There are basically two main sections to a guitar amplifier, the preamp, and the power amp; both using tubes designed for these sections. Preamp tubes are the smaller tubes and are mainly dual triodes such as 12AX7, 12AT7, and 12AU7. These preamp tubes are voltage-amplifying tubes and form the gain stages, and are used to amplify the small signal from the guitars pickups to something more useful. The gain control is usually placed after the first gain stage and the signal is then shaped with the tone control circuit.
The power stage consists of much larger tubes and are usually power pentodes or tetrodes such as EL34, 6L6, 6550, 5881, EL84. They receive the signal from the phase’s splitter and amplify it to produce enough power to drive a speaker.
There is also another type of tube called a rectifier tube used in the power supply of some amps and it’s used to convert the AC voltage into DC. Common types are 5AR4, 5U4G, 5Y3, GZ34. Modern amps use silicon diodes, but the softer sound obtained from tube rectifiers is still preferred by some amp makers and guitarist.
ALL TUBES ARE NOT MADE EQUAL
A tube is a delicate, intricately constructed mechanical device. Although made under strict design and manufacturing rules each tube turns out slightly different to the next one, even though they are all the same types. So you end up with the same tube type, an EL34 for example, but with a spread in their characteristic (the electrical differences in the same type of tubes). To compensate for the differences, we need to adjust bias voltages to suit the individual tube or select the tube for the bias voltages.
TUBE GRADING OR RATING
As we spoke before of the spread in tube characteristics, it is possible to use this to our advantage. Tubes at one end of this range or spread, will conduct a current quicker and require a lower bias voltage than tubes at the other end of the range, which are slower to conduct and require higher bias voltages. The ones that require lower bias voltages will sound dirtier with less headroom and more saturation, great for power amp distortion. At the other end of the range with higher bias voltages required, these tubes will sound the cleanest with maximum headroom and minimum distortion. Some tube suppliers use a scale from 1-12 or colored dots or just the current value marked on the tube.
BIAS ADJUSTMENT
Amps are made with either a fixed bias voltage or an adjustable bias voltage, usually with a trim pot operating in a small range. Amps with a fixed bias voltage, i.e. Mesa Boogie, need the tubes to be selected for the amp. This is done so that the amp sounds the way its manufacturer wants it to sound both before and after a re-tube. To re-tube this amp properly, the same grade tubes are needed. Amps with adjustable bias voltages can use a range of graded tubes without too much change in sound after a re-tube. Using a Marshall as an example, you would select a matched set in its bias range, and then adjust them for the correct current. If you want the amp to sound the same after a re-tube, use the same grading and bias settings as used with the old set.
