What is the inrush current of a good filament or does the slow warmup
process mitigate that? Ive never looked at that so havent an answer.
Carl
KM1H
----- Original Message -----
From: "Paul Christensen" <w9ac@arrl.net>
To: <amps@contesting.com>
Sent: Friday, January 23, 2009 1:58 PM
Subject: [Amps] Alpha 77Dx Filament - No Free Lunch
> Earlier, I had posted a comment about being able to "bring life back"
> to my
> 8877 tubes where they had developed an intermittent short between
> cathode
> and filament.
>
> One astute observer thwarted my thinking with this set of
> hypotheticals.
> I'm withholding the name as he replied privately: Here are the two
> possibilities presented:
>
> "Example 1: 8877 has a short between left pin filament and cathode.
> A hard connection to the left pin and the world seems to be OK.
>
> Example 2: The tube has a short cathode to the right pin. A hard
> connection cathode and left pin will make a short of transformer via
> choke.
> Current flows from the right pin to the wired connect to the left.
> Both pins are now tied together. Danger for burning out, who is
> stronger choke or transformer?"
>
> So, depending on the location of the intermittent short between
> filament to
> cathode, tying the cathode & filament together at the tube socket may,
> or
> may not work. I then replied with this:
>
> "How about a SPDT relay at the tube socket, controlled by a front
> panel
> switch to select which side of the filament the cathode is switched to
> !!!!
> Now there's an interesting thought!"
>
> So, there really is no guaranteed free lunch in trying to breath life
> into
> the 8877 this way. Success is wholly dependent on the location of the
> intermittent contact.
>
> For those of you with 8877 amps where the cathode and one side of the
> filament are tied together -- have any of you experienced a
> catastrophic
> fault as a result of Example 2 above? Seems like the limiting factor
> will
> be either the transformer winding, hook-up wire, or the filament
> itself to
> see which of the three finally opens up first. Now I'm thinking that
> the
> filament secondary to the 8877 should be fused with a 15A automotive
> fuse as
> the primary fuses may not blow in time.
>
> Paul, W9AC
>
>
>
>
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