled
down by the output tubes turning on.
In fact what you say isn't quite the total picture; indirectly heated
rectifiers turn on much faster than output tubes.

Many old amps have caps rated for 500V or more, allowing the PS to be
turned on
without the output tubes present.



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)


The W with a Partridge was one of the very fastest amps with no global FB,
going
with more bw than many other designs relying on FB to make their bw wide.

In fact, there is simply no need to have open loop bw = 200 kHz, since
to have 20 dB of NFB applied at 200 kHz is a real problem
if the load becomes capacitive.

Hence W's addoption of the 470 pF + 4.7 k gain reduction phase tweaking
circuit
applied across the 47k load of V1.

Open loop bandwidth is thus *deliberately* reduced to around 15kHz
and gain is also reduced about 15 dB at 200 kHz, resulting in stability
with pure cap loads between say 0.05uF and 0.47 uF.

The sound does not suffer from such measures.



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.


The efficiency of any speaker varies with F, but all are designed to
make a given constant SPL at all F required providing the voltage of the
signal is held constant for all F.
Current is allowed to vary between lots, and hardly any, and whatever F.



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.


I produce dual chassis amps.

There is a purpose.

And hum problems are rarely from B+ filtering.
The W amp has adequate B+ filtering.

Hum can occur from a variety of mistakes in building any amp,
not just a W.




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.


So what of the 300B?

You don't even mention them.

But beam tetrodes and pentodes were here to stay.

They are hear to stay.

I recently tried a pair of KT90 in a Quad II amp.
They worked fine, giving about 20% more maximum power.

And in a Williamson, KT90 also do very well.


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.


The OPTs I make go from 2Hz to 200khz when i want them to.
Its easy to make something equal or better than Partridge.

The 300 watt OPTs I made went 270kHz.

Partridge wasn't the only maker who knew how to get wide bw.

The only reason why 99.99999999999999999999999999% of ppl
didn't buy Partridge is that the costs of raising a family did not permit
Partridge luxury. Hi-fi was seen as pretentious BS activity
for the layabouts with too much time and money.

These do littles soldered their amps together, and they still do,
why its better than doing a whole range of other silly things.

But a well done Williamson isn't such a bad amp.

The OPT doesn't need to be quite as W specified.

A bigger core, and less turns per volt dramatically improve the outcome.

The first large amp I made was a W with a quad of EL34 in triode.
It was terrific, 30 good watts, and later I went for UL
when i got better at OPT winding.
Nobody could tell me there was any sound change between UL
and triode.

Williamson had a lot of very bright ideas, many of which were ignored
by the makers of so many compromised amps after 1950.

Patrick Turner.









  #7   Report Post  
Patrick Turner
 
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Phil Allison wrote:



** Is there a Part Three yet to come ?

The false allegations re phase inverter imbalance and inductive coupling
caps has got off scot-free so far.


Not for long.

Patrick Turner.



.......... Phil


  #8   Report Post  
robert casey
 
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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.


Sounds like the World Wide Web... :-) My own web site
may have its share of bad ideas... :-)
  #9   Report Post  
Ian Iveson
 
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...Also, as far as is known, no one is making the Partridge...

You mean as far as *you* know. Partridge are still in business, and
still make valve output transformers.

cheers, Ian


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Phil Allison wrote:




** Is there a Part Three yet to come ?

The false allegations re phase inverter imbalance and inductive coupling
caps has got off scot-free so far.


Yes.



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Ian Iveson wrote:
...Also, as far as is known, no one is making the Partridge...


You mean as far as *you* know. Partridge are still in business, and
still make valve output transformers.


Perhaps you could provide a link...

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Patrick Turner wrote:
Snip a bit,


The Williamson Theory

In 1950, Hi Fi bugs didn't know what "feedback" was.


But many ppl did know what it was, and how to use it in various ways.

Today, many ppl still don't know what NFB is, or how to use it.


Those people by definition are not schooled in the electronic art.

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.


This isn't correct. The original 1947 design had a 6SN7 3 stage voltage a=

mp
which
did have a bw exceeding 2 - 200 kHz. I know, I have built williamsons.

The other thing about feedback is that many folks used it with OPTs
and tube stages with maybe only 20 kHz of bw.
The simply used slightly less FB and used phase tweaker networks.

Hardly anyone in Oz purchased partridge transformers,
and nearly everything made in 1950 was crap with poor bw, but you could b=

uy
it,
use it, and it was cheap.





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.


Yes they are.

They are dangerous.


Not if its done properly.

Agreed.


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.


But its a good idea to have a separate power supply.
It makes the amps able to be lifted around easily.
A remote PS is good for noise.

And the two chassis can mount in a frame, PS on the bottom,
so its all one unit when set up.



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.


But the grid current initially flowing in the CPI stage was limited by the
cathode R of the stage. And it wasn't for a long period.
I have used SS rectifiers, the voltage comes up high within 3 seconds,
and sure that direct coupled grid goes high, bit it comes down
to about 100v ok after 15 seconds.



It's not challenging at all today to design a power supply without
tube rectifiers, that cycles the voltages needed for operation of the
amplifier in the proper order and moreover ramps each up in a
controlled, benign fashion. It is simple cookbookery, no engineering
per se is really needed. Most tube builders are electronically below
any reasonable fitness standard, however, and so it does not happen.

Such a power supply would be good for running slightly modified
vintage amps, test bench work, or running radio apparatus as well as
hi-fi.



When the Williamson was turned off, it fed a large very low frequency
power pulse to the speaker.


This slow pulse from the collapsing B+ voltage didn't last long.

Many Hi Fi speakers couldn't take this
very low frequency pulse and blew out.


Its no worse than running the amp to clipping with a signal at LF.



History records few if any such blowouts.


Any speaker that blew after one pulse from the amp
wasn't a hi-fi speaker; maybe an ex radio speaker.
I have had a real hi-fi 12" woofer get +65 v from the rail of an SS amp
when a mosfet shorted without a protection circuit.
I heard a bang, and a hum, and instinctively reached across and turned off
the amp.
The 100,000 uF rail cap discharged through the speaker.
No damage was done.

The qualification that a speaker is hi-fi includes the ability to take the
full power
of the amp for some specified time.
The williamson could only make 10 watts, and with a 37:1 OPT ratio, the
full 450 v B+ rail voltage translates to a max 12 peak volts that is
possible at the
speaker. It cannot be a long lasting voltage, as it can be with an errant
direct coupled SS amp.

Its a very low quality speaker than can be fused by a williamson turning
off.

The ones I built did not display the problems you speak of.


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 williamson was only barely stable as originally presented.
And with a preamp with bass boost, and a phono stage
and with all the power taken from the power amp,
indeed the amp could oscillate at LF.

