Argiriadis Analogue Electronics DU User manual

DU USER GUIDE
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Table of Contents
DESCRIPTION & FEATURES ......................................................................................................................... 5
Input and output stages .......................................................................................................................... 5
Distortion 'tone’ in relation to frequency ............................................................................................... 6
Differences between triode and pentode sound. .................................................................................. 6
Off Load' mode operation ....................................................................................................................... 7
Positive feedback (PFB) ........................................................................................................................... 7
FRONT PANEL CONTROLS AND SWITCHES ................................................................................................ 9
OPERATING CONTROLS FROM LEFT TO RIGHT AND THEIR FUNCTION .................................................. 9
PREPARATION BEFORE SWITCHING ON ................................................................................................... 13
(1) PLACEMENT ................................................................................................................................... 13
(2) GROUND LIFT SWITCH .................................................................................................................. 13
(3) STAND-BY SWITCH ....................................................................................................................... 14
FIRST TIME SWITCHING ON ...................................................................................................................... 14
(4) The output attenuation switch ........................................................................................................ 15
(5) Inputs, outputs & BY-PASS rotary switch (6) .............................................................................. 15
PRECAUTIONS ............................................................................................................................................ 16
BIAS CONTROL AND HOW IT AFFECTS DISTORTION AND CLIPPING IN THE OUTPUT STAGE ....... 17
HOW TO USE THE BIAS CONTROL (6) IN TRIODE ULTRA-LINEAR AND PENTODE MODES ......... 18
SWITCHING ON ...................................................................................................................................... 20
TESTING THE UNIT AND OPERATING GUIDELINES ................................................................................... 21
CASCODE versus TRIODE ................................................................................................................... 22
TECHNICAL NOTES ..................................................................................................................................... 30
SAFETY .................................................................................................................................................... 30
GROUND LOOPS ................................................................................................................................ 30
GROUND LIFTING ............................................................................................................................... 30
BALANCED AND UNBALANCED LINES............................................................................................... 31
TUBE ISSUES ...................................................................................................................................... 32
NOISE, MICROPHONY & AGE ............................................................................................................ 32
MICROPHONY TEST ........................................................................................................................... 32
EMISSION LOSS ................................................................................................................................... 33
IONIZATION ........................................................................................................................................ 34

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HOW OFTEN AND WHEN IS IT NECESSARY TO REPLACE TUBES .................................................... 34
FUSES ................................................................................................................................................... 35

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DESCRIPTION & FEATURES
The signal that enters this Distortion Unit (DU) can be adjusted to produce a wide variety of clean,
mildly or excessively distorted sounds. This is an analogue sound processor and familiarity of the
unit and experimentation by the user are essential in creating a particular 'tone signature'.
Input and output stages
The unit has balanced and unbalanced inputs and outputs but only one input or output can be
used at a time.
The DU consists of an input stage and an output stage. The signal level that enters the input stage is
set by the input gain control, and the output signal level is set by an output control plus an
attenuator. Vacuum tubes are used all throughout the signal chain, except the input balanced to
unbalanced converter and the positive feedback stage. The balanced (XLR) input can accept a signal
of an amplitude of 6.4VRMS (18dBu) maximum, and this signal level must not be exceeded.
The DU has been designed to accept standard studio input signal levels of between 0.775VRMS
(OdBu) to 1.228VRMS (+4dBu), and its input impedance is 50k. It can also accept lower level signals
but if high distortion levels are desired the INPUT control and other level controls must be set higher
than usual. The input impedance of the equipment that the DU is connected too must not be less
than 2K.
The input stage can operate in two modes triode or cascode. Cascode is a low noise pentode made
out of two triodes (=CASCaded triODE).
The output of the input stage can be attenuated if necessary before it is fed to the drive
control. This enables a finer adjustment of the signal that enters the output stage and makes it easier
to blend the tone created by the two stages for mild coloration purposes instead of hard distortion.
The signal from the drive control is fed to the output stage which is a Single Ended (SE) mini power
amp using the power section of the PCL86/14GW8 triode/pentode tube and can be set to work
in triode, ultralinear and pentode low or high distortion modes.
One of the factors that affect the output distortion is how high the drive control is set, and the output
of the unit must always be reduced each time the drive control is set high for more distortion. The
output stage is centred around a high quality 1492s SOWTER output transformer.
The output stage also has a bias control that sets the amount of DC current that flows through the
PCL86/14GW8 power tube and output transformer under idle (no signal) conditions. This gives great
flexibility in altering the shape of the output waveform, its symmetry and the way that it clips, see
relevant section. Clipping occurs when the signal's maximum output is constricted by an amplifier's
circuitry.

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Distortion 'tone’ in relation to frequency
* The distortion of a signal composed out of two or more different frequencies sounds harsher than
the harmonic distortion of single notes (frequencies) and it is called Inter-Modulation (IM) distortion.
*Any type of distortion in the low single frequencies sounds smoother and "warmer" than distortion
in the mid frequencies.
* Mild Intermodulation (IM) distortion of low frequency notes can also sometimes generate
‘warmth’.
* Harmonic distortion in high frequencies (above a few kHz) in single frequency signals is difficult to
detect due to the range of the human hearing. However, IM distortion caused by high frequency
signals is detectable.
* The output transformer and the way it interacts with the power tube also generates distortion
especially in the low frequencies.
Differences between triode and pentode sound.
At low input signals triode mode produces predominantly second harmonic distortion which is more
musical. Drive and gain controls must be set low to drive the DU softly to avoid clipping if this kind
of sound coloration is desired. As the drive level is turned up, more distortion will be generated, first
low even, then low odd (like third) and a smaller amount of higher orders both even and odd,
until eventually clipping occurs.
In pentode mode distortion starts earlier, as far as the drive level is concerned. Pentode output will
produce more third and higher order odd harmonic distortion products, in addition to second and
higher even products. By overdriving the power stage in higher drive settings and with gain switched
high, harsh edgy sound and a boost in the high mids and treble occurs.
In ultra-linear mode the tube operates somewhere in the middle between triode and pentode. This mode
was widely used in 50's and 60's hifi’s, the sound is a bit more like in triode mode but with more volume
and a bit more distortion (odd harmonic too) especially in the low and high frequencies.

