Go wild! Here's the scheme, as you can see it's the standard setup now-a-days of opamp with clipping diodes. Paul C, the designer of the timmy has commented alot on the subject of his pedal over at freestompboxes.org. He still sells the originals directly from himself for around $100 I believe - that is simply the deal of the century when your looking at the bootweek industry! GET ONE FROM HIM NOW! If you fancy trying to build one yourself, here's the scheme!
The Timmy has been cloned the world over due to it's great versitility and tone - the Danelectro Cool Cat Transparent OD 1 was a direct clone and the Lovepedal Amp eleven and OD eleven are both Timmy clones. It's a very nice little circuit and well worth playing with. Here's a nice demo video for you:
And here's another interesting mod - using a feedback arrangement ala the BJF Honeybee and my own Hummingbird to reduce the overall gain and brightness of the unit:
And some interesting options for clipping diodes using a couple of switches. Point A is the - input of the opamp point B is the opamp output:
"I've stated this above, but if you do play around with other diodes/no diodes you have to adjust the output amp or else you may get nasty rail clipping. As is with the gain on zero you've got 6db of gain on the first stage feeding into 6db of gain on the second stage for a total of 12db. Without diodes (and ignoring rail clipping) that 1st stage swings from a gain of 2 up to about 153 in the Tim (500k pot), and 304 in the Timmy (1M pot). This will totally puke out the output amp.
If you want to leave out the diodes for a clean boost, but keep the tone controls I'd mess with dropping the output amp, and just make it a buffer. Then back down the gain of the 1st stage so you don't get clipping. Now you've got a simple opamp booster with bass/treble controls which is actually the original circuit (what a minute - does that mean it didn't start out as a yats??). If I was going with LED's or true mosfet clippers I'd also switch the output amp to a buffer so it wouldn't clip under the larger signal. Better yet if you've got things on a switch have the switch also change the gain of the output amp when you're switching different diodes. You could remove the gain for the large diodes, and put it back for the small. That would solve the loss of volume you get from the standard diode selectors. You could also have some EQ'ing around the output amp to shape things even more. Have it set up to short out the feedback resistor for a flat buffer when using LED's/boosting, or add a cap across the feedback loop and another in series with the inv input to maybe put in a slight mid bump along with the extra gain when using small threshold diodes. Those caps would be added to the circuit, and one pole of the switch would just short out the loop.
I really wouldn't just pull the diodes if I was wanting it to be just a flat booster. There's better ones with more headroom for that.
And some advice on a possible opamp substitutions from twangquack:
"What was not fruitless was experimentation with op-amp substitutions. Unlike a OCD clone on which I did some further tweaking, it was hard to find an op-amp that was a total failure -- that's how sane and well-behaved this circuit is (but in a good way). After trying about fifteen options, alone and dual-stacked, I found that I really liked the TL072 (socket soldered on top) with a BB OPA2134. This is a combo with a bit of added "oomph" that retains the nice treble qualities without adding somewhat extreme tonal qualities that sometimes happens with certain stacked op-amp combinations. I also liked a RC4559 (alone) but it seemed a bit too dark to me, although it had really nice drive -- sounded pretty cool at higher drive levels. But I think I like this pedal to be spankier and used at lower drive levels (hint of grit, or a bit more than a hint) so the TL072 + OPA2134 works very nicely in this particular build."
Here's a vero layout for you: