Review Notes Guitar Amp Tube Reviews written by John Templeton.
The tubes used in this review were selected at random from thetubestore.com's inventory of untested tubes. For each tube used in the test, two were taken since there was no pre-screening involved. The aim was to get a sample that would be practical to work with but allow for variations in the tubes or prevent picking the only dud in a lot.The test amplifiers used were very different. One was a 100-watt Trace Elliot Speed King with 4 x 12 cabinet, and the other was a Fender Blues Junior combo amp. This allows the high power, high gain crowd and the more conservative players to get the fairest evaluation possible. Some tubes were clearly better suited in one application or usable in both. All tubes were used at the input amplifier stage of the amp since this seems to be where most people develop their perceptions of how good a pre-amp tube is.
What Makes A Good Tube? The musical detail or ability to reproduce the sound of the instrument is a key factor in assessing a tube for guitar amplifiers. There is no perfect tube available. Each one has strengths, weaknesses and certain factors that contribute to its overall ratings. Usually a compromise is arrived at in the search for premium tone. All tubes will exhibit some degree of microphonics. Microphonics do not mean that a tube is unusable. You just have to screen them a little closer and determine where they are best suited for use. Input pre-amps are the most sensitive areas of the amplifier. When used in this application most tubes will generate some noise if you tap on them with a pencil during operation. Keep in mind that doing so can actually damage the tube and make it more microphonic or cause it to fail if you hit it real hard. Although they are screened prior to shipment a tube is an electromechanical device and can be damaged during shipment. A microphonic tube will ring, howl or produce general feedback problems. It will be more noticeable at louder volumes or when used in close proximity to a speaker, typically in combo amps. If the tube has good tone at lower volumes and is free from unwanted noise, you use it in a less sensitive part of the circuit, such as tone recovery or phase inverter applications.
Noise is more of a problem than microphonics. A noisy tube will make random popping noises, crackle occasionally or just hum. All tubes have a certain noise floor; this is the inherent background noise that the tube makes in operation. Typically, you will notice this as a soft hiss or "white noise". Tubes designed for high gain can exhibit more background noise. Other components can cause noise problems that may be blamed on a bad tube. Plate resistors are notorious for causing hiss and crackling as they age and begin to fail. A new tube may better amplify these defects, so try substituting another new tube to be sure of the source of the noise.
Information from thetubestore.com
