Crate VFX5112,VFX5212 User Manual

Important Information About Tubes and Tube Products (continued):
You can check your 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 chop stick (my favorite). You will be able to hear
the tapping through your speakers, which is normal. It is not normal for a tube to ring like a bell after it’s
tapped. If it does ring then it’s 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, you should check them anyway. The power tubes can be
checked for microphonics just like pre-amp 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, (under
biased) 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 your amp is like setting the idle
on your 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
(unless the amplifier is equipped with "self-biasing circuitry) and readjusted to accommodate the operating
parameters of the replacement tubes.
Depending on the model and amplifier type, there may be hum balance controls, trim pots, or bias adjust-
ment controls on its rear panel. However, the bias adjustment should be performed only by qualified service
personnel with the proper, calibrated test equipment.
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VFX5112/5212 Guitar Amplifier

Crate VFX5112,VFX5212 Specifications