Here's a reverb circuit that doesn't require a Belton brick–the Equinox II by Valve Wizard. It uses 2, PT2399 delay chips with a pretty standard input/output buffer built around a TL072. The original uses a J112 for switching the tails function, but I've simplified that with a just DPDT footswitch. Use one side of the DPDT for turning the reverb on/off, and the other side for LED indication. If you don't want/need tails with your reverb, just jumper the 2 switch pads. It's gonna be a snug fit in a 1590B, but it will fit.
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I was able to verify this over the weekend and it works great and will fit nicely in a 1590B.
