VAC Vintage Mk II User Manual

VAC VINTAGE WILLIAMSON POWER AMPLIFIER

Operating Instructions

DO NOT OPEN THIS UNIT - THERE ARE NO USER SERVICEABLE
PARTS INSIDE. DO NOT REMOVE THE BOTTOM PLATES. LETHAL
VOLTAGES ARE PRESENT WITHIN THE CHASSIS. DO NOT EXPOSE
THIS UNIT TO MOISTURE OR OPERATE IT IF WET.

VACUUM TUBES BECOME HOT ENOUGH TO CAUSE SERIOUS
BURNS. NEVER TOUCH A TUBE WHEN THE UNIT IS ON. IT MAY
TAKE SEVERAL MINUTES FOR THE TUBES TO COOL DOWN AFTER
THE UNIT IS SWITCHED OFF.

DO NOT PLUG INTO AN AC SOURCE UNTIL ALL CONNECTIONS
ARE COMPLETED.

Introduction

The Vintage Williamson is a single chassis wide-band low-feedback stereo amplifier
conservatively rated at 35 watts per channel. It is to be operated only from the voltage
labeled on the back of the chassis (100, 120, 220, or 240), 50-60 Hz. Please contact VAC
or your dealer if the voltage in your area does not agree with the label. The fuse is a slow
blow type of the size labeled on the back of the chassis. A tube cage is available as an
option.

The basic Williamson design was first published in England in 1947, and was the mainstay
of the original high fidelity movement. Notable features of this circuit include:

-direct-coupled input amplifier and phase splitter, resulting in low phase shift

-inherent precise balance of the phase splitter (if properly executed) at all frequencies
and level, resulting in excellent push-pull balance and low distortion

-a unique bias/balance circuit that allows monitoring and precise adjustment of the
output tubes for maximum power, minimum distortion, and natural sound.

In the refined form presented in the VAC, this circuit delivers true state-of-the-art
performance, and repudiates the more complex designs seen even to this day. The output
stage features independent cathode bias for each tube, which results in excellent sound and
the most stable and maintenance free operation available anywhere.

Premium parts are employed throughout, including 14 pound 22 section bifilar-wound ultra­wideband output transformers, polypropylene and polystyrene signal capacitors, high current
rectifiers, low ESR power supply, and proprietary wiring. The quality of parts and careful
layout allows the use of very little feedback for high sonic purity and low distortion.

Unpacking

Each tube socket on the amplifier is covered by a small round sticker. The color of this
sticker corresponds to the color of the sticker on a tube. Fit each tube into the matching
socket, first removing the sticker from the tube and socket. The bias levels for the output
tubes have been factory set, but should be checked during the installation procedure.

Installation

Physical requirements:

1)Provide adequate ventilation - allow at least 3 inches above and 1 inch to each side.

2)Do not place in a completely enclosed cabinet.

3)Do not stack other equipment on top of the VAC units.

4)Do not operate on carpet or any other surface that might block air flow.

5)The chassis and power transformers will become hot in normal use.

6)Do not allow the chassis of the VAC components to touch any metal parts, such as
the frame of an equipment rack. This might create a parallel ground path that will
degrade the sound of your system.

Electrical connections:

1)Complete all installation steps before plugging the amplifier into the AC mains
supply.

2)Connect signal inputs to the power amplifiers. Single ended interconnect cables
equipped with RCA phono plugs provide the input to each channel.

3)Connect loudspeakers between the appropriate pair of binding posts on the back of
the power amplifiers. Most popular speaker cables can be accommodated by the
output terminal strips. Output impedance matching is available for loads of 2, 4, and
8 ohms. Connect one lead of the speaker cable to the "G" position, and the other to

either 2, 4, or 8. The "G" terminal is at ground potential, and the amplifier does not
invert phase.

4)Do not connect two amplifier outputs to each other.

5)Connect the AC cord to the power source indicated on the rear panel (100 volts AC,
120 volts AC, 220 volts AC, or 240 volts AC at 50 or 60 Hertz).

6)Follow the BIAS/BALANCE procedure described in this manual to set the idle
current. Check the idle current at 1 minute, 30 minutes, 1 hour, and 2 hours. Also,
read the section entitled INSTALLING NEW OUTPUT TUBES.

Operation

Continuous operation is not recommended. If the amplifier will not be auditioned for a few
hours it is best to turn them off. This avoids undue stress to the EL34 output tubes.

As with all high fidelity products, the sound characteristics of the VAC amplifiers change
somewhat as they warm up. Best sound will be achieved after 15 minutes of operation, with
subtle changes occurring for up to one hour. However, we advise against leaving the
equipment on at all times because of the attendant acceleration of output tube wear. Life of
the output tubes averages between 2,000 and 5,000 hours depending upon brand fitted and
random variations within the tubes themselves.

