Current Q&A Topic:  TUBES 

 

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Does this sound like something you've experienced before?
...Recently, my amp has higher noise levels and last night it lost most of the gain and volume. I checked all the cables, and tried different guitars which didn't fix the issue. Also, the pilot light and LED lights are all working. I felt the back of the amp and it didn't seem very hot. My guesss is that the tubes are going. Does this sound right?
Or perhaps-
I plugged my guitar into it, and I started hearing this strange clipping on harder plucked notes. And, as the amp warmed up, there was a sound like an intermittent fuzz or crackling after/during a sustained note, if that makes sense.
Or this-
...When I turn up my amplifier with the gain cranked, I sometimes get a squealing noise, even if my guitar volume is turned off. What could that be?

The active ingredient in all tube amplifiers is TUBES. Tubes are an old technology, but their implementation has yielded some great vintage and modern amplifiers.
However, tubes have a reputation for being fragile (Probably because they ARE made of glass), and all tubes used in guitar amplifiers WILL fail eventually...
The fact that tubes can, do and WILL fail should not be alarming. Tubes often mean a far simpler circuit than a comparable solid state amplifier, and tubes can be quickly replaced by the user. Tubes simply unplug, and in most cases, you just unplug the old tube and plug in the new one. A few seconds later, your amp is like new again.POWER TUBES- Bad power tubes need to be replaced right away, as power tube failure can cause resistors and other passive components inside the amp to fail when the tubes fail. Signs of bad power tubes- Low output, a tube (or tubes) with the metal plate structure glowing (Not the small heater filament, but the big, metal part visible through the glass) glowing orange), rattling or crackling noises when the tube is gently tapped and the amp is operating, especially when the amp is putting out 1/2 power or greater. Failing output tubes do not generally cause microphonic squealing noises.

PREAMP TUBES- Generally, failing preamp tubes can't cause any damage to the amplifier, but they can cause bad sound, or a loss of sound altogether. Signs of bad preamp tubes include: electronic rattling noises, muted high frequency response, low on gain, a hum that is more noticeable than usual, squealing noises, especially at high volume and or gain levels. The most common preamp tube today is the 12AX7, 9 pin dual triode type, followed by the 12AT7. While they are not interchangeable, they can be swapped in some circuits with varying degrees of sucsess.

PIN ALIGNMENT- Preamp tubes in most modern amplifiers made since the late 1950s have 9 pins. Current power tubes in most amplifiers either are a larger socket with 8 Pins (6L6/5881, EL-34, 6550/Kt-88, etc), or are of the EL-84/6BQ5 type and have bases identical to the common preamp tube types. In every case, the tubes are keyed such that there is only one way to plug them in, as each pin has a particular application in the circuit, and they are not interchangeable. With 8 Pin power tubes, it is necessary to look at the center "Key" to make sure it is intact, so you do not plug the tube in incorrectly and inadvertently cause damage. When changing tubes, notice which positions the tubes came out of, and more importantly, notice which way the pins are keyed, so that when you plug the replacement tube in, the pins mate with the proper holes. When plugging a tube in, it is important the the pins are CENTERED before you start pushing the tube in, and that you push straight and steady to make the tube seat into the socket. If the tube does not seem to be going in smoothly, pull it out, inspect the pins for straightness, inspect the tube socket for any obstructions and try again until you get it right.