The Katana is a highly regarded clean boost pedal that can give a nice clean boost or push your tube amp into breakup. The circuit itself is fairly simple, but achieves extra headroom thanks to the internal voltage doubler. For this you can use a ICL7660 or MAX1044. You may want to socket the two 4.7k resistors or replace them with 10k trimpots. The drain of both transistors need to be around 9 volts. On original units, the switch is a push/pull on the pot (which is why it's mounted to the side of the enclosure), but a regular toggle can be used. If the switch wiring doesn't make sense, reference the General Layout Notes tab for switch numbering. It should be possible to fit this in a 1590A if you're not claustrophobic. Here's the schematic for reference.
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