## Product Description

The new PS-24 high-voltage power supply uses a full-wave voltage doubler rectifier configuration and an RC filter to provide suitable B+ voltage for a tube power amplifier. The PCB requires a 100Vac to 170Vac secondary winding to power the power supply. The RC filter consists of a 20 to 300 ohm series resistor (R3) and two 470uF capacitors (C3 & C4) in series, effectively making a 235uF/500V capacitor.

Zener Z1 is optional; it works much like a swinging choke in a power supply, as at idle the RC filter scrubs away ripple, but at full output the zener engages, limiting the voltage drop across the RC filter's resistor to 12V; thus, the output tubes do not see their B+ voltage collapse. When the zener engages, the RC filter falls out of the circuit and all the rail reservoir capacitors are in parallel with each other, bringing each rail's capacitance up to 470uF at 500V. At idle, we want to drop about 6V to 8V across the RC resistor. To find R3's value, divide 7 by the idle current draw. For example, if the total amplifier current draw at idle is 200mA, we get 35 ohms, with either 33 or 36 ohms being close enough.

On the other hand, we could leave resistor R3 off the PCB and attach the leads from an external choke instead. (I would leave the zener in place as long as the choke's DCR isn't too high or use a 1N4007 rectifier to catch inductive voltage spikes at turn-on and turn-off.)

A secondary voltage of 170Vac will result in a B+ voltage of 475Vdc before the RC filter, the result of multiplying the AC voltage by twice the square-root of 2 (2.83). Of course, if we are doubling the voltage, we are also halving the potential current delivery by 2; then we must include the current-reduction factor due to rectification, so the reduction becomes 3.6. For example, if the current rating of the secondary is 300mA, we divide 300mA by 3.6 and get 83mA. If you do all the math you see that we get same final power-supply efficiency of a conventional full-wave bridge rectifier circuit, i.e. about 78%. For example, a 100VA transformer will yield about 78W of DC power, but 100W of AC power with a resistive load.

The 470uF/250V capacitors are rated for peaks of 300V, which is useful in a tube power amplifier, as the B+ voltage will drop after the tubes heat up, but at start-up, the voltage will prove much higher. The PCB dimensions are 3 inches by 3.4 inches. The PS-24 kit includes all the parts needed to populate the PCB, including many alternate RC resistor values.