A friend of mine once said, “If you don’t spend at least as much time designing the power supply as you spent designing the amplifier, you’re probably not doing it right…” There’s a lot of truth to that. A lot of design effort has gone into this power supply. One of the main challenges was to find one power transformer which could supply all the voltages needed. The Classic Tone 40-18069 fits the bill nicely. It’s available from Triode USA and other Classic Tone distributors.



The 4×6 inch (10×15 cm) supply board contains the necessary circuitry to implement soft start, B+ rectification, negative bias voltage regulation, and two rectified supplies to be used with Universal Filament Regulators for filament/heater supplies.

Unpopulated circuit boards for this 300B SET Power Supply are available for purchase. Prices are in US Dollars and payment is handled via Paypal.

PCB ID Description Price each QTY in stock
300B_PSU_R2p0 300B SET Amp – Power Supply Board 50 > 10

Solution Graphics

Below is shown an example of such a board when fully assembled.



Design Collateral & Quick Reference

  • Schematic & Bill-of-Materials: 300B_PSU_2P01_SchematicBOM.pdf
  • The board measures 4.0 × 6.0 inches (102 × 152 mm). Mounting holes are intend for use with M3 (#4) screws. See mechanical drawing below.


Circuit Design

The power supply has four main circuit blocks:

  • B+ Supply – provides 525 V unregulated DC
  • Bias Supply – provides -220 V regulated DC
  • Filament Regulator Supplies – two outputs of 20 V unregulated DC
  • Soft-start and mains voltage selector options

The schematic is shown below (click on the schematic for a larger view).

300B PSU SchematicThe schematic and bill-of-materials (BOM) are available in this .pdf document: 300B_PSU_2P01_SchematicBOM.pdf

The unregulated B+ supply is generated by D1, C1, and C2. Resistors R1 and R2 serve two functions; they ensure that the total B+ voltage is evenly shared by C1, C2, and they bleed off the charge when the circuit is turned off. D2 is a high efficiency LED indicating that charge is stored in the reservoir caps, C1 and C2. The B+ supply is fed by a 380 V transformer winding, resulting in roughly 525 V DC on the output of the supply. This is intended for use as the raw supply to a 21st Century Maida Regulator regulating to the final B+ voltage of 400 V. A lower B+ voltage can be accommodated by swapping the connections on the HV secondary, such that the 330 V tap of the transformer is used rather than the 380 V tap. This will result in an unregulated DC voltage of roughly 450 V on the supply output. This is ideal for regulating to the 350~380 V used with the more conservative 300B operating points.

The bias supply is a bit more complex. Capacitors C4, C5, C7, C8 and diodes D3~D6 form a voltage quadrupler turning the 60 V AC from the transformer into roughly -300 V DC. R3 limits the peak current drawn from the transformer. High efficiency LED, D10, indicates that charge is stored in the capacitors in the voltage quadrupler. P-channel MOSFET, Q1, zener diodes, D12~D14, and associated components form a simple zener voltage regulator that provides approx. -220 V for the bias supply.

The two filament supplies connecting to J3 and J4 should not require much introduction. They are standard full-wave rectified supplies providing approx. 20 V DC each. This is perfect for use with the Universal Filament Regulators.

A soft-start is provided by NTC resistor RT1. This resistance of this resistor is about 90 ohm at room temperature. Hence, on turn-on, the current in the transformer primary is limited to roughly 1.3 A with 120 V mains (2.5 A for 230 V mains). Relay, RL1, shorts out the NTC resistor after the time delay set by R12, C19, Q3, and associated components. The time delay is roughly half a second. After the delay, RT1 is shorted by RL1, hence it is ensured that the soft-start circuit re-activates in the event of a momentary loss of mains power.



Capacitors C3, C6, C9, C13, and C17 are optional. They are used to minimize the switching transients from the diode bridges. However, after much experimenting, I found that I was not able to hear or measure any effect of using these capacitors. Hence, they are marked “DNP” for Do Not Populate. I consider them to be optional.

When used with the Classic Tone 40-18069 transformer, the circuit board allows for use at mains voltages of 120 V, 230 V, and 240 V by soldering in different wire options. This is illustrated below. Click on the image for a closer view.


Q1 will need a ferrite bead draped on its gate lead. Otherwise, the bias regulator will be prone to oscillation. The assembly steps are shown below. Click on the image for a larger view.

  1. Prepare the components – heat sink, FQP3P50 PMOS device, thermal grease, and mounting hardware.
  2. Line Q1 up with the mounting hole on the PCB and bend the leads to a 90deg angle using pliers. Coat the metal tab of Q1 with a light coat of thermal grease. Drape the ferrite bead over the gate lead (pin 1; marked with a square pad on the board).
  3. Carefully place the transistor/ferrite bead/heat sink combo on the footprint for Q1 and tighten the mounting screw.
  4. Solder the device to the board.

If a ferrite bead is not available, a 10 nF capacitor mounted from gate to source (pins 1, 3) on Q1 is an alternative method to keep the regulator stable.

Q1 assembly

The remainder of the assembly process is fairly straight-forward. Note that one terminal in J1 and one in J2 are removed. These are the terminals marked KEY in the schematic and are identified by a smaller pad on the PCB. It’s pin 3 in J1 and pin 2 in J2. These terminals are removed to avoid violating the breakdown voltage spec. on the terminal block connector. It takes a little work, but with some gentle prying with a screwdriver, it is possible to get the terminal out of the plastic housing.

Pay attention to the polarity of the bridge rectifiers — especially on D1. Ensure that they are connected correctly.

On the five capacitors around the voltage quadrupler, the square pad marks the positive anode (+) pin of the capacitor.

The rest of the assembly process shouldn’t raise too many questions. I start with the 1/4 W resistors and work my way up to the taller components.

Unpopulated board: