New high-voltage power supply with tube rectifier

PS-Tube
PS-Tube
Item# PS-Tube Kit
$49.00
JJ 5Y3 Rectifier Tube:  Heatsink Height: 
Availability: Usually ships in 5-7 business days

Product Description

PS-Tube
The PS-Tube-SS has a brother kit. Where the PS-Tube-SS was designed for those who wish to build a tube power amplifier, the PS-Tube kit was designed for those who wish to build a tube-based line stage or phono stage—using a tube-rectifier-based power supply.

The PS-Tube was designed for use with classic tube-amplifier power transformers that hold a center-tapped high-voltage secondary, a 6.3Vac heater winding, and a 5Vac secondary for the tube rectifier. Such a transformer is perfect for a tube power amplifier. For tube-based line-stage amplifiers and tube phono stages, however, such a transformer is a poor fit. Why?

The 6.3Vac heater winding cannot readily be used to power a regulated heater power supply, as 6.3Vac rectifies up to about 7.5Vdc, which does not allow any voltage headroom for a solid-state regulator, even an LDO type. The PS-Tube circuit overcomes this problem by using a voltage-doubler-rectifier circuit, which develops about 16Vdc from 6.3Vac. With the 16Vdc we can easily use the LD1085 LDO voltage regulator to get a clean 12Vdc or 12.6Vdc to the heaters.

The formula for finding out how much DC current a 6.3Vac heater winding can yield in this circuit is:

Imax = Iac/3.6

where Iac is the the heater winding's AC current rating. For example, if the 6.3Vac winding is rated to put out 3A, we can get 833mA of regulated 12.6Vdc voltage.

The high-voltage portion of the PS-Tube is the classic setup, with one embellishment. Capacitor C1 is a 47µF 450V capacitor. Why not use a much larger capacitor? In spite of how fashionable big power supply capacitors are these days, they are a bad idea when it comes to tube rectifiers. Why? The tube rectifier cathode cannot take the high current spikes that a large capacitors imposes on the rectifier. Most tube rectifier specifications state a maximum capacitor value of 50µF or 60µF, so the 47µF plays it a bit safe. In addition, capacitors C2 & C3 add a total of 94µF after the R5 RC resistor.

Remember that, in a rectifier circuit, the larger the load capacitance, the shorter the charging time and the bigger the current spike. In other words, the average current draw from a power supply might be only 50mA, but the peak current spike through the reservoir capacitor might be 500mA, or much higher. Resistors R3 & R4 are 100-ohm 1W types that limit the inrush current into the tube rectifier. Resistor R5 and C2 form an RC filter. The embellishment is jumper J1, which can be replaced by an external choke (inductor). Capacitor C5 is a 0.68µF 550Vdc bypass capacitor.

Resistors R6 & R7 define a two-resistor voltage divider that allows referencing the heater power supply to some positive voltage. This is an important feature when the circuit holds one triode atop another, as in the Aikido and SRPP circuits. I like to split the difference; for example, if the bottom triode's cathode sits a few volts above ground potential and the top triode's cathode rests a few volts above 100V, I would reference the heater power supply to about 50Vdc.

The PS-Tube holds a simple half-wave rectifier circuit for establishing a negative bias voltage. This portion is optional, but it may come in handy.

Many tube-amplifier transformers, however, do not hold a bias-voltage tap on their high-voltage secondary.A good fit for the PS-Tube is the Hammond 270CAX power transformer.

The maximum secondary voltage is 600Vac center-taped (i.e. 300V-0-300V).

The kit includes all the parts, including many alternate resistor values for the two RC filters, and six aluminum hex standoffs and user guide; an option to include the JJ 5Y3 rectifier is offered as well.