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Off Load' mode operation
In 'off load' mode the load that is connected to the output of the DU is removed (reduced to zero)
and this alters the characteristics of the amplified sound wave, introducing high distortion that
is extreme, harsh and shrill. This mode can be switched on or off.
Positive feedback (PFB)
This feature is also used for an extreme type of distortion where the DU output amplifier can
become unstable and in high PFB settings oscillate. This is because the output signal re-enters the
input of the output stage in such a manner (phase) so that it increases the amplification. This mode
can be controlled and switched on or off.

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FRONT PANEL CONTROLS AND SWITCHES
(1) In the middle of the front panel (from top to bottom) and about two thirds of the distance from
left to right there is a large toggle switch labeled as S-BY/ON. When this switch pressed down (ON
position) it connects the high (B+) voltage lines from the internal power supply to the audio circuits
of the unit.
(a) The S-BY switch must be pressed down (ON) approximately one minute after you have
turned on the mains. The mains on/ off switch is at the back, situated next to the IEC mains input
socket. This delay is necessary to ensure that no high voltage is applied to the circuits until the
tubes have reached their operating temperature.
(b) This switch must also be used to disconnect the high voltage from the tubes every time you use
the (12) TRI L/H, PE L/H and the (13) UL mode switches, as discussed later.
(2) The red LED on the far left of the logo (on the center- line that separates the top and bottom
channels) which will light up as soon as the mains switch at the back is pressed on, and it indicates that
the mains has entered the unit and the tube filaments are lighting up.
(3) The green B+ LED on the same center-line on the right of the S-BY/ON switch that lights up when
the (B+) high voltage is connected to the audio circuits and the unit is ready to operate.
OPERATING CONTROLS FROM LEFT TO RIGHT AND
THEIR FUNCTION
THE BOTTOM CHANNEL IS THE MIRROR IMAGE OF THE TOP CHANNEL
(1) INPUT: this control adjusts the amount of signal that enters the input stage of the unit and it is
directly wired across the unbalanced (jack) input which is also the output of the balanced to
unbalanced converter stage. The position of this control enables the unit to accept a high range of
input signal levels.
The more you turn up the INPUT control the more harmonic and intermodulation distortion is
generated by the input stage.
This control has no effect on the amount of signal that enters the (XLR) balanced input
converter stage, but it adjusts the level coming out of the XLR input converter, and thus the signal
that enters the DU input stage.
The balanced to unbalanced converter does not amplify the signal, it has a gain of 0dB
(2) TRI/CASC: this switch selects the input stage operating mode. In the up position is triode (TRI) in
the down position is cascode (CASC) mode.

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(3) ATT: an attenuation switch that when pressed down the signal level coming out of the input
stage is reduced by approximately 18 dBs.
(4) PFB: is the Positive Feedback control that adjusts the amount of in-phase signal that can re-enter
the input of the power stage and can be used to generate extreme distortion and/or oscillation.
(5) PFB ON: a switch that when pressed down the PFB effect comes on, when pressed up is off.
(6) BIAS: this control sets the amount of DC current that flows through the PCL86/14GW8 power tube
section. The higher this control is set the higher this DC current is.
(7) BY-PASS/IN: a two way rotary switch with a pointed knob; when this knob points to IN, and to do
that you must turn the switch clockwise the XLR output socket is connected to the output of
the unit so you get the DU processed sound. As far as the balanced XLR inputs and outputs are
concerned this is a true BY-PASS switch, but it only affects the balanced (XLR) input/outputs.
(8) DRIVE: it controls the signal level that enters the output power stage, and how hard the
output tube and transformer are being driven.
(9) OUTPUT: it adjusts the amount of signal that comes out of the unit and it works in conjunction
with the (15) OUTPUT ATT (output attenuator) rotary switch situated at the right end of the
front panel.
(10) LIM LED: it is situated right under the OUTPUT control on the top channel and right on top of the
OUTPUT control on the bottom channel.
This is the output limiter LED that lights up when the output signal coming out of the unit has reached
its maximum level of 6.7VRMS ,18.8dBu (approx.) and a hard clipping action takes place at the output
to stop any further signal increase in order to protect the equipment that the unit is connected to.
(11) LOAD ON/OFF: a large toggle switch; OFF LOAD (switch set in the UP position) is
an extreme harsh and shrill distortion effect when the output stage works in pentode (PE) mode,
especially in the high distortion setting.
In triode and ultra-linear modes the effect is more subtle and not as strong.
(12) TRI L/ H,PE L/H: a 4 step rotary switch that selects the mode that the output stage operates.
From left to right:
step 1: TRI L= TRIode Low distortion, the signal level in TRI/L is higher than in TRI/H mode.,
step 2: TRI H=TRIode High distortion, or to be more precise 'higher' distortion, because triode
mode offers mild coloration (more than serious distortion) and the difference in tone between
TRI L and TRI H is not huge anyway.
step 3: PE L= PEntode Low distortion
step 4: PE H= PEntode High distortion, and this mode has been especially designed for high
distortion, and the signal level in PE/H is higher than in PE/L