Found this:

http://www.angelfire.com/electronic/...-DB_Heart.html

The Williamson Circuit.
The Williamson amplifier circuit was very popular through the 50s and
60s. It was developed in England and quickly adopted and adapted to
American valves, er, tubes. The British version of the 6SN7 valve had
slightly different characteristics than the American 6SN7 tube. A few
value changes made it work as well as it had in England and the circuit
was used in many commercially made amplifiers as well as home brewed
projects.

The output stage usually employed tubes that were somewhat higher power
than the operating point would require, such as 807s in a 25 watt
amplifier. It was felt that this gave lower distortion than for a pair
of tubes pushed to their limit. It certainly gave longer tube life.

The sections that made a Williamson amplifier, a Williamson amplifier,
were the split load phase inverter and driver. The circuit is given
below.


see web page
The two RC networks at the input are to keep infrasonic frequencies
from entering the amplifier. These inaudible signals, mostly from
turntable rumble, eat up amplifier headroom and can lead to audible
distortion.

The circuit will look familiar but different. The plate of the first
amplifier is directly coupled to the grid of the split load phase
inverter rather than through a capacitor. Although this saves one
capacitor and two resistors, reducing component count is not the
purpose of this modification. It is to eliminate one RC time constant
from the signal path. The fewer there are the better global feedback
will behave. In the circuits above the grid is already at a positive
potential and of course so is the plate. Why not make them the same and
eliminate the capacitor.

The phase splitter is followed by a push-pull triode amplifier. Those
familiar with the Williamson circuit will see that I have added a
modification to it. Actually I have modified someone else's
modification. They had placed the balance control in the plate load of
one of the triode amplifiers. A practical potentiometer has quite a lot
of parasitic capacitance. Adding this extra capacitance in only one
plate could cause imbalance at high frequencies even if balanced at low
and mid. Moving the balance control to the cathode circuit permits use
of a lower resistance pot which moves the roll off beyond the audible
range. The pot in the cathode circuit introduces a small amount of
degeneration and close to equal amounts of stray capacitance in both
sides of the driver.

The performance is as follows.

Driving 6L6s to 26 watts output requires 16 volts RMS per grid. A pair
of 6550s driven to 55 watts requires 17 volts RMS per grid. The
distortion produced by the driver at 17 volts is 1.3% at the upper
plate and 1.35% at the lower plate.

A pair of 6CA7/EL34s requires 25 volts RMS per grid for 54 watts
output. At 25 volts output the driver produces 1.85% from the upper
plate and 1.92 from the lower.

Most of this distortion is taking place in the driver. With the output
still set to 25 volts and measuring distortion at the output of the
phase splitter it was 0.16% at the plate and 0.14% at the cathode.

The voltage gain from input to one of the outputs is approximately 140.

I wanted to give the much touted Williamson circuit another chance. I
had originally constructed it on an old DeVry breadboard which can be
seen in the crystal set article. This breadboard provides a rather poor
ground plane and its high frequency performance leaves a lot to be
desired. I reconstructed it on what I call The Best Breadboard. It
didn't do that much better until I started swapping tubes. I finally
hit one that gave 0.7% at 17 volts, and 1% at 25 volts. It's high
frequency response is -3 dB at 300 kc and still giving a perfect
streight line on the lissajou display. You could get pretty good
distortion figures with a new tube but after it ages a little you may
not be in as good a position. You had better plan on the distortion
figures above. I recommend a circuit that is more forgiving of aged
tubes. I'll work on that circuit and let you know. Stay tuned.

Suppose you are going to build a Williamson amplifier using EL34s. The
tube manual data for a 6CA7/EL34 gives 1.6% distortion at 54 watts
output. Distortion does not add by simple addition but as the square
root of the sum of the squares. Thus the overall distortion would be
1.9 squared + 1.6 squared equals 6.17 and the square root of that is
2.48%. 20 dB of global feedback would reduce that to a respectable
0.248%.

Now here's a real kicker! If you leave out the second 6SN7 and drive
the output tubes right from the phase splitter the distortion figures
are as follows.

17 volts ...... 0.9% at both cathode and plate.

25 volts ...... 1.3%.

With 100 k ohm resistors on the other side of the coupling caps instead
of the 470 k the distortion is,

17 volts ...... 0.86%.

25 volts ...... 1.25%.

YOU CAN LEAVE OUT THE DRIVER AND GET LOWER DISTORTION. Of course there
is not as much overall gain for global feedback. It may be necessary to
add a stage before the 6SN7 to bring the gain up to a sufficient level.
This stage should not contribute much distortion as it will be
operating at a very low level.

The distortion for a pair of 6CA7/EL34s becomes 2.03%.




This seems a well done little page. I am not going to post any more of
it nor critique it, I just think it's more intelligent than a lot of
garbage out there. There is more information here which appears well
done and useful. This is so much more interesting than arguing with
Arny and Pinky or dealing with Margaret and "her" smelly female bits.

said:


This seems a well done little page. I am not going to post any more of
it nor critique it, I just think it's more intelligent than a lot of
garbage out there. There is more information here which appears well
done and useful. This is so much more interesting than arguing with
Arny and Pinky or dealing with Margaret and "her" smelly female bits.


Then stop cross-posting seriously intended audio posts to
rec.audio.opinion.

--

"Audio as a serious hobby is going down the tubes."
- Howard Ferstler, 25/4/2005

Sander deWaal wrote:

snip


Then stop cross-posting seriously intended audio posts to
rec.audio.opinion.


You will note, I didn't.

wrote in message
oups.com...
Found this:

http://www.angelfire.com/electronic/...-DB_Heart.html

(snip)

What a great website! I've bookmarked it for future study. Actually, it's
very timely for me as I've got a home brew tube amp on the go. The main
components and tube sockets are already mounted on the chassis but I'm still
desiging the circuit... (Don't ask!)
Cheers,
Roger