Power Tubes

Power Tubes are the type of vacuum tube that takes a preamplified (line-level) signal and amplifies that signal so as to push the speaker magnets of the speakers. The traditional doom rock setup would be one or several double-stacked 4x12 speaker cabinets (described elsewhere) but can be as few as a single speaker.

In extreme cases like Eddie Van Halen, especially for his early albums, the output of his 100-watt Marshall SuperLead amplifier was dimed (all controls turned to 10) and then that output was attenuated back down to line-level and then power-amplified again. By doing this, Eddie was exploiting something that is usually (and more safely) confined to preamp tubes: Functional failure.

Power Tube Failure Types

There are a few different ways in which tubes fail. Of these failure types, it is functional failure, that is typically desired by musicians. There are other failure types, and they can be very dangerous. But these failure types don't refer to the signal going through the amplifier. Instead, they refer to an actual physical breakdown in or an overworking of the componentry in the vacuum tube itself.

Functional Failure

Distortion!

Functional failure occurs when the level of the signal (not power!) going through the tube exceeds the operating parameters of the tube. When the operating parameters of the tube are overcome, the result is that the peaks of the sine wave of the signal are clipped, resulting in overdrive, or when more severe, distortion. Ironically, the mighty sound of heavily distorted guitars is actually based on a functional failure of an amplifier!

In cases like Eddie Van Halen, the act of cranking the amplifier (along with other tweaks of his) are done purposefully to saturate, to the point of functional failure, the power tubes. As previously described, this causes them to distort. This is one of the main aural differences of Eddie's "Brown Sound" as compared with purely pre-amp tube based distortion.

Physical Failure

There are two kinds of Physical Failure: Wear Failure, and Redplating.

Wear Failure 

Wear failure is just like it sounds. Imagine a tire on your car or bike wearing out. There is no single "event" in this kind of tube failure. Use of any tube over a long period of time results in a diminishing ability of the tube to amplify sound with fidelity. Over time, the highs in the tube go away. Bass notes get flubby, sometimes weak. Generally the "life", or the "sparkle", or the present-ness of the sound being created has lost or is continuing to lose its vibrance. This is how most tubes fail. When the sound characteristics coming out of the amplifier become unacceptable, the remedy is to throw away the worn tubes and replace them with new ones.

Redplating 

The first indications of approaching catastrophic tube failure typically start with red-plating. Redplating manifests as a red glow on the metal of the "plate" of the tube. This happens when the current moving through the plate exceeds the ability of the metal in the plate to handle that current. This phenomenon is known as "resistance" and isn't any different than a glowing burner on an electric range that you might cook food on. Actually, the difference is that an electric burner is actually designed to repeatedly function that way. A typical tube can typically handle very short periods of redplating. Redplating can usually be fixed by a technician adjusting the bias in the tube, or diagnosing some other failing piece of equipment in the amplifier (for example, a plate resistor that has gone out of spec).

Catastrophic Failure

Catastrophic failure occurs when the flow of electricity through the tube exceeds the ability of the tube to handle that flow. Think of a dam breaking, or a bridge collapsing. Catastrophic tube failure isn't that much different. Something is blowing up. With tubes, this is usually manifested by the tube exploding or arcing.

Catastrophic or "meltdown" failures of power tubes, more often than not, result in burned electronics and wrecked (melted or shorted) transformers. Especially with vintage gear, these failures can be very, very pricey propositions. Musicians and amp owners are advised to be meticulously careful of the ongoing operation of their vacuum tube amps!

Tube types

• 5881
• 6550
• 6L6
• 6CA7
• 7027
• EL34
• E34L
• KT66
• KT77
• KT88
• KT90
• KT120

Plate Impedance Characteristics

The following groups of tubes share the same plate impedance characteristics and can more easily be swapped out for each other. Swapping out for other types of tubes will not only require rebiasing, but will also require changes to speaker impedance and may harm your electronics. Do not swap out tube types casually. You need to know what you are doing. If you don't, go to a qualified technician.

Per Michael Soldano (as declared by him in a JetCity Amps Youtube video):

Tubes that share similar plate impedance characteristics:

• EL34
• 6CA7
• 6550
• KT88

Tubes that share similar plate impedance characteristics:

• 6L6
• 5881
• 7027
• KT66