Just in case you can't tell a tube model just by glancing at it, here is a little bit about the preamp tubes we encounter most often.

Preamp tubes are easily identified; they tend to be the smaller bottles in your amp and are almost always close to your amp’s inputs. Sometimes they are covered with metal shields, which can easily be removed. Since the mid 1950s, preamp tubes have mostly been of the smaller ninepin variety, although some older amps will still have bigger eight-pin (or “octal”) tubes that fit the same sockets used by many types of output tubes. The most common preamp tube type by far is the 12AX7 (also known as the ECC83 in Europe, or the high-grade US alternative 7025). This is what they look like:

Some other types you will occasionally see look much the same, other than the numbers printed on them. These are: the 12AT7, often used in reverb driver and phase inverter stages; the 12AY7, used in the first gain stages of many early Fender tweed amps of the 1950s; and the 5751, a lower-gain replacement for the 12AX7. All of these are what we call “dual triode” types, because they contain two independent tubes within the same bottle. They are mostly differentiated by their gain factor— the degree with which they increase the signal they are given. The 12AX7 has the most gain of the bunch, and the 12AY7 and 5751 are direct substitutes with less gain, which in many cases means they’ll distort the early stages of the amp less. The 12AT7 also has less gain than the 12AX7 but requires a slightly different bias voltage for optimal operation (it can be directly substituted in a pinch).

12AT7 12 AY7 5751

The only pentode preamp tube seen with any regularity in amps today is the EF86 (or 6267), which appeared in early Vox amps and has more recently been used in models from Matchless, Dr Z, 65amps, and a few others. Another less frequently seen, but much admired, pentode preamp tube is the 5879, notably used in Gibson’s GA-40 Les Paul amp of the late fifties. Both of these pentodes fit the same 9-pin bottle as the dual triodes but require very different circuitry, and are known for their thick, robust sound. Both have higher gain factors than even a 12AX7, but aren’t prone to distorting the way that dual-triodes can, and instead pass their fattened-up signal on to the next stage. They also have a reputation for handling effects pedals very well. Drive a 12AX7 hard, however, and it will induce quite a bit of sizzling, slightly fizzy-voiced distortion of its own. This can be a great thing if you’re looking for a super-fried overdrive tone that’s cooking at all stages, but not at all desired if you want more headroom and clarity, or the fatter distortion that’s generated in the output stage of the amp when a cleaner preamp signal is driven into clipping at the output tubes.

EF86 5879

The section on power re tubes is being prepared...