Hammond Model A, Manual, Page: 289 / 298

5-44. Volume or Swell Control.
5-45. The volume control is activated by the swell pedal connected by an appropriate
linkage. (See Rear View of consoles in General Descriptions section) The volume
control assembly consists of two sets of stator plates, similar to those used in the
scanner assembly. (See figure 5 in Vibrato and Percussion section.) A rotor assembly
of similar size is moved by the swell pedal and is capable of meshing with either
stator or a portion of each. The degree of mesh determines the strength of the
entire signal.
5-46. The signal is further amplified by the second section of V4 and sent to driver
tube V3 which in tum drives the 12BH7 output tube.
5-47. PERCUSSION SYSTEM. (See console schematic).
5-48. The Touch Response percussion feature is controlled by four tilting tablets
(figure 8 in General Descriptions section). It is available only on the upper manual
and only when the B adjust key is depressed. Percussion tones are produced by
borrowing the second or third harmonic (depending on position of the Percussion
Harmonic Selector tablet) from the corresponding drawbar of the upper manual B adjust
key group, amplifying it, returning part of it to the same drawbar, and conducting
the balance through push-pull control tubes which, when keyed, cause the signal
to fade away at a pre-determined rate.
5-49. With the percussion tablet on, B adjust key pressed, and an upper manual
playing key pressed, the second or third harmonic signal goes to percussion input
terminal H on the preamplifier chassis and is amplified by T4 and T5.
The percussion input transformer T5 not only provides push-pull signal for the
control tube V7 but also has a third winding which feeds signal back to the 2nd or
3rd harmonic drawbar through equivalent key circuit resistor R50
and terminal J.
5-50. When a key is depressed, the note first sounds loudly, after passing through
the control tube V7, transformer T6, a high pass filter, and terminal D to the grid
of V4. Immediately, capacitor C31 in the control tube grid circuit begins to
discharge, causing the signal to fade away.
5-51. This circuit works as follows: Terminal K (Approximately -25 volts) is
connected to the 8th harmonic B adjust key drawbar wire, which is connected through
the adjust key contact to the manual busbar. Pressing any upper manual key connects
this busbar to a tone generator terminal and virtually grounds terminal K through the
tone generator filters. This virtually grounds the plate of V6, stops conduction,
isolates the cathode of V6, and thus isolates the grid circuit of control tube V7.
The grid then drifts from about -25 volts to about -15 volts, at a rate determined by
the time required for C31 to discharge through R57 and R58. At the completion of this
sequence, the percussion signal is blocked so that it is no longer audible.
22