[ARC5] Filament Power Sources

[Fuqua, Bill L]

  By the way, a lot of these 6 volt and 12 volts tubes were operated off automobile or aircraft starter batteries
which can provide loads of current. I don't think inrush current from the PC supply would be an issue. 
73
Bill wa4lav

________________________________________
From: arc5-bounces at mailman.qth.net [arc5-bounces at mailman.qth.net] on behalf of Scott Johnson [scottjohnson1 at cox.net]
Sent: Saturday, August 24, 2013 1:09 PM
To: 'Ian Wilson'; 'Robert Eleazer'
Cc: 'ARC5'
Subject: Re: [ARC5] Filament Power Sources

The PC supply is voltage regulated, and probably current limits at 30-50 A.
The cold resistance of the tungsten filament is about 10% of its operating
resistance, so the recipe for disaster is there, especially if the tube is
old and has thinned spots in the filament (common, and the usual reason for
failure of the filament.  It would be better to change the current loop
resistor to a value of perhaps 120% of the operating current of the
filament, which will result in a nice slow warmup.

Scott W7SVJ

-----Original Message-----
From: arc5-bounces at mailman.qth.net [mailto:arc5-bounces at mailman.qth.net] On
Behalf Of Ian Wilson
Sent: Saturday, August 24, 2013 9:59 AM
To: Robert Eleazer
Cc: ARC5
Subject: Re: [ARC5] Filament Power Sources

I have blown a filament on a 4-125A when using a PC power supply. While a
single case doesn't prove anything, this has never happened when using a
transformer for filament power.

I wonder whether the inductance of a real transformer acts to limit the
starting transient, whereas a switching PSU is much stiffer and might stress
a brittle filament on switchon. I am told that PC PSUs can be made somewhat
soft-starting by adding a capacitor across the reference voltage resistor,
but haven't tried it.

73, ian K3IMW



On Sat, Aug 24, 2013 at 5:35 PM, Robert Eleazer
<releazer at earthlink.net>wrote:

> If you needed a lot of filament power at 6V, would there be anything
> wrong with adjusting a 5VDC power supply up to 6V?  I have one of
> those that will adjust up to 6V via a pot control and it'll put out 100
amps.
>
> Wayne
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