Heritage B-15 Bass Amplifier

The nature of tubes continued...

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 factory bias is adjusted by George Metropoulos himself. 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 50 Ampeg Heritage B-15s were hand-built, 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 (N.O.S. JAN Philips ECG 6188) 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!

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