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Cap size for Hg Rectifiers

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Steven-H

Member
Joined 2007
All,
I have tried to look at some data sheets in the last couple of days, but I haven't been able to find this answer....
What is the acceptable input cap value for Mercury Vapor rectifiers (Namely interested in the 83/816) to make it live a happy life ?

Thanks !
Steve
 
S

Steven-H

Member
Joined 2007
Thanks Gents....This leads me to another question.

In looking here:
http://www.diytube.com/st35/ST35_5AR4PS.pdf

One can see the PSU arrangement. I will be subbing in an 83 tube, so by taking your recommendations I will be ditching C9, and increasing the choke to 5H or so. Any other changes ? I already have a timer delay circuit in place, so the preheating issues are taken care of.

Secondly, the main cause for me asking this question is I wish to use a set of 816s in my PSU that will power my preamp/phonostage. I've been playing around with PSUd for a while, but I haven't been able to come up with anything too impressive. I guess I have a couple of issues; first one being that I am using an 83 tube to design the circuit; as PSUd doesn't have the 816 (But I believe it would be pretty close...). Secondly, I was planning on using a set of 0D3 regulator tubes; of which PSUd doesn't have as well. Do any of you have some tried and true suggestions ?

Thanks again,
Steve
 
sorenj07

sorenj07

Member
Joined 2006
Taking out the input cap from the power supply will drop your voltage by a LOT. instead of multiplying the transformer's AC voltage by root 2, you decrease it by 10%. Unless you have a much higher voltage power transformer than the one DIYTube specifies, you won't be able to use this schematic.
 
S

Steven-H

Member
Joined 2007
Thanks ! I thought that I had read/seen that somewhere. Furthermore, I could have SWORN that I've seen some tube amp PSU's with a 2uF cap in them.....

Edit - more info:
http://www.shinjo.info/frank/sheets/021/8/83.pdf

"....Filter circuits of either condenser-input or choke-input may be employed, providing that maximum voltages and currents tabulated under the CHARACTERISTICS are not exceeded"

http://www.shinjo.info/frank/sheets/093/8/83.pdf

Lists specs for both cap-input and choke-input filters. Also states:

"*When a filter-input capacitor is used larger than 40uF is used, it may be necessary to increase more plate supply impedance than min. value shown to limit the peak plate current to the rated value"

So...I am thinking a 20uF cap would be fine....
 
EC8010

EC8010

Ex-Moderator
Joined 2003
rdf said:
Why the big fat 'zero'?
Click to expand...

If you've read some Hg vapour rectifier data sheets, you'll have seen that a temperature range is also included. What it comes down to is that the correct mercury vapour pressure within the envelope is crucial. Because PV = RT, and the volume is pretty well constant, P = RT/V, making pressure directly related to temperature. The pressure is important because the rectifier uses gas amplification to achieve the low forward drop, and that's what leads to the negative resistance. If you're unlucky, the (unknown) value of negative resistance could exceed the positive resistance in the circuit, causing those capacitor current pulses to run away and strip the (oxide-coated) cathode in short order. In a similar vein, fluorescent lights (which also use mercury vapour) have negative resistance and the ballast choke is there to prevent current runaway.

I've been doing a bit of reading around on this one and conclude that choke input is safest...
 
S

Steven-H

Member
Joined 2007
EC8010 said:


If you've read some Hg vapour rectifier data sheets, you'll have seen that a temperature range is also included. What it comes down to is that the correct mercury vapour pressure within the envelope is crucial. Because PV = RT, and the volume is pretty well constant, P = RT/V, making pressure directly related to temperature. The pressure is important because the rectifier uses gas amplification to achieve the low forward drop, and that's what leads to the negative resistance. If you're unlucky, the (unknown) value of negative resistance could exceed the positive resistance in the circuit, causing those capacitor current pulses to run away and strip the (oxide-coated) cathode in short order. In a similar vein, fluorescent lights (which also use mercury vapour) have negative resistance and the ballast choke is there to prevent current runaway.

I've been doing a bit of reading around on this one and conclude that choke input is safest...
Click to expand...

In my circuit, I have included the following:

.
An externally hosted image should be here but it was not working when we last tested it.


So....Being that the tube will be preheated for 90s; wouldn't this usually increase the Hg vapor temp enough @ room temp ? Naturally this is a rhetorical question - but a thought...
 
rdf

rdf

Member
Joined 2004
Informative technical backgrounder, thank you, though I didn't expect an appearance by Herr Boltzmann! Given the relatively large peak current ratings of these tubes, is the 50 ohm recommendation then more a form of 'plate stopper', adding resistance to reactance?
 
rdf

rdf

Member
Joined 2004
Do you mean the minimum 50 ohm recommendation for capacitor input? The Brimar, GE and RCA include it. On second look though the GE sheet explicitly states it's to limit peak current 'to the rated value' so it's apparently due to the very low plate impedances. Now I'm curious what value inductor is small enough to work with small value input caps 'low DCR / LC style'. It's worth some playing around with PSUD2.
 
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