Any information out there on the so called Robben Ford mod..?
Does Nic maybe offer this..?
From what I`ve read, there`s a power resistor instead of a choke in the power supply to induce more sag in the amp.

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=8520

Good luck experimenting. Remember you are messing with lethal high voltages!