A 2A3’s PSE Amplifier

(click on images for enlargement)

          A structurally simple but effective design about a powerful and “cheap” Parallel Single Ended power amplifier is shown in Figs. 1 and 2. As a rule a 2A3’s PSE yields 7-8 Watts in Class A1 operation with 5% of THD and 10 Watts in Class A2 operation with greater distortion.

Simple single ended amplifiers with two or three gain stages always exibit  any form of harmonic cancellation due to inherent opposite phase characteristics, therefore the designer’s target is to emphasize this aspect without twisting the circuital semplicity. 

From Fig. 3, by slightly overloading the first stage, i.e. reducing its load resistance in order to facilitate the production of higher order harmonics, you can attain to an optimal figure on the THD and an increase in the useful power, provided this operation is specific and prudent on the same time.

Fig. 1 Amp Schematic
 

In order to alter the first stage load you can act on R10 or R6. Nevertheless a reduction in R6 values forces you to increase the capacitive value of C1 ito avoid a premature cut of the reproduced low frequencies. Since good sounding caps

Fig. 2 Power Supply Schematic
 

becomes more and more expensive when the capacitive value increases, I prefer the solution that aims to reduce  R10 although  it can drive the tube bias into dangerous areas with large dissipations and resulting useful

  Fig. 3 THD Dependance on R10

life reduction. Fortunally such effects are mitigated, partially, by the cathode resistor that acts like a current limiter. In this design, the first stage builded around an E188CC (7308) Vacuum Tube dissipates 1.17W vs. the 1.65Wmax allowed.

  Fig. 4 THD on Output in Class A1 Operation Mode