Percussion tones are produced by borrowing the second or third harmonic signal from the corresponding manual drawbar, amplifying it, and returning part of the signal to the same drawbar, and conducting the balance of the signal through push-pull control tubes where its decay characteristics are controlled.
The Percussion signal is then combined with the signal from the mauals after the vibrato but before the expression control. The control tubes are keyed through the 8th harmonic key contacts and busbar.
The "Touch Response" percussion feature is controlled by four tilting tablets. 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 "Perchssion Harmonic Selector" tablet) from the corresponding drawbar of the upper manual "B" adjust key group, amplifying it and returning part of it to the same drawbar, and conducting the balance through a push-pull control circuit (which we would call a VCA today) which, when keyed, causes the signal to fade away at a pre-determined rate.
With the percussion tablet on, "B" adjust key pressed, and an upper manual playing key pressed (i.e a note on the upper manual), the second or their harmonic signal goes to percussion input terminal H on the preamplifier chassis and is amplified by T4 and T5 (not to mention V5). The percussion interstage transformer T5 not only provides the push-pull signal for the control tube V7 but also has a third winding which feeds signal back to the 2nd or 3rd haromic drawbar through equivalent key circuit resistor R50 and terminal "J".
When a key is depressed, the note first sounds loudly, after passing through the control tube V7, transformer T6, a high pass filter (C32, C33, C34, C35, R63, R62), and termnal D to the grid of V4A. Immediately, capacitor C31 in the control tube grid circuit begins to dishcarge, causing the signal to fade away. V4 is the mixer stage, but since one of its inputs is the capacitive vibrato scanner all of the other inputs to it must be made to look capacitive, to avoid frequency response anomalies.
The percussion control circuit works as follows: Terminal K (Approximnately 25 volts from ground) is connected to the 8th harmonic "B" adjust key drawbar wire, which is connected through the adjust key contact to the manual bus bar. Pressing any upper manual key connects this bus bar 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 (note that V6 is wired as a DIODE), and thus isolates the grid circuit of control tube V7. The grid then drifts from about -25v to about -15v, 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.
No further percussion signal can be heard until all keys of the upper manual are released so that the control tube V7 grids can again drop to -25v (the rate of this drop is fixed by the time required to charge C31 to -15v through R55 and R56). Thus the percussion effect is heard only when keys are played in a detached manner; that is, when all keys are released before pressing the next one.
n.b Although the grid voltages are shown as negative here, in reality, they are positive with respect to ground, but negative with respect to the cathodes of V7, which are 37v above ground.