Here at MayosMusic, in addition to offering great guitar, bass, ukulele, banjo and autoharp lessons, plus complete luthier services that include guitar, string instrument and amplifier repair/restorations, we are also a great resource service. Below is a discussion on the difference of each amp design and the effects of that design on your sound
Instrument amps for guitar, bass, keyboards and such, come in several different design topologies. Some are designed with Tubes, some with Bi-polar transistors, some with FETs, some with Op-Amps, some with discrete IC chips, and others in various combinations. We will mostly concentrate on the instrument amplification side of amps and discuss their general differences in sound (clean and distortion), tone, headroom, power, sustain, durability, customizability, weight, cost, and such.
Tubes, or Vacuum Tubes, also know as Valves are devices used to amplify a signal. Tube amps are generally more expensive, higher end, heavier and require more maintenance than their solid-state counterparts. Tube amps require an output transformer (along with the power transformer, the output transformer is where the bulk of the weight comes from in an amp), and some form of power rectification.
Tubes come in three primary categories, rectifier, preamp, and power amp. There are triodes (three parts), beam tetrodes (aka kinkless tetrodes" 4 parts), pentodes (5 parts), and beam pentodes. Most preamp tubes are dual triodes, such as the 12ax7, and related tubes. Power tubes are usually beam power tetrodes, such as the 6L6GC, or beam pentodes like the 6V6, the 6BQ5 (EL84), and the 6CA7 (EL34-ish), or the slightly higher distorting true "screen pentode" the EL34. Changing the tubes in a tube amp can be (along with speakers) a great method for players to custom shape their tone and sound. Different brands and models of the same tube type will all sound different. There are in many cases "tube type swaps" that can be made with no other circuit changes, and if you are willing to modify the circuit a bit, even more varied tube types can be interchanged. This is most significant to preamp tubes but can also apply to power tubes and even rectifiers. But, caution is needed when doing any modifications to any amp, especially a high voltage tube amp where voltages and currents can be painful, or worse, lethal.
Tubes give a nice and personal touch to your music. They impart a tone and liveliness that adds realism, and depth, with a delicate and touch sensitive nature to your tone. Tubes usually have much more head room than any other form of amplification, especially when combined with a solid state rectifier. Tube amps have more sustain, especially when a tube rectifier is used as the "sag" factor is greatly increased. Tube amps will appear to sound more linear in frequency over more varied volumes.
Tube amps do require more maintenance than a comparable solid-state amp. Both amp types will have electrolytic capacitors that dry up over time (about 8 to 12 years old), and require replacing, however the damaging effect they have when they go bad is more dramatic on a tube amp (due to the much higher voltages). Tubes themselves also have a finite lifespan. The more you play, the more times tubes are powered on and off, the hotter they get, the more vibration, and the cheaper the tube manufacturer, all shorten the lifespan of the tubes. Power tubes usually have the shortest lifespan, with preamp and rectifier tubes generally more long-lasting and dependable. Also, in many amp designs the power tubes will need to be biased, and if there is a pair, or a quad of them, you need to get matched sets, as it will ensure the safety, stability and sonic maximum of the amp. Also, all tube amps require a speaker load properly connected before turning on and using.
Due to tube amps increased weight, more expensive internal parts, and generally more labor involved assembly, tube amps are much more expensive than solid-state amps given the same category and/or power output. Parts can also be more expensive when repairs are needed, and maintenance is generally more expensive due to the tubes.
Solid-state amps use some form of a semi-conductor device to amplify a signal. In the early days of solid-state, this was done with germanium transistors, but the vast majorities are made of silicon. There are several forms of solid-state devices: Diodes, Bipolar transistors (NPN, PNP), FETs (such as MOSFETs, VMOS Chips, etc.), Op-Amps, ICs, Microprocessors, and now Modeling amps. Solid-state amps are much less expensive to manufacture than tube amps, and they also weigh significantly less. There are generally less parts to go bad, compared to a tube amp. However, since most modern solid-state amps are built in Mexico at best, and more likely in China or other Asian country, they are much less reliable overall (this can also be an issue with any tube amp built overseas). Some exceptions to this would be the old Roland Jazz Chorus series of solid-state amps, the USA made Peavey solid-state amps, most Yamaha products, Fender solid-state amps not made in China (90's Fender Bronco amp for example), and many other vintage American made solid-state amps.
Solid-state guitar amps are generally more of a student model amp, compared to the more professional tube amps. However, solid-state is the design of choice (due to weight, size) in the PA realm, and has the majority rule in keyboard and bass amplification too. This is for several reasons, mainly, cost, weight/size, heat, and power.
In cases such as bass guitar, the lower the frequency, the less efficient our ears hear, and the less efficient everything else is from amps to speakers. So the lower the frequency, the more power is needed, and not just a bit more, but significantly more. Thus, a guitarist might be loud enough with a 22, 50, or at most 100Watts tube amp, a bassist will need 200, to 1200Watts or more to keep up. This kind of power output is pretty much unheard of in tube amps, but very common for solid-state bass amps.
There are some amps that utilize the benefits of both tubes and solid-state into one incorporated design. Fender, Randall, Roland, Peavey, Marshall, Vox, MusicMan, Kustom, Hartke, and many others have all made hybrid amps. Some have tube preamps, and transistor outputs, while others have transistor preamps and power tubes. Either version is generally considered better than an all solid state design, due to the character, depth, and realism tubes add to the sound.
Solid-state amps tend to have a compressed sound quality to them, and usually have a biting, edgy or harsh distortion quality. This is due to two factors, one, the crossover distortion much more present in solid-state devices, and the hard clipping of solid-state devices (compared to the soft clipping of tubes which creates even order harmonics), which creates more odd order harmonics, and distortion which our ears and brains do not like. Solid-state amps also appear to sound less linear, from highs to lows, over varied volumes (especially at lower volumes). However, solid-state devices enable many more effects to be incorporated inside an amp, or even a smaller device such as a pedal or rack effects unit. Though again, the higher end equipment will often be made with tubes, or at least incorporating tubes for part of their circuit.
The newest class of solid-state amps is the modeling amp. These are amps that use software analysis of known great sounding amps and attempt to emulate their magic and "model" their tone. These are typically lower priced student model amps, most if not all of which are made in China or other Asian country, they do not sound like the amps they claim to and they are usually trouble prone and unreliable due to their inferior internal parts. They do however give a moderate variety of tonal options that are all "ok" (albeit they lack depth, realism, and authentic, genuine quality tone) and at a low price. The higher models from Vox are the better examples in this class, while Fender, Marshall, Line6, Peavey, and others are slightly less realistic from our research. Remember that having one amp that can do all things, usually means it doesn't do any one of them all that well.