Octave JUBILEE Mono, Руководство пользователя

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7. TROUBLESHOOTING
7.2 Identifying defective tubes
There are especially four l reasons why tubes
can become defective.
1. Breakage of the heater filament and
internal contact problems
The tube's operating current normally
drops to 0 when this fault occurs. Because
of this, the electronic protection does not
cut in. You can confirm this type of fault
using the bias measurement facility while
the amplifier is on. The problem tube will
give a very low reading, or a reading of 0.
Use the rotary selector switch to determine
the location of the faulty tube. Then
replace it (see chap. 6.2 Replacing tubes).
2. Leaking glass envelope
To function properly, the inside of a tube
must be a vacuum. Tiny hair cracks or
defects in the glass envelope can allow
small amounts of air to enter the tube.
Initially, the effect is not noticeable.
However, the bias measurement system
allows you to trace such tubes because
they produce a strongly fluctuating reading.
Larger amounts of air penetrating the tube
cause the shiny silver surfaces in the glass
envelope to turn white. The readings for
these tubes will be around 0. Once air has
entered the tube, the heater filament will
burn out. This type of tube failure will not
result in damage to other parts of the
amplifier. You should replace any tubes
suffering from this problem (see chap. 6.2
Replacing tubes).
3. Signs of tube aging
It will normally be necessary to re-bias
output tubes at some time during their
service life to bring the idle current back to
the prescribed setting. Depending on the
age, quality, type of construction and the
materials used inside the tube, the bias
setting may be quite different after a
number of years from that of a brand new
tube.
To monitor this effect, we have included a
Control measurement setting in the bias
measurement system
The meter shows the setting required to
achieve the correct idle current. With 'bias
high' and new tubes, the value is around -
400 (for engineers: - Ug1 = - 40 V). The
amount of adjustment needed depends
greatly on the manufacturing techniques
used by the tube manufacturer. You can
safely use the tubes in your amplifier until
there is insufficient adjustment remaining to
set the bias correctly. The specified limits for
adjustment are between - 200 and - 800 on
the display.
4. Uncontrolled rise in tube idle current
Mechanical faults, particularly loose
connections, can lead to an increase in tube
idle current. This increase can be very rapid
and is sometimes accompanied by sparking
inside the tubes. When such a fault occurs,
the amplifier's electronic protection reacts
within < 0.2 seconds. The amplifier will
switch itself off, making it impossible for the
user to locate the tube that has caused the
problem.
For this reason, we have incorporated a fault
detecting circuit in the amplifier. This
subsystem monitors the output tubes
alongside the electronic protection system.
However, the fault detecting circuitry will
recognize which tube is responsible for
triggering the protection system and display
this information via the 8 Status LED’s F in
the display. Each LED is allocated to a
single output tube. The LED for the faulty
tube will remain lit even after the electronic
protection has switched the amplifier off.
You should replace this problem tube (see
chap. 6.2 Replacing tubes).