Octave V 70 Class A, Owner'S Manual

Page: 30 / 35

31
11. TROUBLESHOOTING
Tube faults that trip the protection system
The protection system continuously measures the current flowing into the four power tubes.
Depending on the problem, this current may exceed a specified limit and cause the protection
system to switch the power stage off. The red protection LED will light up to show that this has
happened. Once the protection circuit has been tripped, you will no longer be able to measure the
BIAS of the power tubes and your amplifier will not produce any output. Exceeding the specified
current limit can have a variety of causes.

Faulty output tube
Aging and mechanical stresses from rough handling during transport can create an unwanted
connection inside the tube, which can lead to failure of the tube.
How can you recognize a faulty tube? If you do not know what fault caused your amplifier to cut
out, it is a good idea to disconnect the speakers and switch the V 70 Class A off and on again.
Before switching it back on, turn the mode selector to the BIAS position. Now check the correct
operation of the tubes via the BIAS display. If the tubes are good, the sequence of events will be as
follows:
-
Four LEDs illuminate until the soft-start phase is complete.
- After the soft-start phase, the power tubes will begin to heat up and will visibly glow as current
flows into the tubes. After approximately one minute, the BIAS display should read "green".
- A fault in one of the power tubes would result in an uncontrolled rise in current, which would
cause the top, red BIAS LED of the relevant tube to illuminate after a short time. A further rise
in current to the tube would then cause the protection system to trip. Should this happen,
switch the V 70 Class A off and replace the problem tube.

Faulty driver tube
In very rare cases, a problem with one of the driver tubes can cause the V 70 Class A to switch off.
You can use the BIAS display to troubleshoot problems in the driver stage. The procedure is the
same as the one described earlier, although the display will behave differently after the amplifier
has warmed up. If one of these tubes is indeed responsible for the problem, both output tubes in
the affected channel will behave erratically. This behavior may take the form of short, rapid
changes in LED color - from yellow to green and red and back again. If the BIAS of both output
tubes in one channel appears unstable, one of the driver tubes is generally responsible.
 Tube faults that degrade the sound
These faults are relatively uncommon and usually a result of a fault already described above.
Thanks to the controlled soft-start circuitry, the tubes will retain their tone throughout their working
life.
The V 70 Class A's sophisticated, low-noise power supply circuits push hum and noise down to
negligible levels. Tubes do not inherently produce hum or hiss: these problems are simply side
effects of older, classic tube technology.
Noise, crackling or similar undesirable noises can be caused by residual gas or other residues in
the tubes. This kind of noise does not often reach a level that could be considered annoying,
although the nearer to the "front" the particular tube is, the more it will tend to make itself felt. In the
V 70 Class A, this would be tube V 5. As it is rare for both systems in a double triode to be equally
affected, the noise levels in each stereo channel will be different. If you experience noise that is
louder on one channel than on the other, the first thing to do is to change the V 5 tube (ECC 83).