GENERAL INFORMATIONS
13
PRE-AMPLIFIER TUBES
The pre-amplifier tubes are not normally subject to wear and tear and may last more or less for "the entire lifespan of
the device".
Possible problems on this kind of tube are related to microphonic effect and noise.
Microphonic noise effect
Defined as a secondary effect of the tube of producing external vibrations as well as electrical signals, generating horrible
frequencies (whistles). This phenomenon is even more evident on high earning levels such as distorted channels.
With the amplifier on and the load correctly connected, if knocks are made to the glass of the pre-amplifier tubes, it is
normal to hear reproduction of this sound through the speaker.
Microphonic noises are clear and different because it will be possible to hear (whistles and interference) and in some
cases also distortion of sound even without hitting the tubes.
It is important to point out that the first tube of the chain is the one that more than any other will tend to reproduce
percussion generated by ourselves. Therefore it is the one that is subject the most to the microphonic part of the chain.
Noise
An intrinsic characteristic of the tubes is the background noise that borns from the natural operation.
During the lifespan of a tube, this noise may increase dramatically by creating problem of various kinds.
The pre-amplifier tubes do not necessarily need to be selected electrically (this is normally performed by the manufacturer),
but it is important to make sure that they are of excellent quality.
POWER TUBES
The power tubes are the last stage of amplification (before the output transformer and the speakers) and are subject to
wear and tear.
On average a final tube that works in class A/B may last between 800 and 1,000 working hours, considering that their level
of wear and tear is also connected with the volumes of sound.
On an amplifier of the same power working in class A, the same tube may last between 600 and 800 hours. In this working
class the tube is subject to greater stress.
When the final tubes should be replaced
It is possible to see when a tube is exhausted because it has a general loss in power and therefore in volume, sound has
less impact and low frequencies tend to lose power.
The main problems related to this kind of tube are:
-
a short circuit within the tube that normally blows the high tension fuse (HT)
- one of the tubes does not light, a fault that can be easily identified by a visual control on the internal filament of the
tube itself
- the tube becomes incandescent, in this case an increase in the background noise of the amplifier (hum) will be registered,
a visual control of the tubes will confirm this kind of defect.
In all of the fields mentioned above, it is important to replace the faulty tube with a new one.
If the faulty tube is not new, therefore if it has worked for approximately 1 year, we recommend that all of the tubes are
replaced.
This is done to optimise performance. If, however, the tube is relatively new, then simply replace it with an identical one.
Caution!
Make sure that only coupled tubes are assembled, with the same selection characteristics that can generally be
identified on the tube with the indications given by the manufacturer (in our case MATCHING REFERENCE).
All of the final tubes fitted onto an amplifier must have this common characteristic. If sets of tubes are used
with a different matching reference the BIAS must be realigned on the amplifier.