But where the preamp had its own supply, this didn't occur.


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.


Rubbish.

Cap inputs with chokes, then more caps make brilliantly good PS.


When the Williamson circuit was published, inexpensive electrolytic
capacitors were already available in the post war market.


They were regarded as unreliable.

The oiler caps were bullet proof.
High values were not needed, because the output stage stayed in class A.

Quad even used only 16 uF to anchor down the CT of their
class A output stage.

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.


Today that all is the case and I routinely use 470 uF caps where once the
same space
was taken up by a 4 uF paper in oil cap.
I still use chokes in my designs, but they don't need to be such large
values as used
in 1950.


The oil caps will last far longer,as proven by the continuing
operation of much WWII and Korean military gear. However, the hobbyist
can always replace them himself cheaply, which is why photoflash caps
enjoyed such popularity in the Early Audio Amateur Era. Amps so modded
in the 80s should be recapped now.



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.


Not utterly clean, some 2H was made by the SE input tube.
Pentodes usually make at least as much thd, but have a broader spectra.

It didn't matter because 90% of the thd produced by most tube power amps
is produced by the output tubes.
There is 20 dB of NFB.
This reduces **ALL** of the thd including the 1/10 of it from the input
stages
by a factor of about 0.1.
And because the output stage was a triode satge thd at 10 watts
was 1% with no loop FB and 0.1% with FB.
Dyancos and many other amps with UL, pentode, tetrode stages,
or class AB1 operation had thd at up to 5% without loop FB
so with FB they could only get down to around 0.5%.




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 Williamson CPI works fine.


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.


But it only becomes unbalanced at above 20 kHz.

If you wish to extend the balanced bw, a 15 pF across the Rk of the CPI is
about
all you'll need.

The anode output of the CPI sags as F rises before the low Ro of the
CPI cathode sags.
A little compensation to the Rk thus boosts the output from the CPI
cathode.

Ppl didn't need to know this because W amps work without the compensation
at this point.


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.


And the distortion gets reduced by a factor of about 0.1.

Its only important to reduce distortions below 20 kHz.

Above 20 kHz, it matters less, since the first possible harmonic
of say a 20 khz tone is 40 kHz, and we cannot hear that.

Some would say distortion is OK of signals above 10 khz.

If you have 5% thd of a 10 khz tone, and switch it on and off while
listening to a 10 khz squeal, you will hear no change.


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.


I dispute this as well.
The open loop bw of the W amp was about 80 kHz.
With NFB applied, the bw could be 100 kHz.
But many W amps are now trimmed to go to 65 kHz,
and at full power into the rated load, and without any stage saturating
with
grid current.

The resulting HF response is fabulous, and a reason why tube amps are so
detailed and
fine sounding; they have no problems with musical transients.
It does depend on clipping never occuring, but ppl with hi-fi systems
never go near clipping.


Maybe they -shouldn't-. They do!



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.


Wrong, the W amp has a fast drive amp including the CPI.
Its bandwidth with 6SN7 was over 200 kHz.

Many other designs were slower, such as a mullard 520, which has a
****ant EF86 driving a 1/2 of a 12AX7 of an LTP,
and the response at the output of the 520 12AX7 anodes was much
reduced below what W did.
The CPI acts as a buffer between the SE input stage and the balanced drive
amp.
the CPI has its own large local amount of current FB.
To make a maximum of 30 vrms at the balanced amp output to drive the outp=

ut
triode grids,
only about 3 vrms needs to be applied to each grid of the balanced amp.
This is really easy for the CPI to do, even at HF, since the current need=

ed
to
charge and discharge stray and miller C is low, since the signal voltages
are low.


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.


Many W amps were made and worked as W said they would.


Many were. Many more were deviated from and still usually worked. In
the US no amateur wound his own output transformer. Most had a pet
builder whom they attributed Godlike powers to, such as Ercel Harrison
at Peerless, and would use their product over all others based on some
mantra. Much of the "secret wisdom" of these "gurus" is quietly sitting
on library shelves if one knows where to find it-a hint: The U.S. Navy
is the most ardent audiophile organization in history.

6SN7's have poor high frequency response
further limiting the high frequency response of the front end.


Wrong, a 6SN7 has a fairly decent HF capability; typical of many medium =

=B5
triodes.
You are confused; its the high gain high Ra types that have the limited b=

w=2E



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 Partridge was an item better had, than not had;
the OPT market was riddled with crap OPTs.

the OPT market is still riddled with crap OPTs, and nearly every major
maker
between 1950 and 1970 tried to cheapen and ruin OPT construction down to
crap, eg, Leak, QuadII, etc....



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.


Wrong.

Correct! He's on crack.

The signal voltage across the common Rk of the W amp wastes almost
no power at all because this common cathode signal is so small.
Ever measured the signal power generated in the Rk?

While in class A the common Rk does not spoil the bw.

One can even use a CCS a s a common Rk, and the amp will work
as well as ever; but only in class A, as it it was meant to.


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.


RTFM, Stan. The source document, available from UK and US sources,
specifies that the KT-66, 807 or 6L6 may all be used. US builders
usually followed Sarser and Sprinkle, who used the 807 strictly because
WWII surplus ones were cheap. Later surplus ones, by NOS standards,
still are. Triodes were not used for reasons clearly stated in the
source document, not for lack of them. The Brits had such jewels as the
PX series and the DA100.

But KT66, and 6L6 and 6V6 were very common tubes; they still are.

Brook began making an amplifier with
the 2A3 shortly after the advent of the Williamson. The Miller effect
(grid to cathode capacitance)


The Miller effect is the gain x capacitance between grid and anode, not
including to the cathode.
Triode gain for KT66 was low, so Miller C was low

reduced the high frequency response of
the Williamson amplifier as the 6SN7's had fairly high plate impedance
for a triode.


No, the 6SN7 was and is regarded as having a low Ra.
The data suggests 7.7k at 10mA of Ia, but at 3 mA,
its more like 10k, but that's low enough to allow the W amp to work ok.

12AX7 with Ra =3D 65 k was regarded as high Ra.


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.


So what?

Triodes seem to work well into reactive 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.


So what?

The result of UL is that Ro of the output stage about equals RL.
With triode RL is a lot more than Ra, and with pentode
Ra is a lot more than RL.
UL was used to maintain pentode power but allow
enough plate signal fed back into the tube to linearize the
current flow.
Its because to get a wide plate voltage swing from a triode,
you have to operate class AB2, which was a pita and cost another 6SN7
used as a CF driver.



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).


Where is Kroomel when you need him?

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.


