Amplifiers FAQ's

[OctaveAdmin]

Yes. The MRE 220, like all OCTAVE amplifiers, is fully protected against open circuit
operation, i.e. the amplifier will come to no harm if it is operated without loudspeakers
connected and the input level is in the magnitude of normal listening levels.

[OctaveAdmin]

No. Tubes normally sound the same throughout their service life. Our soft-start technology
contributes greatly to extending the service life of tubes. You can tell when an output tube has
reached the end of its useful life: it becomes impossible to adjust it correctly. Driver tubes
cannot be checked, but these will generally last for well over 10 years.

[OctaveAdmin]

Tube faults caused by output tubes

There are 3 different symptoms indicating a faulty output tube:
1. Broken heater filament: the tube stops glowing.
2. Defective cathode layer: the tube glows, but no current can flow. You can confirm this fault
using the bias display LEDs - no amount of adjustment will be able to extinguish the minus
LED.
3. Internal short circuit: Normally, the electronic protection cuts in and the red "off" LED
illuminates, or the tube will not respond to BIAS adjustment (the display keeps jumping
from plus to minus or vice versa).
With faults 1 and 2, the amplifier will still operate, although the channel containing the faulty
tube will be quieter than normal. At low listening levels, the fault may not be obvious, but
distortion will become evident at higher listening levels.
If fault 3 occurs, the protection circuits will normally switch off the amplifier. You may also hear
loud background noises just before it switches off, although these will not harm the amplifier.
You can find the faulty tube by removing one tube at a time. Operating the power amplifier with
just a single output tube is allowed for test purposes and will not damage the power amplifier.
Only a qualified technician must carry out this test.

Tube faults caused by driver tubes

Increased noise in one channel.
Normally the noise level of a driver tube is stable during its lifetime. Depending on the
production and type of tube, the noise can noticeably increase over time. The most sensitive
tube in the MRE 220 in this regard is the input tube V5 and in rare cases V6.

[OctaveAdmin]

In principle, Octave power amplifiers will also operate without output tubes. Of course, it is not possible to
play music under these circumstances.
For test purposes, or as a temporary measure, one channel may be fitted with just a single
power tube, although its power output will - of course - be reduced. The amplifier will come to
no harm if it is operated continuously like this.
Operation without driver tubes is also possible for test purposes, although, for obvious
reasons, music playback is not possible.

[OctaveAdmin]

The impedance and efficiency of modern loudspeakers is not an issue for OCTAVE amplifiers.
The often-quoted damping factor is not normally a guarantee that an amplifier will exert tight
control over the loudspeakers. In practice, speakers of 85 dB efficiency and above are suitable
for use with tube amplifiers. The high stability of the OCTAVE power amplifier technology even
allows the use of speakers whose impedance dips as low as 2 ohms (except for EL34 with 4
ohms)

[OctaveAdmin]

The cable manufacturers are now offering cables that have supposedly been designed
specifically for tube amplifiers. Although such cables may be of good quality, there is no need
to use special cables with tube amplifiers. Speaker cable can exhibit high values of
capacitance and inductance, and tube power amplifiers deal with such loads better than
transistor power amplifiers. The only exception would be if you needed to use a tube pre-topower
amp interconnect cable longer than 5 meters. In that case, a low capacitance cable
would be advisable.