User Manual

V-4B Bass Guitar Amplifier
10
Power tubes are the largest tubes used in an amplifier, generate the most heat and wear out the fastest.
These tubes convert the low-level, conditioned signal from the preamplifier into a level that is sufficient
to drive the speakers. There are several types of power tubes available, each of which offers a different
performance/sound characteristic. For example, the EL34 power tube produces a great classic rock
sound. When an EL34 is driven into distortion, it produces a unique sound (“crunch”). When compared
to the 6L6, the EL34 distorts more quickly, exhibits a “looser” low-end response and produces more
harmonics at mid and high frequencies (“creamier” sound). These differences become more noticeable
at higher volumes.
The 6L6 tubes produce a big low-end thump and have very good dynamic range. They offer a more
traditional “American Rock” sound. The 6V6 tubes produce a creamy sound with nice distortion.
On the other hand, the KT88 produces a big low-end but sounds more like an EL34 in the mid and high
frequencies.
The 6550 power tubes are more rugged and stay cleaner sounding, even at full power. When they
do distort, the sound produced is more solid and has a tighter low end; more of a “heavy metal” type
distortion with lots of power.
Some power tubes are available in matched sets. These tubes have been extensively tested for optimum
performance and longevity.
When replacing power tubes, please replace them with a tested, matched pair and have the amp rebiased
by a qualified service center. It is possible to change the sound of the amplifier by changing the type of
power tubes. When making any modification to your equipment, it is highly recommended that you consult
with a qualified service center.
The nature of tubes – why (and when) to replace them:
Tubes are made up of a number of fragile mechanical components that are vacuum-sealed in a glass
envelope or bubble. The tube’s longevity is based on a number of factors which include how hard and
often the amplifier is played, vibration from the speakers, road travel, repeated setup and tear down, etc.
Any time you notice a change in the amplifier’s performance, check the tubes first.
If it’s been awhile since the tubes were replaced and the sound from the amplifier lacks punch, fades in
and out, loses highs or lows or produces unusual sounds, the power tubes probably need to be replaced.
If the amplifier squeals, makes noise, loses gain, starts to hum, lacks “sensitivity”, or feels as if it is
working against you, the preamplifier tubes may need to be replaced.
The power tubes are subjected to considerably more stress than the preamplifier tubes. Consequently,
they almost always fail/degrade first. If deteriorating power tubes aren’t replaced, they will ultimately fail.
Depending on the failure mode, they may even cause severe damage to the audio output transformer
and/or other components in the amplifier. Replacing the tubes before they fail completely has the potential
to save time, money and other unwanted troubles. Since power tubes work together in an amplifier, it is
crucial that they (if there is more than one) be replaced by a matched set. If you are on the road a lot,
we recommend that you carry a spare matched set of replacement power tubes and their associated
driver tubes.
After turning off the power and disconnecting the amplifier from the power source, carefully check the
tubes (in bright light) for cracks, white spots inside the glass or any apparent damage. Then, with the
power on, view the tubes in a darkened room. Look for the preamplifier tubes that do not glow at all
or power tubes that glow excessively red.
Tube types and usage continued...