The reactive nature of speakers cause currents to flow through the amp
which are not producing audio power, but which rob the
amp of its maximum current ability.
But despite all you say about a W triode amp, used at normal levels into
almost any speaker, they manage to sound and measure well.

This BS about reactives being so evil is just a myth.
They are an inevitable part of converting electric power into sound.
If you have a speaker with an ill concieved Xover network with a
series resonance at say 500 Hz, and a 2 ohm impedance, then
when a 500Hz note plays loud, the amp can be overloaded, or clip at that =

F,

thus intermodulating all the other musical notes.
But while the music contains no 500Hz, all is well.

Speakers need to be designed well.


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 too many variables I have not got time to discuss here.
There are no general rules that favour UL exclusively.




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.


Perhaps you are a leading light in this trend.....


Given
everything else held constant, no triode or pentode tube operation
equals "ultra linear", (constant power) operation. The physics is
against it.


But the power in music is ever changing, and the RL ever changing.....
Power levels vary regardless of triode, UL, ot pentode/beam.




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.


The PV rating of many of the capacitors I have seen in W and other
tube amps and countless radios is quite adequate.
What killed caps in PS was a saturated output tube, and the ripple current
took out the cap.





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.


In a recent upgrade of a Quad II with 5AR4, the B+ soars up to about 440V
before being pulled
down by the output tubes turning on.
In fact what you say isn't quite the total picture; indirectly heated
rectifiers turn on much faster than output tubes.

Many old amps have caps rated for 500V or more, allowing the PS to be
turned on
without the output tubes present.



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)


The W with a Partridge was one of the very fastest amps with no global FB,
going
with more bw than many other designs relying on FB to make their bw wide.

In fact, there is simply no need to have open loop bw =3D 200 kHz, since
to have 20 dB of NFB applied at 200 kHz is a real problem
if the load becomes capacitive.

Hence W's addoption of the 470 pF + 4.7 k gain reduction phase tweaking
circuit
applied across the 47k load of V1.

Open loop bandwidth is thus *deliberately* reduced to around 15kHz
and gain is also reduced about 15 dB at 200 kHz, resulting in stability
with pure cap loads between say 0.05uF and 0.47 uF.

The sound does not suffer from such measures.



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.


The efficiency of any speaker varies with F, but all are designed to
make a given constant SPL at all F required providing the voltage of the
signal is held constant for all F.
Current is allowed to vary between lots, and hardly any, and whatever F.



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.


I produce dual chassis amps.

There is a purpose.

And hum problems are rarely from B+ filtering.
The W amp has adequate B+ filtering.

Hum can occur from a variety of mistakes in building any amp,
not just a W.




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.


So what of the 300B?

You don't even mention them.

But beam tetrodes and pentodes were here to stay.

They are hear to stay.

I recently tried a pair of KT90 in a Quad II amp.
They worked fine, giving about 20% more maximum power.

And in a Williamson, KT90 also do very well.


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.


The OPTs I make go from 2Hz to 200khz when i want them to.
Its easy to make something equal or better than Partridge.

The 300 watt OPTs I made went 270kHz.

Partridge wasn't the only maker who knew how to get wide bw.

The only reason why 99.99999999999999999999999999% of ppl
didn't buy Partridge is that the costs of raising a family did not permit
Partridge luxury. Hi-fi was seen as pretentious BS activity
for the layabouts with too much time and money.

These do littles soldered their amps together, and they still do,
why its better than doing a whole range of other silly things.

But a well done Williamson isn't such a bad amp.

The OPT doesn't need to be quite as W specified.

A bigger core, and less turns per volt dramatically improve the outcome.

The first large amp I made was a W with a quad of EL34 in triode.
It was terrific, 30 good watts, and later I went for UL
when i got better at OPT winding.
Nobody could tell me there was any sound change between UL
and triode.

Williamson had a lot of very bright ideas, many of which were ignored
by the makers of so many compromised amps after 1950.


Generally in the US if you homebrewed audio you were either retired or
young and technically curious-same as ham radio. But whereas hams were
generally cheap, audio guys went first rate. Dynaco and other cheap
kits were the beginning of the end for scratchbuilders.

  #13   Report Post  
 
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Margaret von B. wrote:
ignore


Only if you have a gas mask....

  #14   Report Post  
Ian Iveson
 
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wrote

...Also, as far as is known, no one is making the Partridge...


You mean as far as *you* know. Partridge are still in business,
and
still make valve output transformers.


Perhaps you could provide a link...


Not too hard to find...

http://www.transformers.co.uk/

cheers, Ian


  #15   Report Post  
 
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This website says nothing about audio transformers of any type nor
does it in any way indicate they are heirs to Partridge.



  #16   Report Post  
Ian Iveson
 
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wrote

This website says nothing about audio transformers of any type nor
does it in any way indicate they are heirs to Partridge.


Step into the light...
Hmmm, your right.

I had noticed, but thanks anyway.

cheers, Ian


  #18   Report Post  
Patrick Turner
 
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snip a bit,


But the grid current initially flowing in the CPI stage was limited by the
cathode R of the stage. And it wasn't for a long period.
I have used SS rectifiers, the voltage comes up high within 3 seconds,
and sure that direct coupled grid goes high, bit it comes down
to about 100v ok after 15 seconds.


It's not challenging at all today to design a power supply without
tube rectifiers, that cycles the voltages needed for operation of the
amplifier in the proper order and moreover ramps each up in a
controlled, benign fashion. It is simple cookbookery, no engineering
per se is really needed. Most tube builders are electronically below
any reasonable fitness standard, however, and so it does not happen.


B+ ramp ups don't need to be installed. tube amps work OK without them.
But all that needs to be used is an inrush current limiting R so the mains fuse
value
can be kept as low as possible, and therefore provide adequate protection
if the other measures in the amp fail.


Such a power supply would be good for running slightly modified
vintage amps, test bench work, or running radio apparatus as well as
hi-fi.


But 99% of tube amp buyers don't tinker with power amps.



When the Williamson was turned off, it fed a large very low frequency
power pulse to the speaker.


This slow pulse from the collapsing B+ voltage didn't last long.

Many Hi Fi speakers couldn't take this
very low frequency pulse and blew out.


Its no worse than running the amp to clipping with a signal at LF.


History records few if any such blowouts.


So why did you suggest thse pulses would blow speakers?


Snip a bit more


The resulting HF response is fabulous, and a reason why tube amps are so
detailed and
fine sounding; they have no problems with musical transients.
It does depend on clipping never occuring, but ppl with hi-fi systems
never go near clipping.


Maybe they -shouldn't-. They do!


Nobody I know uses their amps at an average level much above 1/20 the maximum
power at onset of clipping.



