[R-390] R-390A Use of Stand By on Function switch

[Larry H]

Jim,  As I understand it, a typical pentode's plate current varies as the
screen voltage varies.  In the 390A, the screen voltage is always higher
than the plate voltage, so when the audio B+ goes up, the screen and plate
go up about the same amount.  This causes the cathode current to rise quite
a bit when they go up.

When the screen voltage is at a constant specific voltage, then varying the
plate voltage drastically does not cause the plate current to vary
that much.  But, that is not the case in the 390A, the screen voltage goes
up when the B+ goes up.

When the Navy converted the tube rectifiers to silicon diodes (without the
dropping resistor), the audio B+ went up causing the audio output tubes to
deteriorate at an unreasonable rate, thus eib 895 to fix that problem.

Regards, Larry

On Wed, Oct 12, 2022 at 10:50 PM Jim Whartenby via R-390 <
r-390 at mailman.qth.net> wrote:

> Larry I looked for maximum cathode or plate current specs but did not find
> any listed in the 6AK6 RCA datasheet.  The European tube manufacturers
> quote 21 mA as the maximum cathode current.  The curves that accompany the
> tube data indicate that plate current will increase less then 1.0 mA when
> the plate voltage goes from 200 to 300 volts.  This is what one would
> expect from a pentode.
> I have no long term maintenance experience with the R-390A.  Perhaps
> someone who maintained a group of them will share their military experience
> with the reliability of the 6AK6.  The only thing that comes to mind is a
> leaky coupling capacitor driving the 6AK6 stage.  Other then that, I am at
> a loss to understand what the issue is and why the Navy issued the EIB 895
> maintenance tip on the 6AK6 as used in both the R-390 and
> R-390A.  Regards,Jim
> Logic: Method used to arrive at the wrong conclusion, with confidence.
> Murphy
>
>
>