Mercury Wetted Relays

Is there anything "special" about these for handling audio signals ?
.....and...
...are they implemented in much the same way as electro-mech. types ?
(the ones I have, need to be directionaly pointed in the "up" position)
I've found little info so far.

I believe these are not made any more....too toxic to be in the care and trust of humans :razz: .

But they still can be found in the surplus market.
--------------------

The ones I have are:

The Adams and Westlake Co.
"High Speed Mercury Wetted Relay"
AWCB-16632 D045

I can't find any documentation, but the PC pin type pinout (not octal plug) arrangement is:
(w/o dots, "on..on" is the vertical top position)

NC...........NO
.................
.................
.................
................
-..............+
- ............+
.......C........

These are the markings on the underside.

I haven't tested them yet to determine the coil pins, but do you think it's a single coil ?
(edit) no, it's a double coil.

=FB=

This PDF seems close to what I have, and it looks as if I might have a double coil......but according to that manufacturer, the lower pins should be
2 pluses
2 minuses
and a pole


that's not what is marked on mine.
????

=FB=

[quote author="Freq Band"]Is there anything "special" about these for handling audio signals ?
[/quote]
Mercury wetted rellays = very low noise.

It is metalurgic contact. It behaves like soldering.
Nothing is better (... and nothing is harder to find)
Very long ago mercury relays was rather common.
Before invention of reed relay, mercury ones was the only way
to get smal and good relay. Robert Hermeyer in Berlin was producer
of mercury relays in 1950-ies. That relays performed like thermometer. If some current was fed into control branch, gas inside expands and moves mercury in the capilar. Mercury then connect two platinum wires inside capilar.

Today, we use MOSFETS instead that relays, but for special purposes mercury relays could have better dynamic range.

xvlk

I designed in Hg-wetted reeds, switching low-Z bulk metal foil Vishay resistors, for the Harman R&D group's fancy computer-controlled attenuator, although we never finished the project as they were having some budget problems. The design achieved about a 144 dB signal-to-noise ratio, so that I could say we were doing a real 24 bits.

The relay's on resistance is a little higher than direct clean gold or palladium reeds etc. sometimes, but as xvlk says it's like soldering and represents a new metallurgical junction each time you make. They are usually position-sensitive although there were some "mercury-film" ones that were not.

Another advantage of Hg-wetted: essentially no contact bounce on make-break. People have used them as fast risetime pulse generators (with of course huge jitter and very slow repetition rates).

For switching high power there are big clunky things called mercury displacement relays, where a plunger is pulled down into a mercury bath to push the mercury up to make contact with the electrodes. Magnecraft makes them, and the big ones have about a milliohm of on resistance and switch a hundred amps. They are used for "mission-critical" applications and for situations where there could be explosive atmospheres. But they have the same "dry" contact behavior as a mercury reed. When the original Harman design was going to switch speakers and amplifiers, I specified these (or maybe the somewhat smaller ones) for that portion of the switching, and was considering showing how good the switch was by running an MC cartridge signal through them and noting the lack of degradation :green:

Neumann used mercury wetted relays in the later versions of the SAL/VG74 cutterhead drive electronics. They disconnect the 600w power amplifiers from the cutterhead when the temperature of the cutterhead drive coils exceeds 200 degrees centigrade. The earlier versions used reed relays.