Snip a little more,


Many W amps were made and worked as W said they would.


Many were. Many more were deviated from and still usually worked. In
the US no amateur wound his own output transformer.


Huh?
I have had quite some emails for advice about winding OPTs.

Some ppl are doing it.

Most had a pet
builder whom they attributed Godlike powers to, such as Ercel Harrison
at Peerless, and would use their product over all others based on some
mantra. Much of the "secret wisdom" of these "gurus" is quietly sitting
on library shelves if one knows where to find it-a hint: The U.S. Navy
is the most ardent audiophile organization in history.


Navies were not usually interested in purchasing audiophile grade OPTs for their
aircraft carrier PA systems.



snip more,


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.


Wrong.

Correct! He's on crack.


Prove you are correct then.

Snip the rest, since I think I have addressed the contentious
issues.

Patrick Turner.


  #20   Report Post  
Patrick Turner
 
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John Stewart wrote:

wrote:

Not sure where this article came from but the style is reminiscent of Stan
White & his POWRTRON Amp.

See
http://www.cosmos2000.org/audio/powrtron1.htm

Cheers, John Stewart


The dissertation posted by wasn't written by
calcerise, although the way he presented it made it look that way,
but out calcerise wouldn't have been capable of drafting such a document.

The article which I spent so long largely debunking came from

http://www.cosmos2000.org/audio/william1.htm

and under the heading:-

D.T.N. WILLIAMSON AMPLIFIER
Copyright © 2001 Stanley F. White

I hope that might leave calcerise feeling less mauled.

He really should learn to post material with the source, or else
he breaks copyright, and could be accused of plagerism, and being an idiot.

Patrick Turner.





  #21   Report Post  
Marcin Slawicz
 
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Uzytkownik "Patrick Turner" napisal w wiadomosci
...
*** Snip ***

Williamson had a lot of very bright ideas, many of which were ignored
by the makers of so many compromised amps after 1950.

Patrick Turner.


Thanks Patrick for your great explanation. I was familiar with White's
article and really resisted some statements I found there. There is quite a
number of misguiding articles regarding the Williamson amplifier on the web,
and because of my nearly no-experience in tube electronics, I am not always
sure what true and what false is.
If you don't mind, please take a look at my first tube project I am
assembling these days
(http://www.echostar.pl/~slawicz/conc...oncertino8.htm). The site is in
polish and it's not completed yet, but you'll find the schematic there.
The biggest difference between Williamson's amplifier and my one are
(despite the UL output stage) the constant current draw first two stages in
Aikido style described many times by John Broskie in TubeCad Journal
(http://www.tubecad.com/april99/page6.html). I hope it will work properly.
The simulation shows about 14 dB better PS noise and ripple rejection
comparing to typical Williamson front end.
Best regards,
Marcin

  #22   Report Post  
Clyde Slick
 
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"Patrick Turner" wrote in message
...

I hope that might leave calcerise feeling less mauled.

He really should learn to post material with the source, or else
he breaks copyright, and could be accused of plagerism, and being an
idiot.


Are you implying that he is Ferstler's sockpuppet?



----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
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  #23   Report Post  
Patrick Turner
 
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Marcin Slawicz wrote:

Uzytkownik "Patrick Turner" napisal w wiadomosci
...
*** Snip ***

Williamson had a lot of very bright ideas, many of which were ignored
by the makers of so many compromised amps after 1950.

Patrick Turner.


Thanks Patrick for your great explanation. I was familiar with White's
article and really resisted some statements I found there. There is quite a
number of misguiding articles regarding the Williamson amplifier on the web,
and because of my nearly no-experience in tube electronics, I am not always
sure what true and what false is.


At all times in the past, nearly all men with a slightly different way of
building an amp
would routinely say how bad the other guy's amp was.
The same BS goes on today.

I have built just about all types of amplifier, and all are derived from what my
father's
generation thought up.
For example I like the Quad II idea of CFB from the OPT, but I think the way
Quad
implement their idea is very lack lustre, and its a chic amp which has been
dumbed down by accountants.
It still sounds ok though, at a few watts.
I criticise many of the sacred cow brands such as Leak, Quad, Dynaco,
and I don't care whose ego I bruise; if they can't see that these old bits of
junk
**could have** been a lot better pieces of engineering had the makers used a
little more labour
and material instead of buying that new Mercedes for the boss, then they'll
never see anything.

Anyway, I use the Quad II idea in preference to the normal screen taps of UL.
But I use much more CFB, since the amount Quad use doesn't do enough to reduce
Ra-a and thd. Same goes for UL; one still has an output stage with Ro = approx
RL,
but its better than nothing.



If you don't mind, please take a look at my first tube project I am
assembling these days
(http://www.echostar.pl/~slawicz/conc...oncertino8.htm). The site is in
polish and it's not completed yet, but you'll find the schematic there.
The biggest difference between Williamson's amplifier and my one are
(despite the UL output stage) the constant current draw first two stages in
Aikido style described many times by John Broskie in TubeCad Journal
(http://www.tubecad.com/april99/page6.html). I hope it will work properly.
The simulation shows about 14 dB better PS noise and ripple rejection
comparing to typical Williamson front end.
Best regards,
Marcin


There is always a constant current drawn from the PS to any kind of SET
V1 and CPI V2 stages, simply because class A stages do not have a changing
supply power, other than that due to the 2H produced in signal, and in your case
that
will be below 0.1% evan at 2vrms from V1, so the DC voltages at
all electodes won't change much.

The use of the R19 bypassed with C5 and R24 is a step in the right direction
because it makes the set up conditions of
V1 and and V2 easier to arrange so that the anode voltage at V1 is best for
linearity,
and the grid voltage at V2 is best so Ek isn't too high above the heater supply.

Also at very LF, where the williamson margin of stability becomes
much less than at 1 kHz, the R19 and R24 become a voltage divider reducing open
loop gain
by 6 dB below 20 Hz, because C5 graduallly becomes a large and open impedance at
LF.

You could further stabilise the amp by using more LF attenuation in the open
loop response than you have.

Go to

http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm

There you will see a schematic for 50 watt class AB1 UL channel using KT88/6550
and including an integrated input preamp stage.

It isn't a williamson, but is a derived topology from what Mullard used
back in the 50's, and which has been addopted by 1,001 makers since then.

However, I have a few secret things in there that would NEVER have been done in
1955, such as the gain&phase shift network between the first tube in the power
amp, and the
second, which is 1/2 of the longtail pair.

This network should be of great interest to anyone building their
PP amp especially if the OPT isn't up to the **best** of the Partridge models.
About 99.99% of OPTs sold on this planet are a lot worse than
Partridge OPTs.

