The basis for use of the 308 was made clear in an excerpt from Art Thompson's "Stompbox" book reprinted in Guitar Player some years back (
http://www.amazon.com/Stompbox-History-Guitar-Flangers-Phasers/dp/0879304790#_ ). They reprinted the interviews with several pedal designers, including the folks at Proco Part of the origins of the Rat was essentially a component-value error. The designer thought h was sticking in a larger value ground-leg resistor, but inadvertently used a low-value one and set the gai skyrocketing. Most importantly, the gain it was set to vastly exceeded the gain-bandwidth product of the chip, and this interesting sort of undertone was produced.
The Rat's tone...at least as much of it comes from the choice of chip...is a function of a) the slew rate, b) the compensation cap value, c) the gain-bandwidth product of the chip, relative to the gain setting. Keep in mind that all chips have a slew rate, and that their capacity topass AC with greater amplitude at the output than at the input (i.e., gain) decreases as frequency goes up.
Look at the graph in the upper right corner on page 14 here:
www.national.com/ds/LM/LM108.pdf where you'll see the open-loop gain - the gain-bandwidth product. With a 30pf compensation cap, you'll see that the chip is simply not capable of producing the gain of over 3000 for content above 1.5khz when the stock Gain control is maxed. Where the LM308 can only produce a max of 50db gain at 1khz, the CA3140 can do 70db, the NE5532 looks like it can do around 78db, the LF441 will do 60, and the NJM4558 will do 70. The datasheet for the OP-07 shows it having an open-loop gain-bandwidth product of 60db at 1khz. maximum gain is obviously lower for all of these at higher frequencies.
So part of the sound of a Rat is essentially the chip collapsing under pressure, and the sound of
that as clipped by diodes.
Certainly, one can get distortion by using ANY op-amp to amplify the signal several hundred-fold and passing that by some diodes. One can also get an interesting quality or tone of distortion by using any old op-amp and the dual ground-leg arrangement of the Rat to produce additional boost for upper mids and beyond. But the true sound of a Rat requires that the chip be as limited in capacity as the 308. There may well be others with that same sort of limitation, but I don't know what they are
