madlee wrote:Since tubes and transistors do the same exact thing, i.e. allow a small voltage control a larger voltage, what's the difference?
They don't do the exact same thing the same way. Transistors are high-current, low-voltage devices and tubes are high-voltage, low-current devices.
The sonic difference is due to their respective distortion characteristics.
Solid-state devices emphasize higher-order harmonics characteristic of 'hard' clipping and a harsher distorted sound. Many of these devices emphasize odd-order harmonics in particular (MOSFETs being the notable exception, though MOSFETs still emphasize higher-order distortion more than tubes).
Tubes emphasize lower-order harmonics, which are more euphonic.
Tubes 'emphasize' lower-order harmonic distortion by having more of it.
Most tube stages that are apprehended as being euphonic have a very high degree of second-order distortion in particular.
By having more of this lower-order distortion, esp. 2nd-order, the higher-order HD (which is present) is masked somewhat.
Also, as tube stages are overdriven increasingly, the lower-order distortion increases disproportionally to the higher-order distortion. This is not true of many solid-state stages.
You can measure all this shit.
When run clean (esp in true high-fidelity applications), it can be extremely difficult to tell the difference between tubes and solid state. And if one has a very clean guitar or bass sound, it is much less likely to make a big difference what kind of amp you use.
A distinguishing characteristic of tube amps is the output transformer. A big hunk of iron w/wire wrapped around it, the transformer allows the high-voltage, low-current output tubes to interface with the loudspeaker, which requires lots of current. Solid-state amps can source a lot of current, so they don't need a transformer. Transformers have all sorts of special distortion characteristics and overload behavior of their own. If you've ever blown up a Partridge in a HiWatt and had to replace it w/something else, you know what I mean.