There is a network containing 0.47 uF, 0.047 uF, 1M, 220k
which acts to reduce LF gain a maximum of 15 dB, starting at 15 Hz.
By 1.5 Hz, at which many amps would oscillate badly without this network,
the LF gain has become about 12 dB less, and as stability is affected by the
amount
of open loop gain and the amount of FB applied, the amp will have far less NFB
applied
at 1.5 Hz than at say 100Hz, so the amp will be rock stable at *all* LF.
The amount of FB that *can* be applied safely without oscillations
occuring is also dependant on the phase shift of an amp, and stablity
is threatened as phase shift approaches 180 degrees betaween input and output.

At LF, the CR couplings between stages and the primary OPT
inductance will produce a max of 90 degrees each.
So its not uncommon for a tube amp to have maybe 180 degrees of phase shift
at 5 Hz, depending on the OPT and CR couplings, because the phase shift
of each reactively affected stage is acumulative.
So 3 lots of 60 degrees adds to 180 degrees.


FB applied, in dB = 20 x log of A / ( 1 + [ A x ß ] ), where A = open loop
voltage gain, ß = fraction of the output voltage fed back to the input,
and 1 is a constant required for all equations to work.

If mid F gain, or 1 kHz open gain A = 50, and ß = 0.1, then FB applied at 1 kHz
=
20 x log of 50 / ( 1 + [ 50 x 0.1 ] ) = 20 x 1.021 = 18.4 dB.

A is the ouput voltage from where the NFB is taken divided by the input voltage
without any loop FB applied.

You need to be totally and utterly familiar with this formula
and how to derive ß or else you just won't know what you are doing
when you are building an amp, like most ppl when they start.
The smoky and lousy sounding amps are a testament to their ignorance.

You can also measure and calculate the amount of FB you have applied.
Applied FB in Db = 20 log ( output voltage without FB / output voltage with FB
).

A typical Williamson amp may need 0.2vrms input for 10vrms output without
loop FB.
Hence open loop gain = 10 / 0.2 = 50.


Now with this LF gain reducing network, or step network, or shelving network
which some folks call it, open gain at LF is reduced about say 4 times at 5Hz,
and then the OPT and other CR couplings will have reduced the gain in the output
stage about 6 dB,
so total open loop gain at 5 Hz might be 50 / 4 x 2 = 6.25.


So the amount of FB in Db = 20 log 6.25 / ( 1 + [ 6.25 x 0.1] ) = 11.7 dB.

This is a considerable reduction in the amount of applied NFB.

The open loop phase shift at below 5 Hz won't be any worse than you'd have with
all CR couplings, so stability is far better with such a network in place than
without,
and I use such a network on all my amps.
Many amps with FB will oscillate at say 1.5Hz, but not with this network
when it is fitted, because open loop gain continues to fall.

The values can be fiddled with make it more effective with crummy low inductance
OPTs,
so that 0.022 uF or even 0.01 uF will begin to reduce the open loop gain
at a higher F.

Such networks do not substantially reduce the bass response at 20 Hz or increase
the
bass F output impedance if they are simply designed to do their dirty work below
20 Hz.
Who cares if the Ro of an amp becomes higher at F outside of the audio band?

The same idea of gain stepping applies to the HF response.

In my schematic there is a network of 3.3k and 330pF which applies itself
effectively
too the anode of the input triode.

The input triode in my case has a bypassed Rk, so its Ra is a low 10k
which is in parallel with the 75k DC RL + the bias R of 220k so the output
impedance of the set input tube is 10k //75k//220k = effectively 8.4k.

Gain for all tubes = µ x RL / ( Ra + RL ).

Now as F rises, the RL seen by V1 anode falls because the 330pF
begins to have a low impedance, and by 500 kHz, Z330pF = 1k.
Thus the load the V1 anode becomes approximately 4k,
and thus gain of V1 is halved at such a HF.

The phase shift effect of any stray C or Miller C at such F is reduced
because the R component of what is a low pass filter circuit has become lower.

Therefore the amp is more stable because at HF the phase shift and open gain has
been slightly reduced at an F where
the amp is otherwise likely to oscillate, especially if a capacitance load is
ever used with the amp.

meanwhile, anything going on at up to 20 khz is totally unaffected by the
gain/phase tweak HF network.

To get the best value for the CR HF step networks, don't just calculate.
You won't succeed. There are far too many unknown C quantities, not to
mention the leakage inductance of the OPT which all affect the signal through
the amp.
I set up the amp to be stable with a 0.22 uF across the output without a load R.

Many amps oscillate violently in such a condition.
But first you have to apply just enough C across the global feedback resistor
to **advance the phase at HF** of the HF signals being fed back, to compensate
for the phase lag the HF signals suffer as they pass by all the R-C and R-L
interfaces.
If the OPT is just good enough, without too much leakage L then you should be
able to just stabilise
the amp with the C across the FBR.
Once the oscillations have stopped with the 0.22 uF,
you then hook up a variable radio tuning cap of 20pF - 365 pF,
and a 25k pot in series with a couple of jumper leads,
and adjust the pot and cap for the least ringing on a 5 kHz square wave with the

0.22 uF cap in place as the sole load on the amp.
If the ringing is only a couple of cycles, and the overshoot is only 3 dB more
than
the peak rise voltage, you have done well, but the sine wave response at full
power
into a resistance load should still be 65 kHz, - 3dB.

Its easier said than done, because you have to consider about 10 interactive
things at once,
and the God Of Triodes arranged the laws of electronics to be dammned difficult,

and bloody frustrating, to make sure only those with a modicum of intelligence
could
work out what exactly has to be done with the bits and pieces in an amp to make
sure they will
always be stable, since idiots who will never understand or have a natural feel
for
these things give up and go away, and become politicians or parking inspectors.

It is vitally important to consider all amplifiers as bandpass filters,
with poles affecting their bandwidth, and in the case of a tube amp.
there are quite a few poles because of all the Miller C in different stages
and the effects of primary and leakage inductances.

Solid state amps also have to be considered similarly,
and in their case the amount of NFB applied is usually
vastly more than is applied in a tube amp because the
poles of the roll offs are further away from the boundaries of the 20Hz to 20
kHz audio band.
But they too will oscillate badly at HF especially without correct HF
compensation
networks placed to control HF open loop gain.
They also have a zobel L&R and C&R network to make sure there is a load on the
amp
at HF, and that the output stage is never directly connected to an extremely low
load value
which would cause the devices to fail easily ( which they do anyway, judging
from the
2 or 3 amps I get to fix each week with fused bjts ).

