The Robben Ford mod, involves installing a large resistor instead of the Choke to simulate the sag of a TUbe rectifier (like GZ34, 5U4 etc). I a misunderstanding that ordinary silicon diodes (like 1N4007) will be slow. I have never heard of a slow silicon component.
A resistor of 330 Ohms 5-10watt will do the job. However, you have to turn the (master) volume quite high, in order to build the sag, since its a product of the powersupply not being able to deliver fast enough, because of resistance in the supply.
You can e.g. read more about in this link:
http://ampgarage.com/forum/viewtopic.php?t=8520Good luck experimenting. Remember you are messing with lethal high voltages!