User Manual

V-4B Bass Guitar Amplifier
11
Whenever replacing power tube(s):
•Alwayshavetheamplifier’sbiasvoltagecheckedbyaqualifiedservicecenter.Improperbiasvoltage
will cause degradation in performance and possibly damage tubes and/or the amplifier. (See the section
below titled “The importance of proper biasing” for more information on this subject).
•Wehighlyrecommendreplacingthedrivertube(s),aswell.Thedrivertubedeterminestheshapeand
amplitude of the signal applied to the power tube(s) and has to work almost as hard as the power tube(s).
You may check the preamplifier tubes for microphonics by turning the amplifier on, turning up the gain
and tapping lightly on each tube with the end of a pencil or a chopstick. You will be able to hear the
tapping through the speakers, which is normal. It is not normal for a tube to ring like a bell after it is
tapped. If it does ring, then it is microphonic and should be replaced. Remember to use only high quality,
low microphonic tubes in the preamplifier section.
Even though power tubes are rarely microphonic, they should be checked, anyway. The power tubes may
be checked for microphonics just like the preamp tubes.
In the case of very high gain amps, you may be able to reduce the amount of noise generated by simply
swapping the preamp tubes around.
The importance of proper biasing:
For the best performance and longest tube life, proper biasing is imperative. Bias is the negative
voltage which is applied to the power tube’s control grid to set the level of idle current. We cannot over
emphasize the difference in warmth of tone and dynamic response that come with proper biasing. If
the bias is set too high (overbiased), the sound from the amp will be distorted at all levels. If the bias is
set too low (underbiased), the power tubes will run hot (the plates inside the tubes may glow red due to
excessive heat) and the sound from the amplifier will lack power and punch. The excessive heat greatly
reducestubelife–fromafewdaystoaslittleasafewhoursinextremecases.Settingthebiasonthe
amp is like setting the idle on a car. If it’s too high or hot, it’s running away with you and if it’s too low or
cold it will choke when you step on it.
The bias is adjusted at the factory in accordance with the type of power tube(s) installed in your
amplifier. It is important to point out that tubes of the same type and specification typically exhibit
different performance characteristics. Consequently, whenever power tubes are replaced, the bias
voltage must be checked and re-adjusted to accommodate the operating parameters of the
replacement tubes. The bias adjustment should be performed only by qualified service personnel
with the proper, calibrated test equipment.
The nature of tubes continued...