Any time that the VAC Power Amplifier has not been used for a few weeks the sound may
be different. This is also normal for high resolution audio equipment. Optimum sound
should return after a few hours of operation, preferably with an audio signal.

Please note that although your VAC amplifiers have been run for 48 hours at the factory,
they will continue to "break in" for approximately 150 hours. Also be aware that many
components display the need for a new break in period after being transported in unheated
cargo aircraft.

Bias and Balance Adjustment

Your VAC Power Amplifier has been shipped with output tube bias preset. This should be
checked when you install your amplifier, and approximately every month thereafter. It must
also be set whenever an output tube is changed.

Monitoring and adjustment of the output tubes is quite easy, and requires a voltmeter
capable of reading in the millivolt range. To start, place the black test lead in the center test
point (TP2) and the red lead in TP1. Adjust the bias control to obtain a reading of 325
millivolts DC (.325 volts).

Next, place the black test lead in TP3. The meter may indicate a positive or negative reading.
Adjust the balance control to read a near to 0 millivolts as possible.

Since the bias and balance controls interact slightly, repeat this procedure two or three times
until the readings are unchanged.

When installing new tubes, adjust the bias and balance controls to the center of their
rotations. Check the bias of both channels as the amplifier warms up - don't wait ten
minutes.

Bias levels should be checked monthly to ensure optimum sound quality. Allow the
amplifier to warm up for an hour before checking. It is not unusual for bias current to change
with time, particularly when tubes are new. In fact, the greatest amount of drift occurs during
the first 200 hours of a tube's life. The drift may change direction periodically, such that the
bias control must be increased and then later decreased, or vice versa. Also check bias if the
sound seems to be lacking in detail or dynamics.

Tube Types

The input/splitter tube V1 and the driver tube V2 may be type 12AU7A/ECC82/E82CC or
type 12BH7 (typically richer sound). The output tubes V3 and V4 may be type EL34/6CA7
or type KT77. Golden Dragon tubes are generally recommended and have been used
extensively during the development of this amplifier. Please feel free to contact VAC if you
require additional information or advice.

Installing New Output Tubes

Replacement tubes are available from VAC and other sources. Output tubes may be EL34
pentodes, 6CA7 beam power tetrodes, or KT77 beam power Kinkless Tetrodes. It is not
necessary that they be matched pairs, although an improvement in measured performance
may be achieved in this way. Make certain that each tube fits firmly in its socket. A tube that
fits loosely may not make correct contact on all pins and might "run away" (read on). DO
NOT MIX TUBE TYPES. DO NOT MIX BRANDS OF TUBES.

ALL POWER MUST BE OFF. Remove the old tubes after they have cooled down (TUBES
BECOME HOT ENOUGH TO CAUSE SERIOUS BURNS WHEN IN OPERATION AND
MAY TAKE SEVERAL MINUTES TO COOL DOWN). Install the new tubes firmly and
fully in the sockets, taking care to observe the direction of the locating ridge on the plastic
center pin of each tube.

Follow the normal turn on procedure and begin the BIAS procedure. While doing this, keep
an eye on the plate (the outermost metal structure) of the output tubes. SWITCH OFF
IMMEDIATELY IF THEY BEGIN TO GLOW RED. This indicates that the tube is
"running away", being destroyed rapidly by conducting excessive current. (Note: with some
KT77s a slight dull orange glow may occur over a very small section of the plate, usually
at an edge. This is acceptable and not the same as running away, in which most of the plate
will become bright orange or red.)

Tubes may run away for several reasons:

1)The tube is not fully inserted in the socket.

2)The tube fits loosely in the socket and thus can not make correct contact. Such a tube
is unusable and should be returned to its seller.

3)The tube is defective.

4)The bias is misadjusted.

5)There is a problem with the amplifier. Contact VAC or your dealer to arrange service.

In the event that trouble is encountered, try another tube. Stop if the problem persists and
consult with your dealer or VAC.

Follow the BIAS procedure described previously in this manual to set the idle current at 70
milliamperes per tube, checking the idle current at 1 minute, 30 minutes, 1 hour, and 2
hours.

Replacement of Low Level Tubes

All power must be switched off. Allow tubes to cool down. Remove and replace with new
tubes of the appropriate types, noting the location of holes in the socket and pins of the
tubes.

Replacement tubes are available from VAC and other sources.

Ultra-linear or Triode?

The input amplifier, phase splitter, and driver stages are triode tubes operated in Class A
with no cut-off or grid current. The output stage is connected for partial-triode, well known
for low distortion, good damping factor, and high efficiency. This technique was invented
by Alan Blumlein of EMI in England, and was popularized in the US as "ultra-linear." It is
simple for a qualified service technician to connect the output stage for triode operation.
Directions will be provided upon request.