Originally posted by Dave Curtis, dB AudioTech
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There are two power supplies in this amp. One for the high voltage tube section and a separate one for the op amps.
The high voltage power supply (B+) is fed from the power transformer secondary via the two red leads. Each side of the center tapped secondary is rectified by two series connected diodes, D3 and D4 for one leg and D5 and D6 for the other. So you can use a 1N4007 to replace any of these 4 diodes.
The low voltage bipolar supply is fed from the bias tap of the power transformer, the blue wire. The 52 volts ac is rectified by the two diode D1 and D2. D1 creates the negative bias voltage for the output stage as well as the negative voltage for the regulated -15vdc power supply. D2 creates the positive voltage for the regulated +15vdc power supply. Because the voltage requirements of the diodes is lower here, you can use a lower rated diode like a 1N4002 here. That doesn't mean that the higher rated 1N4007 wouldn't work here, it just isn't needed here.
Q1, Q2 and their related components are the two 15 volt regulator circuits.
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