[AMRadio] Re: Gas and mecury rectifiers

D. Chester k4kyv at charter.net
Tue Apr 29 21:02:54 EDT 2008

> Do you know anything about the pressure in these tubes?  I'm sure
> the mercury vapor rectifier tubes are supposed to be vacated of air, but
> what is the mercury vapor pressure and what is the difference in pressure
> from cold to hot.  I wonder if it is just a result of a vacuum leak that
> causes some type of corrosion on the anode.

I suspect it has to be a pretty good vacuum to ionise the mercury, just like 
a gassy tube turns blue with high voltage.  I have  never found ary articles 
giving the details, but there may be one in my archive of  IRE Proccedings.

> Maybe the reason they need several days for heat up and then slow
> anode voltage increase is to force air out that has accumulated over years
> of setting cold.  This might not be possible in a vacuum rectifier but in 
> a
> mercury rectifier it might be that the mercury vapor pressure might 
> displace
> enough of the air forcing it back out 5the hole it came in enough to allow
> it to work or perhaps the lengthy time is just causing a more homogenous 
> mix
> of the vapor and atmospheric gasses.

I suspect it would push mercury vapor out as well, and maintaining a 
constant ratio of MV and contaniments inside the envelope, since heating the 
filament enough to push air out of the hole would also vaporise the mercury. 
I think the ash is a reaction between the mercury and the anode metal.  It 
is probably a side effect of some underlying change in the structure that 
causes the tube to arc over, since with even a tiny speck the tube seems to 
become prone to arc.

It also reminds me of  some grades of steel which, when heated red hot, 
sheds its outer layer of slag with a similar looking ash.

Don k4kyv

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