Paul, a number of years back I installed a couple of mobile rigs in my car.
The power leads were fused using the screw in type holders used in our radio
gear. I measured the voltage drop in the holder plus fuse and it was not
insignificant. It also varied a bit from time to time. I had some of the
larger fuses (about the size of your little finger with nice screw in
holders. They were much more stable and had lower voltage drop. Sorry that I
don't have any numbers and do not have the fuse types at hand. I do not know
where automotive blade type fuses fall but would check the voltage drop in
the fuse holders to be sure they are stable
73, Dan, N5AR (77dx owner also)
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]On
Behalf Of Paul Christensen
Sent: Friday, January 23, 2009 10:59 AM
To: amps@contesting.com
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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