I don't think its filament sag so much as the lower voltage drop. An indirect cathode has lower resistance than a directly heated one.
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On 10/22/2021 4:26 AM, Chuck Harris wrote:
The whole point behind the 5AR4 was the indirectly heated
cathode. It was a great benefit over the 5Y3 directly
heated cathode family because it could be used in any
position, without risk that the filament would sag into
one of the plates.
If this 5AR4 is really directly heated, that could be why
the short occurred. The rectifier tube was operated
horizontally, in the wrong rotation, and the filament's
normal sag allowed it to touch, and get welded to, the
plate electrode.
How could it happen that a 5Y3 was relabled as a 5AR4?
Well, fresh manufacture tubes started to wind down in the
late 1960's, through today, and there is pressure to
sell whatever you can/could make to as many customers as
possible.... and hope for the best. A 5Y3 would work just
fine most of the time... a little under rated, perhaps.
And, most engineers from the era automatically oriented
the 5V rectifiers so that if the tube was horizontal, the
two plates were side by side, rather than cocked at an angle.
-Chuck Harris
On Fri, 22 Oct 2021 08:20:19 +0100 "Dave_G0WBX via groups.io"
<g8kbvdave@...> wrote:
From: Dave Wise
<mailto:david_wise@...?subject=Re:%20HP200CD%20oscillator>
Date: Thu, 21 Oct 2021 19:34:09 PDT
If that was my tube I’d say it’s not a 5AR4.? Every datasheet shows an
indirectly heated cathode.
------------------------------------------------------------------------
Hi.
The data sheets I've looked at, such as:-
Show an indirectly heated cathode, but joined to one side of the
heater at Pin 8.
But, if it has an internal short, intermittent or for any other
reason, it is as we would say "Junk"...
73.
Dave G8KBV.
--
Richard Knoppow
dickburk@...
WB6KBL