Such zobel networks can also be applied to tube amp output stages, but
experience and practice
is needed to get the values right to make sure they don't make the amp less
stable,
and don't help overload any stage of the amp by causing too large an error
signal at HF.

With some intelligent use of R&C parts, your Williamson will sing.

But every OPT has a different amount of leakage and stray C so there is never
going to be an easy way to make your amp unconditionally stable,
ie, any load whatever can be connected including any L, or C, or no load at all,

and oscillations will not occur, ever.

When all the conditions above have been addressed properly,
usually tube amps sound just fabulous.

Patrick Turner.





  #24   Report Post  
Patrick Turner
 
Posts: n/a
Default



Clyde Slick wrote:

"Patrick Turner" wrote in message
...

I hope that might leave calcerise feeling less mauled.

He really should learn to post material with the source, or else
he breaks copyright, and could be accused of plagerism, and being an
idiot.


Are you implying that he is Ferstler's sockpuppet?


Was he Lastler's?

Blessed are those that come last, for they shall come ferst to the Kingdom ;-)

So saith God.

Promises, promises, ha, I still won't go to church on a sunday.

Patrick Turner.



----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----


  #25   Report Post  
John Stewart
 
Posts: n/a
Default



Patrick Turner wrote:

Marcin Slawicz wrote:

Uzytkownik "Patrick Turner" napisal w wiadomosci
...
*** Snip ***

Williamson had a lot of very bright ideas, many of which were ignored
by the makers of so many compromised amps after 1950.

Patrick Turner.


Thanks Patrick for your great explanation. I was familiar with White's
article and really resisted some statements I found there. There is quite a
number of misguiding articles regarding the Williamson amplifier on the web,
and because of my nearly no-experience in tube electronics, I am not always
sure what true and what false is.


At all times in the past, nearly all men with a slightly different way of
building an amp
would routinely say how bad the other guy's amp was.
The same BS goes on today.

I have built just about all types of amplifier, and all are derived from what my
father's
generation thought up.
For example I like the Quad II idea of CFB from the OPT, but I think the way
Quad
implement their idea is very lack lustre, and its a chic amp which has been
dumbed down by accountants.
It still sounds ok though, at a few watts.
I criticise many of the sacred cow brands such as Leak, Quad, Dynaco,
and I don't care whose ego I bruise; if they can't see that these old bits of
junk
**could have** been a lot better pieces of engineering had the makers used a
little more labour
and material instead of buying that new Mercedes for the boss, then they'll
never see anything.

Anyway, I use the Quad II idea in preference to the normal screen taps of UL.
But I use much more CFB, since the amount Quad use doesn't do enough to reduce
Ra-a and thd. Same goes for UL; one still has an output stage with Ro = approx
RL,
but its better than nothing.


If you don't mind, please take a look at my first tube project I am
assembling these days
(http://www.echostar.pl/~slawicz/conc...oncertino8.htm). The site is in
polish and it's not completed yet, but you'll find the schematic there.
The biggest difference between Williamson's amplifier and my one are
(despite the UL output stage) the constant current draw first two stages in
Aikido style described many times by John Broskie in TubeCad Journal
(http://www.tubecad.com/april99/page6.html). I hope it will work properly.
The simulation shows about 14 dB better PS noise and ripple rejection
comparing to typical Williamson front end.
Best regards,
Marcin


There is always a constant current drawn from the PS to any kind of SET
V1 and CPI V2 stages, simply because class A stages do not have a changing
supply power, other than that due to the 2H produced in signal, and in your case
that
will be below 0.1% evan at 2vrms from V1, so the DC voltages at
all electodes won't change much.

The use of the R19 bypassed with C5 and R24 is a step in the right direction
because it makes the set up conditions of
V1 and and V2 easier to arrange so that the anode voltage at V1 is best for
linearity,
and the grid voltage at V2 is best so Ek isn't too high above the heater supply.

Also at very LF, where the williamson margin of stability becomes
much less than at 1 kHz, the R19 and R24 become a voltage divider reducing open
loop gain
by 6 dB below 20 Hz, because C5 graduallly becomes a large and open impedance at
LF.

You could further stabilise the amp by using more LF attenuation in the open
loop response than you have.

Go to

http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm

There you will see a schematic for 50 watt class AB1 UL channel using KT88/6550
and including an integrated input preamp stage.

It isn't a williamson, but is a derived topology from what Mullard used
back in the 50's, and which has been addopted by 1,001 makers since then.

However, I have a few secret things in there that would NEVER have been done in
1955, such as the gain&phase shift network between the first tube in the power
amp, and the
second, which is 1/2 of the longtail pair.

This network should be of great interest to anyone building their
PP amp especially if the OPT isn't up to the **best** of the Partridge models.
About 99.99% of OPTs sold on this planet are a lot worse than
Partridge OPTs.

There is a network containing 0.47 uF, 0.047 uF, 1M, 220k
which acts to reduce LF gain a maximum of 15 dB, starting at 15 Hz.
By 1.5 Hz, at which many amps would oscillate badly without this network,
the LF gain has become about 12 dB less, and as stability is affected by the
amount
of open loop gain and the amount of FB applied, the amp will have far less NFB
applied
at 1.5 Hz than at say 100Hz, so the amp will be rock stable at *all* LF.
The amount of FB that *can* be applied safely without oscillations
occuring is also dependant on the phase shift of an amp, and stablity
is threatened as phase shift approaches 180 degrees betaween input and output.


LF Phase/Gain networks such as you refer too were not very secret at all in the
50's. For example, they were well understood & used in both the McIntosh MC40 &
MC75.

Cheers, John Stewart



  #26   Report Post  
John Stewart
 
Posts: n/a
Default



John Stewart wrote:

Patrick Turner wrote:

Marcin Slawicz wrote:

Uzytkownik "Patrick Turner" napisal w wiadomosci
...
*** Snip ***

Williamson had a lot of very bright ideas, many of which were ignored
by the makers of so many compromised amps after 1950.

Patrick Turner.


Thanks Patrick for your great explanation. I was familiar with White's
article and really resisted some statements I found there. There is quite a
number of misguiding articles regarding the Williamson amplifier on the web,
and because of my nearly no-experience in tube electronics, I am not always
sure what true and what false is.


At all times in the past, nearly all men with a slightly different way of
building an amp
would routinely say how bad the other guy's amp was.
The same BS goes on today.

I have built just about all types of amplifier, and all are derived from what my
father's
generation thought up.
For example I like the Quad II idea of CFB from the OPT, but I think the way
Quad
implement their idea is very lack lustre, and its a chic amp which has been
dumbed down by accountants.
It still sounds ok though, at a few watts.
I criticise many of the sacred cow brands such as Leak, Quad, Dynaco,
and I don't care whose ego I bruise; if they can't see that these old bits of
junk
**could have** been a lot better pieces of engineering had the makers used a
little more labour
and material instead of buying that new Mercedes for the boss, then they'll
never see anything.

