WhitakerAudio J C Whitaker 20 W Stereo Audio Amplifier, Manual

Page: 15 / 194

15
20 W Stereo Audio Amplifier
Q301. When RYL301 is energized, a voltage source from the 35 V dc input is applied to the gate
of the SCR, triggering it. The SCR (and therefore the B+ voltage) remains energized once
triggered until the amplifier is switched off. Zener diodes D301 and D302 are included to keep the
voltage at the SCR below 400 V, regardless of whether a load is connected. The SCR chosen for
Q301 is rated for operation at up to 600 V dc.
The low voltage supply to the preamp power supply PWB is taken from a switched source on
the power management PWB. The +35 V dc (approximate) source is activated when the B+
detection circuit determines the high voltage power supply has reached at least +300 V dc. In this
manner, the high voltage for the amplifier is allowed to stabilize before power is applied to the
separate preamplifier. The typical time delay is about 20 seconds following amplifier turn-on.
Filament transformer T301 is rated for 6.3 V ac at 7.6 A. The filament supply is fused at 6.3
A. The B+ supply line is fused at 0.1 A.
1.2 Power Management Module
The power management circuit board is a supervisory device that adds additional layers of
protection to the built-in protection already provided on the power amplifier. The power
management system adds the following features:
• Automatic Shutoff – While is possible to operate the amplifier continuously for long
periods of time, most consumers would likely prefer to not do so. Experience has shown
the amplifier may be tuned on to play one or more records and then inadvertently left on
after the record ends. Automatic Shutoff provides a fixed maximum run time for the
amplifier—adjustable to about one hour, +/– 15 minutes. This time delay can be defeated
at the option of the consumer.
• Over Temperature Shutdown – The amplifier is designed to operate reliably at normal
room temperature. If operated at an elevated temperature, component damage or
shortened tube life could result. This feature shuts down the amplifier if the temperature
inside the chassis exceeds 170°F. This is well below the maximum rated operating
temperature of the amplifier components.
• Under-Voltage Shutdown – The power supply is designed to operate reliably under a
wide range of conditions. It is possible, however, that because of the failure of a tube or
another component, the power supply could be loaded to the point that continued