Figure 1 Circuit Schematics - Crowbar circuits. This article was originally written in March 2008. Additions were made in January 2013. One way to protect a circuit from over-voltages is to use a "crowbar circuit".. Dynaco Push-Pull EL84 / 6BQ5 or 6V6 / 6AQ5 Tube Amp Schematic. The EL84 (6BQ5) / 6V6 Push-Pull tube amplifier schematic and text below is from the Dynaco Super-Fidelity Output Transformers catalog circulated around 1955..