ekco EV55SE, User Manual, Page: 10 / 12

Valve Types
A triode valve has three elements The Anode and Cathode are
connected between the Power supply and Ground. When an input
is applied to the
Control Grid , the amplified output appears at the
anode.
Triodes are linear and robust but their amplification factor is low
which makes them inefficient and limits their output power.
To overcome this drawback an extra grid is added. This greatly
increases the amplification factor and so the efficiency of the valve
and makes large
outputs a reality. The drawback is that
these valves are inherently non-linear and their distortion is high.
Triode and Beam Tetrode Output Stages
High power valve output stages are push pull . In this design the input signal is split into
two halves via a phase splitter . Each half of the signal goes to one output valve and the
signal is combined in the output transformer which is then coupled to the loudspeaker.
The triode push-pull output stage is the choice of
many audio purists. The drawback is that because
triodes are inefficient the output is limited and
insufficient to drive many modern low sensitivity
loudspeakers to high volumes.
A basic diagram of our triode output stage is given
here. By connecting the screen grid to the anode a
beam tetrode valve effectively becomes a triode with
all its benefits and drawbacks.
For many years the beam tetrode output stage was
relegated to public address amplifiers because of its
inherent non linearity and high distortion. Many
attempts were made to improve tetrode operation
(notably by Quad with their cathode-coupled design)
but their output transformers are complicated and
very expensive to manufacture.
Enter the Ultra Linear output stage. By connecting
the screen grid to a special tapping on the output
transformer a
local feedback loop is incorporated
around the output valves, lowering their output
impedance, linearising the output and massively
reducing distortion. The result is an amplifier with
most of the advantages of the triode combined with
the efficiency and power of a multi-grid valve.
Negative Feedback (NFB)
Overall Negative Feedback (NFB) is a process where a percentage of the output of an
amplifier is fed back in reverse (negative) phase. This lowers the output
impedance, reduces distortion and improves frequency response. The amplifier has better
control over the loudspeaker and a flatter frequency response and lower distortion bring
obvious benefits. However valve amplifiers have output transformers and valves are high
impedance devices High levels of NFB may at high frequencies in extreme cases (especially
with awkward loudspeakers) cause instability. Long before instability arises however the
listener can experience discomfort - a phenomenon known as
listener fatigue.
In our amplifier you may select low or high NF . Our amplifier is inherently stable so if you
have, for example, horn loudspeakers you may find that triode operation with low NFB is
the way to go. With conventional loudspeakers (especially low sensitivity designs) you may
find that Ultra linear operation with high NFB offers the best operating mode.
We feel it is preferable to have a relaxed generous sound stage which you can enjoy over
long periods than a ruler-flat response which may become tiring to listen to over time.
Experiment with different sources at different volume levels. Allow a generous time period
to elapse before changing modes and then use the one which offers you the best result.
power
to the input
ANODE
CATHODE
CONTROL GRID
ANODE
CATHODE
CONTROL GRID
SCREEN GRID
10
+DC
NFB
INPUT
KT88/6550
KT88/6550
+DC
NFB
INPUT
KT88/6550
KT88/6550
TRIODE
AMP
U/L
AMP