Heritage B-15 Bass Amplifier

Tube types and usage continued...

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. The Ampeg Heritage B-15 uses two 6L6GC. As with the preamp tube selection, the 6L6GC was selected for period authenticity. 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.

Rectifier tubes convert the high voltage AC from the power transformer to a high voltage DC supply that powers all the other tubes. Rectifier tubes give the amplifier much of the compression “feel” as maximum output power is approached. This, in turn, is one of the keys to an amplifier’s responsiveness to the musician. The Ampeg Heritage B-15 uses a single 5AR4 (GZ34S) for rectification to mimic the

1964 – 1966 era circuit. It is also possible to use 5U4G to get the vibe of a pre-1962 B-15N. Like all of the other tubes, these rectifier tubes were also chosen for period authenticity. 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.

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