Ampeg HERITAGE B-15 - Manual (Page 9)

Heritage B-15 Bass Amplifier Heritage B-15 Bass Amplifier
9
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 (in fixed bias circuits
such as the 1966 bias setting) 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 Woodinville, WA, U.S.A. factory by the dedicated Ampeg team. 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 bottom line:
Only 100 2012 Ampeg Heritage B-15s were hand-wired and assembled in the USA, with the intention
of replicating the classic sounds of the 1964 and 1966 models. In order to achieve this, the combo
comes standard with 3 6SL7 preamp tubes, 1 5AR4 (GZ34S) rectifier tube and 2 6L6GC power
amp tubes. As mentioned earlier, replacing these tubes with different rated ones is possible, but
the consequence is a completely different sound. Proceed with caution!
The nature of tubes continued...