Filament and Control Power Supplies

Posted on Feb 4, 2014

AC power is supplied via AC Fuse F1 and Power On/Off switch S1 to 6. 3V filament transformer T1. Fuse F2 is provided as an additional safety feature, since a short to the filament transformer may not be enough to blow the main power fuse. Hum balance pot R1 is provided for good measure, though it`s unlikely to have much of an effect given the overa

Filament and Control Power Supplies
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ll design of the amplifier. This supplies filament power to most of the tubes in the unit, as well as the #47 pilot light I1. Note that T1 does not provide filament power for tubes V1, V3, V4 and V18. V4 is a gas regulator tube, and obviously doesn`t need filament voltage. V1 and V3 are in the high-voltage regulator circuitry, and require isolated filament supplies because their cathodes are sitting at high voltage above ground; these will be covered in the HT section. V18, the first gain-stage input pentode, is treated specially to a DC supply (covered in the below) to reduce the possibility of filament-induced hum. D1 and R2 provide rectification and current limiting for metering the supply voltage. Note that in all schematics, "MP" represents a metering point; the metering circuit is further detailed in next section. Switched AC voltage is also applied to a timer relay RLY1. I used a commercial timer block for a standard octal relay, by Releco; this is rather pricey, however, and DIY timer circuits are readily available if you search the web. This timer delays for a variable time (on the order of 20 seconds) to allow all the important filaments to reach full temperature before energizing the main HT supply. The input to the timer relay is also routed through the "safety link" incorporated in gas regulator V4 (pins 3 and 7), preventing energizing of the HT supply (and very probable damage) if the unit is powered up with...

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