(https://i.postimg.cc/nj68GcQQ/pin6.png) (https://postimg.cc/nj68GcQQ)
I found this on Electrosmash.com and I'm trying to understand how this works. It says a transistor can be used to limit the amount of current out of pin 6 and gives an example, but I don't quite understand.
So a transistor is a current-controlled resistor. The base-emitter current controls the collector-emitter current.
The purpose of using a transistor can be to allow remote control of delay time (an expression pedal could be the pot) but most often to add a start-up time delay. The start delay is because the PT2399 delay clock can be made to run faster if the pin6 current is only increased sometime after power-on. If the pin6 current is too high at power-on, the delay clock won't start and the chip is effectively stalled or locked-up.
A problem with that is that if power is lost after a successful start, the capacitor on the transistor base (which provided the start delay) may still hold some charge so the next start when you put the power back means the start delay won't happen.
Version C puts the start delay feature on the second transistor. This will have its emitter voltage higher than 0v and the capacitor on its base can discharge to a lower voltage toward 0v. The diode to +5v helps to discharge the cap when the +5v supply drops when power is lost. Now when power is restored, that cap will take longer to charge up and turn the top transistor on so there is a much more reliable delay.