WhitakerAudio
 
Audio Projects – Regulated High-Voltage Power Supply

This page includes supporting information for the regulated power supply project described Chapters 8 and 12 in The TAB Guide to Vacuum Tube Audio, along with commentary from the author. This page picks up where the book left off.

Shown on the right is the regulated power supply used on the 50 W stereo amplifier.

Power supply with 50 W amplifier

Note 1

A varistor may be added across the primary winding of the power transformer in order to minimize transient disturbance resulting from removal of primary power from the supply. Switches normally exhibit some amount of "contact bounce" when changed from one position to another. This can lead to noise resulting from the high voltages generated by the collapsing magnetic field of the transformers in the circuit. The varistor is essentially invisible in the circuit until a predetermined voltage is reached, at which point the resistance of the device decreases to a low value, thereby shunting the transient energy. Attention to transient disturbances is critical for proper operation of solid-state hardware; taking similar precautions in tube-based equipment is a good practice.

A variation on this approach is to place the varistor across terminals of the power switch rather than in parallel with the transformer primary. The net effect is about the same—reduction in transient disturbances at turn-off (and to a lesser extent at turn-on). Analysis of any design needs to include the impact—if any—on safety should the device fail. In the case of the varistor, the usual failure mode is a short-circuit. That being the case, if the varistor failed the user would be unable to turn the amplifier off from the front panel. While this is an annoyance, it would not raise safety concerns under normal conditions. The author tried both approaches and each accomplish the objective of eliminating an audible pop or click when the amplifier is turned-off. This is an optional modification.

A typical device for this application is the V07E130P from Littlefuse, which is rated for 130 V ac (Allied stock #70184303). Make certain the varistor is placed after the ac line fuse or circuit breaker.

Note 2

As discussed in Chapter 8 of the book, be certain to use hookup wire rated for 600 V dc or above for the rectifier circuit of this supply. Common hookup wire is typically rated for 300 V dc, which is inadequate for the power supply rectifier and filters. Wiring for the voltage regulator can be rated for 300 V with the exception of the plate connections to the 6080 series regulator tube.

The current-carrying capability of the hookup wire used to build this power supply, or any other circuit described in the book or on this site, is usually not a limiting factor except when it comes to the primary and heater circuits. Generally speaking, hookup wire used in the primary circuit within the chassis should be no smaller than #18 (and properly fused). Heater circuits should use wire no smaller than #22; for power output tubes, #18 is usually a good choice. Keep the lengths of these wires to the minimum required by the physical layout. Specifications of standard copper wire for various wire sizes is given in this table (Source: Whitaker, J.C., AC Power Systems Handbook, 3rd ed., CRC Press, Boca Raton, FL, pg 378, 2007.)

Note 3

The "final design" power supply described in Chapter 8 of the book (Figure 8.6) specifies a value of 8 A for circuit breaker CB1. This is more than is really necessary for the expected loads of the supply. A 5 A breaker (or even 4 A) should be sufficient. Consider the following parts:
• 4 A panel mount circuit breaker, TE Connectivity #W58-XB1A4A-4, Allied stock #70198794
• 5 A panel mount circuit breaker, TE Connectivity #W58-XB1A4A-5, Allied stock #70199427

The parts list for the final design power supply (Table 8.7) gives a value for VR1 of 2.5 ohms (cold). VR1 is a power varistor intended to limit current inrush when the power supply is first switched on. After the supply has reached operating temperature, VR1 is taken out of the circuit (shorted) by RYL-1. Upon reflection, a slightly higher value for VR1 will provide greater benefit. A value of 5 ohms (cold) works well with this circuit. Consider using the GE Infrastructure Sensing #CL-40, Allied stock #70181325.

Note 4

In Chapter 8 of the book, Figure 8.6 shows the time-delay relay circuit used to short-out the ac input line varistor, VR1. This circuit consists of a 20 ohm (cold) varistor (VR2) in series with RYL1 (shunted by a large value capacitor) and a power source derived from an available 5 V ac winding on the power transformer. As designed, the voltage across RYL1 builds up as VR2 heats, eventually resulting in sufficient voltage across the winding to pull-in the contacts and take VR1 out of the circuit. The voltage at which RYL1 will activate is about 3.5 V dc. In the book, a pull-in time of about 60 seconds is estimated.

I have used this circuit successfully in all of my amplifier projects. It is important to note that the pull-in time is greatly influenced by the ambient temperature. In all of the implementations I have built, this circuit is contained within the chassis of the amplifier. Due to the other components within the chassis, the ambient temperature quickly rises to a point that the relay will pull in within a reasonable length of time. Having said that, the time delay will be longer on a cold day. For this particular application of the circuit, the actual delay time is of little consequence, since the only function of the relay is to short-out the thermistor in the input ac line. If it takes 30 seconds or 3 minutes to close, there is no impact on the performance of the amplifier. As such, the simplicity of the circuit outweighs the variability of operation. For a critical function, such as actuating some specific device, this circuit would be inappropriate because the pull-in time cannot be closely controlled.

A more sophisticated time-delay circuit could certainly be implemented. The one described in Chapter 8 continues to be used because it is inexpensive to implement and works reliably for the intended application.

Note 5

While checking out the Hammond web site for transformers for a new project, I came across a very useful "Design Guide for Rectifier Use." See: http://www.hammondmfg.com/pdf/5c007.pdf. I hope you find this document helpful.