[HAM] HR-40 spring reverb unit advice?

[Dave Bishop]

The oil-filled spring unit is typically filled with mineral oil, per 
Hammond's original instructions.  I use Baby Oil, because I have it leftover 
from babies, and it has a more pleasing scent during the inevitable spills. 
One of our members (Tim H.) reports interesting results from layering 
different viscosities of oil in the tubes, such as 5 weight and transmission 
fluid, which causes different frequencies to reverberate at different rates. 
The smallest tube has the greatest effect on reverberation, and you can 
change the reverb tails by varying the amount of oil in the tubes, starting 
from a baseline of 1 inch below the top for all of them.

The springs are attached to a crystal diaphram which has a tendency to 
crumble with age.  Many of the reverbs sold on eBay are already broken. 
There is a discussion on one of the lists about fabricating replacements 
through some service that repairs crystal microphones, but I don't know the 
details.  If your crystal is broken, you won't hear a sound no matter how 
much signal you put into the reverb.

The HR-40 schematic shows that the organ pre-amp's approximately 3.5 volt 
signal goes through a reverb preamp consisting of one 6SN7 with an 
amplification factor of 70.  The weak return signal gets boosted again by 
another 6SN7 before hitting the power amp.  So there really isn't any power 
"diverted".  One simple solution would be to just build the two reverb 
amplification stages as shown in the schematic, and mix the result into the 
pre-amp you're using for the rest of your devices.

The locking mechanism is only used during moving, to keep that crystal from 
breaking.  There are no motors associated with it.  You'll get no advice to 
get rid of  your reverb unit from me.  I think the oil reverb is great for 
an organ, with its own distinctive sound, though all kinds of modern reverbs 
are more authentic.

Regards,

Dave Bishop