Hammond Model A, Manual, Page: 112 / 298

RHEOSTAT BOX
The rheostat box contains the expression control rheostat and other components.
including some terminals associated with the tremulant system. Figures 1 to 8 show
various models of rheostat boxes and their circuits. The rheostat box is used only
in console models with tremulant and with non-selective vibrato .
The rheostat itself is actually a variable resistor with no sliding contacts. When
the expression pedal is advanced a bakelite cam moves down, opening in succession a
series of 32 contacts. tipped with precious metal. The contacts are connected to
fixed carbon resistors.
Resistor R2 in figures 2 and 4 forms a constant load on the matching transformer,
while R4 and C4 serve to attenuate the higher frequencies. R4 and C4 were not used
in Model A consoles below serial number 1231. The rheostat, in series with bass
compensating condenser C2, is across the signal line so that when its resistance is
least the volume is least. Condenser C5 avoids excessive tremolo on the lower bass
frequencies. It was not originally installed in Model A consoles below serial number
2311. C3 is a blocking condenser and R3 is a grid resistor for the first
preamplifier tube.
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