WhitakerAudio 60 W Stereo Power Amplifier, Руководство пользователя и по сборке

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Charging resistor R201+R209 must be small enough to supply the minimum trigger current of
Q201 plus the leakage current of the capacitor when the emitter of the unijunction is biased at its
peak point voltage. This would place a limit on practical time delays of only 6 or so seconds. As
shown in the figure, a relaxation oscillator (Q202, 2N491) is included to reduce the minimum
trigger current requirement more than 1000 times by pulsing the upper base of Q202 with a 0.75
V pulse. The negative pulse causes the peak point voltage to drop slightly and if the voltage level
at C201 is greater than this, the unijunction will trigger. R202 is optimized for best stabilization
over the required temperature range. The maximum time delay that can be achieved by this circuit
is mainly dependent upon the maximum values that can be obtained for R201+R209 and C201
consistent with the low leakage requirement. Without diode D201, R201+R209 is limited to 15
megohms for an accuracy of 0.5% at 25 degrees C, but with D201, R201+R209 can be increased
considerably beyond that.
The voltage sensing circuits (Q204/Q205 and Q206/Q207) are identical. The values of R214/
R215 and R218/R219 are determined by the operating voltage being measured. As a general
guideline, R214 + R215 = 800 × voltage being measured. Two voltage sensing circuits are
provided. One is used to monitor the B+ voltage supply and the other is used to monitor the screen
voltage supply (if used). The voltage sensing circuit is designed to latch on after the proper
voltage has been detected. Using the values of R214/R215 and R218/R219 shown in the parts list
(360 k
Ω
total), the circuit will latch on at about 450 V dc.
Because the sensing circuits require time to settle (and the B+ voltages require time to build
up to operating value), a time delay is needed prior to enabling the protection devices. This delay
is accomplished by the combination of R211 and C207 for solid state rectifier circuits used in the
20 W amplifiers. For the tube-based rectifier circuit used in the 40/60 W amplifiers, a longer
delay is required than can reasonably be provided by a simple RC circuit. For such longer delays,
an SCR (Q208) is added to the circuit to provide for a nominal 30 second delay before closing.
2 Parts List
A parts list for the Automatic Protection Option board is shown in Table 1.