Courtesy Steve Bench Purpose This power supply is intended to power a stereo amplifier utilizing KT-90 output tubes and the associated drive circuitry. I-O The following input output description is used to cable between the power supply and the amplifier: 1. +550V at 600 mA max. This is used for the plate voltage to the KT-90s. 2. +450V at 40 mA max. This is used for the driver and input stage circuitry. 3. +300V-1 at 35 mA max. This supplies one side KT-90 screens. It is regulated. 4. +300V-2 at 35 mA max. This supplies the other side KT-90 screens. It is regulated. 5. Power supply ground. 6. -75V at 10 mA max. This supplies the bias for the KT-90s. It is regulated. 7. Power supply ground. 8. 6.3VAC-1 at 10A maximum. Heater voltage 9. 6.3VAC-1 at 10A maximum. Heater voltage. Electrically connected to pin 8. 10. 6.3VAC-CT. Heater center tap. No significant current. 11. 6.3VAC-2 at 10A maximum. Other side of heater voltage. 12. 6.3VAC-2 at 10A maximum. Ditto. Electrically connected to pin 11. 13. 6.3VAC_ref_HV-1 at 1.2A maximum. Isolated heater voltage. 14. 6.3VAC_ref_HV-2 at 1.2A maximum. Isolated heater voltage other side. 15. Power supply ground. 16. Loop around. Connects to pin 17 at power amp. 17. Loop around. Connects to pin 16 at power amp. 18. Chassis. 19. Spare. Not currently used. 20. Spare. Not currently used. 21. Spare. Not currently used. 22. Spare. Not currently used. Description: The HV is developed by using a full wave bridge rectifier consisting of 2 paralleled "damper tubes" in each leg of the bridge rectifier. This is filtered by a choke input filter. The output side of the filter consists of a 10 uF oil capacitor and seried electrolytics with bleeder resistors. The input is in series with a relay contact that opens if the bias voltage fails or if the power supply connection to the amplifier is disrupted. The output side is in parallel with another relay contact that closes to a higher current bleeder resistor if the supply is turned off or if the bias supply fails or if the connection to the amplifier is disrupted. This bleeder is chosen to discharge the HV within a few seconds. This function is served with a DPDT relay. One feature we want to include in the amplifier is triode operation, for cleaner but lower output Power. In order to accomplish this feature, the filtering on the power supply for the KT-90 must be sufficient for this condition, so at a consumption of 150 mA, we want the ripple voltage to be less than a volt. In order to accommodate that, the output side of the LC filter needs to be about 200 uF. Computer grade aluminum electrolytics (Available from Mouser etc) as well as the oil capacitors are used here. There is an additional R-C filter that drops the output voltage to 450 volts and filters the output. This supplies the input and driver stages. (There is additional filtering in the PA for these stages). The ripple coming from the supply will be less than about 5 mV. The screen voltages are developed separately and individually regulated by 2 series connected 0D3 gas VR tubes for each side. There is less than 5 mV ripple on these supplies. The screen supply for each channel will be cut off if the 0D3 for that channel is removed. The bias is derived from a separate 1:1 transformer with a half wave rectifier and RC filtering, coupled into a 0A3 gas tube regulator. There is less than about 10 mV of ripple on this supply. This supply is also used "internally" to mediate shutdown of the HV if something happens to the bias supply. The bias voltage is "cut off" if the 0A3 is removed. This will also cause the HV to be cut off as well. The control relay is a DPDT 24 volt 2000 ohm coil relay (12 mA). It is operated from the regulated bias supply output. There are separate transformers for the main heater voltage, and referenced heater voltage. The plate and HV output supply is fused with a 5A slow blow fuse. The main heater transformer is fused with a 3 amp fast blow fuse. The bias and auxilliary heater supply is fused commonly with a 1A fast blow fuse. The entire supply is switched on/off with a common switch. Tube Complement: 9 - 6AX4 used for the HV, and for the bias supplies. 4 - 0D3 used for the screen grid supplies. 1 - 0A3 used for the bias supply.