For a number of years, I suggested a gas discharge tube to protect the original Quad ESL treble panels.  and they do work well, but I’ve had them get leaky over time and their striking voltage drops.  I have reverted to the stock Quad ESL treble protection circuit.  The original circuit is a rectifier bridge of 3000 volt diodes, where the input to the treble panel is applied across the AC inputs of the bridge.  A stack of zener diodes is placed across the rectified outputs.  

I’ve designed a little board to fit under the input transformer case.  The schematic and layout are shown here:

Clampboard Schematic

Clampboard Layout