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

The vast majority of the original Quad ESL’s have rectifier modules that consist of a bakelite box with a circuit board sticking out the top and filled with bee’s wax.  This module can reasonably be rebuilt by heating the blocks in an oven (at 275F) to melt the wax out, and then replacing the components on the circuit board.  When complete, the molten wax is then poured back in the box with the circuit board and allowed to cool.

However, the early Quad ESL’s came with epoxy potted blocks that are not rebuildable.  And occasionally I’ve come across speakers which are not complete, and the rectifiers are missing.  I designed a replacement rectifier block as an upgrade to the original design.

My design uses the same cockroft-walton voltage multiplier as the originals, but rather than running the panels straight off the multiplier, I isolate the panels through a series resistor and a neon bulb (like the Quad ESL-63 circuit).

SDS Labs Quad ESL EHT Schematic