A lot of solid state amps from the late 70s and early 80s are designed similarly to tube amps. Instead of tube gain stages, there are discreet transistors. The distortion came from actually overloading the transistor gain stages. Occasionally, there would be an op amp or a clipping diode., but all the volume controls and tone shaping were done passively, just like a tube amp. It just so happened that a lot of the transistors and FETs sounded pretty good when distorting, and the preamp would distort before the power amp would distort, and the power amp distortion almost universally sounds bad with solid state amps.
As the 80s progressed, solid state amps became almost all op-amp based. Volume controls went from passive controls to actively controlling negative feedback in a gain stage. Tone controls went from passive to active. Distortion became fully diode based, like a stompbox pedal. The older solid state amps would kind of ease into distortion, like a tube amp. The newer solid state amps were meant to be clean, or they were meant to distort like a distortion or overdrive pedal. If the clean channel ever got overloaded, it sounded horrible, and the distortion channel usually sounded like a crummy pedal.
Now, amp companies tried to fight this by putting a preamp tube or two into the preamp. GK, for instance, puts one FET in the 400RB and 800 RB, right after the boost control. You can tell that they knew the amp distorted terribly, so they compensated by choosing the one solid state device that clips most pleasingly, put it in the area that will most likely distort, and made everything else run cleanly.
It's not as though it's difficult to design a good solid state amp nowadays, it's just a matter of cost. Tube amps can get made in Mexico or China and sell for $300 or less. A really good, well designed, well researched solid state amp is going to cost more than that, and it just won't sell to the average customer. There are a number of very good sounding, very expensive solid state amps aimed at jazz guys and acoustic instrument players. The average player will always choose a tube amp over a similarly priced SS amp with the same feature set.
In short, old Traynors, Randalls, Sunns, Music Man (SS preamps!) and some others are more-or-less reworked tube circuits using solid stated devices. Later SS amps are generally not, and sound differnt.
edit: proofread it
Last edited by benadrian
on Mon Apr 16, 2012 12:34 pm, edited 2 times in total.