The Quad ESL-63 has gone through several revisions of the input protection boards.  The latest design uses a bridge rectifier and zener diode stack similar to the treble panel protection circuit in the Original ESL’s.  This is combined with a simplified arc detection and clamp circuit.  This latest design has been used in all the subsequent Quad ESL models from the 988, 2805, and 2812 models.  

The arcing clamp circuit senses the electrical field from an arc with a short antenna by rectifying the signal with a diode.  This rectified signal turns on a transistor which clamps the input on a 555 timer chip wired as a one-shot.  This then turns on a TRIAC which shorts the input to the speaker.  

The OEM arc clamp board also contains the input ballast resistors as well as a capacitor “zobel” for the input transformers.  In the OEM circuit the input series resistor (1.5ohms) and parallel capacitor (220uf) are mounted to the chassis.  

I have designed a replacement zener clamp board which is much like the OEM unit, but uses surface mount zener diodes.  I have also designed a replacement arc clamp board which uses precision ballast resistors and also incorporates the input resistor and parallel electrolytic capacitor.  And a needed improvement is provisions for adding a healthy sized film bypass capacitor across the input electrolytic.  

The schematics and layouts of both boards are here:

ESL-63 Arc Clamp Board Schematic

ESL-63 Zener Clamp Board Schematic

ESL-63 Arc Clamp Board Layout

ESL-63 Zener Clamp Board Layout

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