AmpWerks Audio Lab

AA334 Power Supply

The AA334 HV transformers are new old stock Canadian Armed Forces surplus Hammond 279Z. The HV windings are rated for 850V CT at 172ma. There is a 5V CT at 3A winding for the rectifier filament, and 6.3V CT at 5A for the other tube heaters. These transformers are over spec but they will run cool and be reliable – and I had 4 of them in stock.

The Z mounting bracket supports the entire transformer above the chassis. This worked out perfectly since it gave us enough room underneath to mount the filter choke, a 5V transformer for the 300B heater, plus a 6.3V 4A bucking transformer (more on this below).

I used the free PSU Designer II software from Duncan Amps. This analog power supply CAD program supports most popular tube and solid state power supply topologies and the results are quite accurate.

I had a few new 5U4GB rectifier tubes in stock. It has a maximum current rating of 300 ma. I expect the AA334 will require about 130 ma. at 450V, so the 5U4 should live a long and happy life.

The power supply filter is a fourth order RCLCRCRC configuration (R is resistance, C is capacitance, and L is inductance).

Class A amplifiers need clean power. They always draw maximum current, even when they are idling, so they are quite sensitive to ripple voltage on the B+. This power supply might seem like overkill, but the additional cost was not significant, and the result is a pair of very quiet amplifers.

Here's a close-up of the power supply filter section under the chassis.

C1 is a 22 uF 630 V Obbligato film capacitor which is mounted above the chassis between the two transformers. The other filter caps are high quality Nippon Chemi Con electrolytics mounted underneath. Each electrolytic is bypassed by a small value film capacitor and a flame proof metal film bleeder resistor. Bleeder resistors ensure that the filter capacitors discharge when the power is turned off.

All the tube filaments are AC powered, including the 300B. A small 5V CT 3 Amp transformer under the chassis supplies filament power for the 300B. The centre tap is connected to circuit ground through a 1,000 Ohm 50 Watt resistor. A cathode current of 74 ma. through this resistor results in a bias of -74 V DC on the 300B grid.

An increase in cathode current would increase the negative bias on the grid, which would tend to reduce the cathode current. Inversely, any decrease in cathode current would decrease the negative bias on the 300B grid, which would tend to increase the cathode current. This helps to promote long term stability as the tubes and other related components age.

The most common primary voltage rating for older transformers, and even many new ones, is between 115 and 117 Volts AC. But in recent years electrical utilities have increased the voltage at our wall outlets to between 120 and 125 V. The higher voltage can put undue stress on power transformers, and can reduce tube life by changing operating points.

Our solution is a small 6.3V filament transformer, connected counterphase, with its secondary winding in series with the amplifier's AC supply. This effectively reduces the AC line voltage by 6.3 Volts, which allows us to use our NOS transformers, delay relays, and other components at their proper design voltages.

Each AA334 monoblock draws about 100 Watts from the power line. Hmmm, let's see... 100 Watts in and 6 1/2 Watts out. That's not terribly efficient no matter how you look at it. But we did specify pure Class A, didn't we?


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