It's a familiar problem: you would like to build a tube power amplifier, but you are daunted by the thought of having to deal with the high-voltage power supply. The amplifier's input and driver stages are simple enough, as is the output stage, but the power supply must offer several B+ outputs and a negative bias voltage for the output tubes. The solution to this problem is the new GlassWare PS-Tube-SS kit. Simple and self-contained, the PS-Tube-SS PCB holds two cascading RC filters for the high-voltage outputs and negative-bias-voltage power supply and 12Vdc power supply for the input stage.
The "SS" stands for solid-state rectifiers. The PCB holds seven rectifiers, five high-voltage, high-speed rectifiers and two low-voltage, super-fast rectifiers. The is PCB is a compact 3 by 6 inches. With four mounting holes.
Tube power amplifiers usually require several B+ voltages, the highest (B1) for the output tubes, the next highest (B2) for the either the phase splitter or driver stage, and the lowest (B3) for the input stage. This cascade of DC voltages results from cascading RC filters, so the input stage receives the cleanest DC voltage.
Many classic tube amplifier power transformers offer a bias-voltage tap on the B+ secondary. This single tap is used in a simple half-wave rectifier circuit. Resistor R14 limits the inrush current and capacitor C20 accumulates the stored negative bias voltage. The capacitor supplied in the kit is rated for up to 50V use, so the negative-bias tap is limited to no more than 33Vac. If the AC voltage is greater than this, a higher-voltage capacitor must be used, say a 100V device.
The PS-Tube-SS PCB holds a simple DC heater power supply that consists of a two-rectifier voltage doubler and an RC filter. Since no tube rectifier is used with the PS-Tube-SS PCB, the 5Vac that would otherwise go to waste can be put to use powering this voltage doubler. The MUR410G rectifiers (D5 & D6) are ultra-fast types with a small voltage drop. Just how much DC develops depends on several factors. First, many old tube-power transformers were built back when the wall voltage was much lower than it is today. I have seen a few rated for 110Vac, for example. Most are were built with 115Vac as the intended input voltage. This means that if your wall voltage is higher (mine is 123Vac), then more than 5Vac will be presented by the old 115V transformer. Second, if the 5Vac has a current rating of 3A and you only draw 0.3A of DC from it, the transformer's poor regulation will result in more than 5Vac of secondary voltage.
What if you plan on using the 5Vac winding on a 300B output tube, could the 6.3Vac winding be used with the voltage-doubler circuit? Yes, but the voltage drop must be greater across resistor R13, so it must be larger in value and it will dissipate much more heat. This brings up the issue of not using a classic tube power transformer, but a two transformers. Say you plan on using a toroidal transformer with two 6.3Vac winding and another toroidal with a secondary of 240Vac. In this case, one 6.3Vac winding can be used to heat the output tube heaters and the other 6.3Vac for powering the 12Vdc power supply on the PS-Tube-SS PCB. The 240Vac will rectify up to about 340Vdc.
Many topologies, such as the SRPP or cascode or Aikido topology, use triodes standing atop another, which makes referencing the heater power supply to the B+ power supply a small chore. With one tube atop another and a single heater power supply, the top and bottom heaters cannot share the same heater-to-cathode voltage relationships. With such totem circuits, the safest path is to reference the heater power supply to a voltage equal to one-fourth the totem-pole stage's B+ voltage; for example, 75V, when using a 300V power supply. Resistors R11 & R12 on the PCB establish the reference voltage, as shown below in the PCB's entire schematic.
The kit includes all the parts, including many alternate resistor values, and hex standoffs and user guide.