Guitar Amplifier PSU

Tubes (thermionic valves) have never departed from the amplified instrument scene and the majority of guitarists, including very young ones, wouldn’t use anything else. Some diehards think that the H.T. (high tension) rectifier should also be a piece of glass-ware and some manufacturers are still producing amplifiers incorporating one. The nett effect is really that a rectifier tube acts as a relatively effective heat-dissipating resistor, causing the HT rail to sag as output signal loading increases, generating a compressive characteristic which is fundamentally added distortion(‘crunch’). The traditional arrangement uses a centre-tapped HT winding on the power transformer but this has a number of drawbacks for an adequately rated core size including increased voltage stress, small wire size and a poor utilisation of the available winding window.

Circuit diagram:

Guitar Amplifier PSU Circuit Diagram

The example arrangement shown here reduces both of these problems and for a given core increases the current delivery capability of the winding by allowing the use of a heavier wire gauge. Normally some resistance is added in series to each anode to limit peak cathode current to minimise cathode-stripping during the high current pulses delivered to the input filter capacitor at each voltage peak.

Even if one includes such resistance (and a single resistor in series with the cathode or winding achieves the same end albeit with double the device dissipation) the benefits to the transformer of reduced voltage stress and increased wire insulation thickness (which scales with wire diameter) along with decreased heating in the windings, are obvious.

Alternatively, a smaller winding window (reduced core size) may be employed with-out diminishing power-handling capacity. The circuit shown here should is typically intended for the amplifier preamp and phase splitter stages. Due to the use of the EZ81 (6CA4) tube its maximum output current is about 100 mA. Higher currents call for a more powerful rectifier tube and diodes to match.

Author : Malcolm Watts (New Zealand) - Copyright : Elektor Electronics