View Full Version : STA amplifiers now featuring valve runaway protection

Radford Revival
26-10-2016, 15:34
We have been incrementally improving our amplifiers over time. The latest improvement is an active protection circuit to protect the amplifier against runaway output valves. This means the risk of a bad valve smoking up components or even damaging an output transformer is significantly reduced. This circuit can be used on either STA25s or 15s. We have previously been using HT fuses which also work well, but this reacts faster. We have also retained the fuses for another layer of output stage protection.

The circuit employs a 'two-tier' approach, where minor over-currents are subject to a time constant so that transients from music do not unduly trigger the circuit, and an instant trigger threshold to instantly cut off the HT during a more severe fault. The high voltage AC secondary is switched rather than the DC side of things as this is much easier on the relay. This has proven to work quite effectively.

The amplifiers will now come supplied fitted with this circuitry neatly affixed to the output transformers and can also be retro fitted to our existing amplifiers.

New PCBs were designed for this purpose. The board with the slots hold a power resistor, and the slots are just for convective air flow.