Anyway, I use the Quad II idea in preference to the normal screen taps of UL.
But I use much more CFB, since the amount Quad use doesn't do enough to reduce
Ra-a and thd. Same goes for UL; one still has an output stage with Ro = approx
RL,
but its better than nothing.


If you don't mind, please take a look at my first tube project I am
assembling these days
(http://www.echostar.pl/~slawicz/conc...oncertino8.htm). The site is in
polish and it's not completed yet, but you'll find the schematic there.
The biggest difference between Williamson's amplifier and my one are
(despite the UL output stage) the constant current draw first two stages in
Aikido style described many times by John Broskie in TubeCad Journal
(http://www.tubecad.com/april99/page6.html). I hope it will work properly.
The simulation shows about 14 dB better PS noise and ripple rejection
comparing to typical Williamson front end.
Best regards,
Marcin


There is always a constant current drawn from the PS to any kind of SET
V1 and CPI V2 stages, simply because class A stages do not have a changing
supply power, other than that due to the 2H produced in signal, and in your case
that
will be below 0.1% evan at 2vrms from V1, so the DC voltages at
all electodes won't change much.

The use of the R19 bypassed with C5 and R24 is a step in the right direction
because it makes the set up conditions of
V1 and and V2 easier to arrange so that the anode voltage at V1 is best for
linearity,
and the grid voltage at V2 is best so Ek isn't too high above the heater supply.

Also at very LF, where the williamson margin of stability becomes
much less than at 1 kHz, the R19 and R24 become a voltage divider reducing open
loop gain
by 6 dB below 20 Hz, because C5 graduallly becomes a large and open impedance at
LF.

You could further stabilise the amp by using more LF attenuation in the open
loop response than you have.

Go to

http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm

There you will see a schematic for 50 watt class AB1 UL channel using KT88/6550
and including an integrated input preamp stage.

It isn't a williamson, but is a derived topology from what Mullard used
back in the 50's, and which has been addopted by 1,001 makers since then.

However, I have a few secret things in there that would NEVER have been done in
1955, such as the gain&phase shift network between the first tube in the power
amp, and the
second, which is 1/2 of the longtail pair.

This network should be of great interest to anyone building their
PP amp especially if the OPT isn't up to the **best** of the Partridge models.
About 99.99% of OPTs sold on this planet are a lot worse than
Partridge OPTs.

There is a network containing 0.47 uF, 0.047 uF, 1M, 220k
which acts to reduce LF gain a maximum of 15 dB, starting at 15 Hz.
By 1.5 Hz, at which many amps would oscillate badly without this network,
the LF gain has become about 12 dB less, and as stability is affected by the
amount
of open loop gain and the amount of FB applied, the amp will have far less NFB
applied
at 1.5 Hz than at say 100Hz, so the amp will be rock stable at *all* LF.
The amount of FB that *can* be applied safely without oscillations
occuring is also dependant on the phase shift of an amp, and stablity
is threatened as phase shift approaches 180 degrees betaween input and output.


LF Phase/Gain networks such as you refer too were not very secret at all in the
50's. For example, they were well understood & used in both the McIntosh MC40 &
MC75.

Cheers, John Stewart


BTW, I have on occasion used LF Phase Correction networks too. You can see an example
of that as it was published in the July 1998 Electronics World over at ABSE under the
heading 6AS7/6080 Amp. A more complete article on the same was published in Glass
Audio Number 2, 1999. The same article also covers a better way to drive low mu tubes.

Cheers, John Stewart

  #27   Report Post  
Phil Allison
 
Posts: n/a
Default


"John Stewart"

LF Phase/Gain networks such as you refer too were not very secret at all
in the
50's. For example, they were well understood & used in both the McIntosh
MC40 &
MC75.



** Learn to trim !!!!!!!!!


You ****ING pig ignorant, Canadian **** !!!!





.......... Phil


  #28   Report Post  
Patrick Turner
 
Posts: n/a
Default


There is a network containing 0.47 uF, 0.047 uF, 1M, 220k
which acts to reduce LF gain a maximum of 15 dB, starting at 15 Hz.
By 1.5 Hz, at which many amps would oscillate badly without this network,
the LF gain has become about 12 dB less, and as stability is affected by the
amount
of open loop gain and the amount of FB applied, the amp will have far less NFB
applied
at 1.5 Hz than at say 100Hz, so the amp will be rock stable at *all* LF.
The amount of FB that *can* be applied safely without oscillations
occuring is also dependant on the phase shift of an amp, and stablity
is threatened as phase shift approaches 180 degrees betaween input and output.


LF Phase/Gain networks such as you refer too were not very secret at all in the
50's. For example, they were well understood & used in both the McIntosh MC40 &
MC75.


I was being slightly facetious about secrets, but a large number of home diyers
have a rather limited knowledge of the finer points involved about amps they try to
make.

Patrick Turner.





Cheers, John Stewart


  #29   Report Post  
Patrick Turner
 
Posts: n/a
Default



Phil Allison wrote:

"John Stewart"

LF Phase/Gain networks such as you refer too were not very secret at all
in the
50's. For example, they were well understood & used in both the McIntosh
MC40 &
MC75.


** Learn to trim !!!!!!!!!

You ****ING pig ignorant, Canadian **** !!!!

......... Phil


There is no need to swear or get upset if someone hasn't
trimmed a long post to reply to just one point raised.
I just scroll down to where they have made their reply, and its no trouble.
And when so many ppl send me emails in html, I just
convert to plain text, no dramas.

Patrick Turner.


  #30   Report Post  
Ian Iveson
 
Posts: n/a
Default

"Patrick Turner" wrote

Its Ian's little way of giving you an indirect message;
he sent you on a wild goose chase


What would you know.

I would be perfectly happy to be the only one with access to new
production Partridge transformers.

I can give mohammed a gift horse, but I'm not going to climb the
mountain for him. He can get his own water.

cheers, Ian




  #31   Report Post  
John Stewart
 
Posts: n/a
Default

Phil Allison wrote:

"John Stewart"

LF Phase/Gain networks such as you refer too were not very secret at all
in the
50's. For example, they were well understood & used in both the McIntosh
MC40 &
MC75.


** Learn to trim !!!!!!!!!

You ****ING pig ignorant, Canadian **** !!!!

