Fluid Type Reverb

The first spring reverb (used in the earlier Hammond ToneCabinets) were oil-damped.

Notes from BobSchleicher:

The FluidTypeReverb is interesting in that really only one spring is used for echo. Most of the energy applied to the springs is lost in the fluid.

The delay time in the springs is about 2/15th second. Each reflection is 2/15th second apart, until all the energy is absorbed by the oil. The only critical tube is the short one. The level of oil in this tube controls the reverb time. Normally set at about 3 1/2", it can be altered slightly up or down to decrease, or increase delay time.

What Oil to Use?

Type A transmission fluid and mineral oil (from the pharmacy) are both reported to work well. You can control the reverb time by adjusting the level of oil in the tubes.

Crystals

The old rochelle salt pickups have a finite life, which expired long ago! BobSchleicher reports that Astatic crystal phono pickups can be used as replacements.

You might be able to get crystal phono pickups from here (or maybe even Radio Shack.)

Replacement

Here's a Hammond Technical Service Bulletin which details how to replace an oil-type reverb with a (more) modern dry reverb unit.