Octave V 70 SE [28/33] Troubleshooting

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 SE 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 SE 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 SE 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 SE'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 SE, 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 /

12AX7).