What Sound Level Can Quad ESL57 Produce?

[sheep]

Not sure if my sample is bad, but I don't think it can do more than 90dB without damaging them.
From measurements a vintage Quad ESL-57 produced 81 dB with 4.4V input @1kHz.
What is max output level for a ESL-57 before arcing occurs? What is typical sensitivity in good condition?

[Bazukaz]

According to spec. max output should be about 100db , but the voltage is not specified. Sensitivity around 85db/1w or so. Arcing voltage is stated as 33Vpk at inputs but IMO its not safe there.I would not drive those above 20-25Vpk unless protection is installed.

Old units can have problems affecting sensitivity(most common) :
1) Failing diodes of EHT supply
2) Loss of coating conductivity
3) Arced treble units
4) Leaky panels

First you must make sure these old speakers are charged properly. This may take hours or even days.
The next thing is to check is voltages of the EHT supply. This could be done by using HV probe. Alternative solution is to connect HV 1GOhm resistor in series with multimeter having input resistance of 10Megs, making a resistor divider. This will show somewhat lower reading than really is(maybe 20-30%) because EHT supply is loaded by 1GOhm measurement device significantly.

Regards,
Lukas.

[arend-jan]

Bazukaz is right. I can say from experience that most unserviced Quad ESL 57s that seem OK will show a sensitivity loss of anywhere from 2 to 5 dB, depending on how they have aged.

First thing is to make sure the power supplies are good. If they are no good everything else is moot.