......... Phil


I must apologize Phil. For a few weeks you seemed OK. Are you off your meds
again? JLS

  #32   Report Post  
Sander deWaal
 
Posts: n/a
Default

Patrick Turner said:



Phil Allison wrote:

"John Stewart"

LF Phase/Gain networks such as you refer too were not very secret at all
in the
50's. For example, they were well understood & used in both the McIntosh
MC40 &
MC75.


** Learn to trim !!!!!!!!!

You ****ING pig ignorant, Canadian **** !!!!

......... Phil


There is no need to swear or get upset if someone hasn't
trimmed a long post to reply to just one point raised.
I just scroll down to where they have made their reply, and its no trouble.
And when so many ppl send me emails in html, I just
convert to plain text, no dramas.



Phil's therapists advised him to vent his anger out on the newsgroups
instead of in real life.

Whether this approach works for him, I don't know, it surely doesn't
work for us.

--

"Audio as a serious hobby is going down the tubes."
- Howard Ferstler, 25/4/2005
  #33   Report Post  
Ian Iveson
 
Posts: n/a
Default

"Sander deWaal" wrote

Phil's therapists advised him to vent his anger out on the
newsgroups
instead of in real life.

Whether this approach works for him, I don't know, it surely
doesn't
work for us.


Works for me, or at least it does when he's on form. I've even tried
emulating his style myself. Feels alright :-) , but I need more
practice.

He's been sickly-sweet with Patrick Turner of late...drivel in
stereo.

Now I've got him back on track, I must admit I'm disappointed. I
guess his imagination kicks in when he's thoroughly warmed up.

If he wasn't so daft he could be dangerous. Better the devil you
know. As long as he's in pompous mode he'll never learn anything.

If you get him really in a tizz, he will look things up for you, and
he is really good at that. Perhaps he missed a vocation in directory
enquiries.

cheers, Ian


  #35   Report Post  
 
Posts: n/a
Default



John Stewart wrote:
wrote:


Not sure where this article came from but the style is reminiscent of Stan
White & his POWRTRON Amp.

See
http://www.cosmos2000.org/audio/powrtron1.htm




It was- I accidentally clipped off the header with the title and name.
It was my intent to post it and up until now didn't notice it had been
omitted!



  #36   Report Post  
 
Posts: n/a
Default

Many/most DIYers are abysmally ignorant today...

  #37   Report Post  
 
Posts: n/a
Default

Partridge transformers have been apparently available on an
intermittent basis in recent years but like yourself a subset of Brit
vendors are "right ****s" so us arrogant colonials just don't want to
**** with them anymore. Partridge can't be too anxious to sell their
wares as they don't advertise, not here and not there either-we get UK
magazines easily at news vendors even in backward markets in North
America.

Japanese, German, and even Italian publications would be more
valuable, but tech pubs in foreign languages are scarce.

  #38   Report Post  
Ian Iveson
 
Posts: n/a
Default

wrote

Partridge transformers have been apparently available on an
intermittent basis in recent years but like yourself a subset of
Brit
vendors are "right ****s" so us arrogant colonials just don't want
to
**** with them anymore. Partridge can't be too anxious to sell
their
wares as they don't advertise, not here and not there either-we
get UK
magazines easily at news vendors even in backward markets in North
America.

Japanese, German, and even Italian publications would be more
valuable, but tech pubs in foreign languages are scarce.


I have several sheets of A4 with specs of current Partridge
transformers from perhaps 2 yrs ago. There was no reason to believe
from their internet presence that they existed, and neither is there
now, so I have no reason to assume they are not still extant.

They seemed surprised to have been discovered. I rumbled them via a
possibly over-zealous marketing executive. They e-mailed me the
specs in a strange format that took some time to decode. I sent them
back my Word version thinking it might be useful. Not heard since.
Either there are a few old guys on permanent secret holiday within a
combine that's forgot they are there, or they supply to OEMs only
and have no ambitions to expand...a small cash cow. Perhaps
somewhere in between.

The only Partridge trannies I have are 60s Carlsbro, same design as
Marshall of the period, so it's not inconceivable (to me who knows
nothing of current Marshall stuff) that they are still connected,
but the list has a variety of usual domestic valve audio specs
AFAIR.

Perhaps I can find it. I guess I posted it here at the time.

yawn,

Ian









in message
ups.com...


  #39   Report Post  
Mark Harriss
 
Posts: n/a
Default

John Stewart wrote:


I must apologize Phil. For a few weeks you seemed OK. Are you off your meds
again? JLS



In Aus.hi-fi we found a correlation between the phases
of the moon and Phil's behaviour: it was a full Moon on Thursday,
so that's all it takes to trigger off a dummy spit.

Mark Harriss

  #40   Report Post  
 
Posts: n/a
Default

I assume that the windery which makes Partridge is either too small to
be able to bother with hobbyists or so large that they know it isn't
profitable. They do however make high-fidelity transformers and supply
them to small manufactories, or at least they did within recent memory.
For instance, a copy of the catalogue of La Maison de L'Audiophile in
Paris, no 24, undated but about 10-12 years old, list Partridge TK4519
output transformers suitable for 300B at 1900 francs and TK6241 power
trx for 2200 francs, also as the standard transformers in their
well-regarded Hiraga-inspired Legend 300B amp. The prices quoted are
similar to what La Maison charged for mid to high-level Tango iron.

HTH.

Andre Jute

Ian Iveson wrote:
wrote

Partridge transformers have been apparently available on an
intermittent basis in recent years but like yourself a subset of
Brit
vendors are "right ****s" so us arrogant colonials just don't want
to
**** with them anymore. Partridge can't be too anxious to sell
their
wares as they don't advertise, not here and not there either-we
get UK
magazines easily at news vendors even in backward markets in North
America.

Japanese, German, and even Italian publications would be more
valuable, but tech pubs in foreign languages are scarce.


I have several sheets of A4 with specs of current Partridge
transformers from perhaps 2 yrs ago. There was no reason to believe
from their internet presence that they existed, and neither is there
now, so I have no reason to assume they are not still extant.

They seemed surprised to have been discovered. I rumbled them via a
possibly over-zealous marketing executive. They e-mailed me the
specs in a strange format that took some time to decode. I sent them
back my Word version thinking it might be useful. Not heard since.
Either there are a few old guys on permanent secret holiday within a
combine that's forgot they are there, or they supply to OEMs only
and have no ambitions to expand...a small cash cow. Perhaps
somewhere in between.

The only Partridge trannies I have are 60s Carlsbro, same design as
Marshall of the period, so it's not inconceivable (to me who knows
nothing of current Marshall stuff) that they are still connected,
but the list has a variety of usual domestic valve audio specs
AFAIR.

Perhaps I can find it. I guess I posted it here at the time.

yawn,

Ian









in message
ups.com...


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