Solid-State Power Supply
for ALTEC 1566 micrphone preamplifier




This high voltage power supply supply uses a half-wave voltage-doubler circuit. The low-voltage filament supply is a full wave (but not a bridge) rectifier.
Note About The Schematic

Regarding capacitance values, a common rule of thumb is for the max voltage applied to the cap to be 75% of the rated operating voltage.  In this case, consider that a solid-state power supply is "instant-on," the raw DC at the head of the supply is going to all caps (until the tubes turn on and lower the voltage).

Relative to what is currently available, remember these are power supply caps with a tolerance that is typically 20%!   So, if you can't find exact values — even with the audio caps — choose the next HIGHER value.

Vacuum tubes require two power sources: a low-voltage, high-current supply for the filaments and a high-voltage low-current supply for the plates. From right to left, Figure 4 (above) shows how wall juice gets converted into tube food. The tall vertical lines represent the iron core of the power transformer. The primary coil on the right gets connected to "117 v / 60 ~" while the two secondary coils create 110 VAC — Volts (of) Alternating Current — and 22 VAC.

A pair of diodes — RS-1 and RS-2 — serve as a half-wave voltage doubler. Each rectifies half of the 110 VAC / 60Hz sine wave, turning it into about 265 volts direct current (DC), after which capacitors C9 and C7 smooth the humps into non-audible, filtered DC. C6A and C6B provide further filtering, while R8 serves as the voltage divider for the different stages of amplification. The "B Plus" voltages are ballpark and unregulated meaning that they will vary with the incoming line voltage and with tube condition. Only after the tubes are in and warmed up will the voltage at points "1" and "2" be approximately 235vdc and 155 vdc, respectively.

The 22VAC winding is rectified by RS-3. Unlike the half-wave voltage-doubler circuit for the plate supply, the two diodes that comprise RS-3 are configured for full-wave rectification. Since the winding is center-tapped, each diode rectifies an 11 VAC sine wave into a positive-going hump. When combined, the "two humps" effectively double the 60 Hz line to 120 Hz. (That’s why power supply failures generate 120 Hz "hum.")

Power Supply Notes:

High voltages can give a buzz that can end your lifetime. Use caution and remember that capacitors will retain their charge even after the power is turned off.

  1. Please note the connection between the third-pin ground from the power cable to the chassis.
  2. RS-1 and RS-2 are oxide-style diodes.  If questionable, they can be replaced with type 1N4007.  The change to silicon will increase the high voltage supply and therefore improve head room.
  3. Changing the selenium rectifier to silicon will increase the filament supply voltage. Unlike the B+ supply, this will shorten tube life and increase self-noise.  Increase R10 (currently 1 ohm) until the voltage is 12volts (+/- .5 volts).

    4. Thanks to Bill Whitlock @ Jensen Transformers for his observations and tips.

    PHANTOM POWER

    Yes, Phantom Power would be a great addition.  In this case, it wouldn't even be necessary to build another supply.  All you'd have to do is add a dropping resistor and a zener diode clamp, but I haven't worked that out yet...

    Meanwhile, you have two options.  Click below and you'll be there in a flash.
     


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