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.
(Thats 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.
Please note the connection between the third-pin ground
from the power cable to the chassis.
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.
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).
Thanks to Bill Whitlock @ Jensen Transformers for
his observations and tips.
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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