Here's my take on the Belton brick. It's pretty standard. I'm indebted to FRW for basically taking his Box of Hall jpg and editing it.
The circuit itself is based very closely on the datasheet and FRW's schematic and came about through fiddling on a breadboard, and some calculations for the final differential amplifier.
Op amps are JRC4558s, simply because I have many of them to use up.
The fairly oversized cap C5 after the regulator is to keep the 5V going while the 9V dies, so you don't get digital power down noises when it's switched off.
Some of the resistors are somewhat arbitrary, I just had them laying around, but the ones determining gain I worked at, as I felt the wet signal wasn't loud enough on full reverb setting. Totally dry is close to unity gain according to the maths and my ears.
The components to play with are the following.
The 78L05 gets a bit warm, but has survived very long on times near a hot radiator. I think there's a bit of variation in how much Belton Bricks draw. Mine was 55mA. You could use a LDO regulator for batteries, but I feel a 9V battery approaching the 78L05's Vin threshold is dead anyway. I managed to get pretty good battery life out of it, by pretty good I mean more than the length of a gig and sound check. C4 is three times what's required. I just needed a 2.5mm pitch cap there, so went for an electrolytic I had.
Input cap C7 is fairly high. You could probably drop it to 47n. I needed 7.5 pitch on my vero and had a 220n green cap that fitted.
I worked at the shelving cap C8 and resistor R7 by ear, not wanting the reverb too boomy, but not without power either. In the end it hardly does anything except maybe prevent very low loud notes from clipping the reverb unit. Maybe, like with C7, it's the bass player in me. Most circuits I've seen have C8 much smaller and R7 higher or not there at all. Ideally R8 should come before C8 and R7, but again vero won over electronics.
C9 was a pain to get right, and I'd like to work more at a tone control. I say tone, but it only controls the wet signal. It's basically shimmer to dark control. I'm making this for a multi instrumentalist whose main instrument is ukulele so it's a bit high, and I think it sounds better with a 150n in there for guitar, and 220n for bass. Either way it's a bit too subtle, but still useful. I tried a few other tone controls but they just sounded bad.
I do think the Belton brick sounds fantastic, particularly with a clean single coil guitar.
