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Power supply

Requirements

The power supply has to deliver the following voltages and currents:

  • +/- 15 V for the low voltage signal parts (current sources and Op-Amp)
  • 6.3V AC 6.4A for the heater-filaments of the power tubes (KT88), 1.6 A each.
  • 12.6V DC 600mA for the heater-filaments of the signal tubes (2 in series per channel)
  • High voltage supply for the tubes:
    • 450V 500 mA for the power tubes
    • 450V 24 mA for the driver tubes (E88CC)
    • Two times 370V 1.2mA for the signal amplifier tubes (ECC83), left and right separated.
    • -65V for the current bias circuit of the power tubes.

The high voltage has to be switched on two minutes after the filaments have started to heat-up. This is better for the lifetime of the tube. A timing circuit with two 555 timer IC's switches the high voltage first through a series resistor in order to limit the peak current, after a few seconds this resistor is bypassed. At the same time the discharge output of the second LM555 is switched from -15V to 0V, enabling the bias current control circuit.

DC heater-filament supply connection

The DC heater-filament voltage for the signal tubes is chosen at 12.6V 600 mA instead of 6.3V 1.2 A for practical reasons. When starting with 6.3V AC the rectifier and voltage stabiliser would take too much voltage resulting in less than 6.3V DC. With double the voltage, the 12.6V AC was just sufficient to get 12.6V DC. This means that the heater-filaments should be connected in series. For the ECC83 this would be easy as the filaments are split which enables series and parallel connection. The E88CC does not have this option unfortunately. Because both tubes require the same current (300mA) it was decided to connect the ECC83 and E88CC in series per channel. Manufacturing tolerances could give problems so this should be checked in the system.

Tubeamp power supply