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

[Jim Whartenby]

Larry
Agreed, G2 will have some affect on plate current.  The tube manual data does not specify what happens when G2 is at the same potential as the plate and both are above 180 vdc.  To determine just how much of an affect this will have, one will need to do some experimenting at various B+ voltages from say 180 to 250 vdc.  That said, the 6AK6 still appears to be operating within design center values.

In addition, cathode self bias is used so the higher the plate current, the more negative the control grid bias.  Add to this that there are three feedback paths really complicates a paper analysis.  For some insight of what the designers took into account, the below is from the R-390 Final Engineering Report.

"Local Output 
One half of a 12AT7 grid is fed from the local gain control. Itsplate is coupled to the grid of the 6AK6 local output tube. Negativefeedback is employed from the plate of the 6AK6 to the cathode of the12AT7 to reduce the output impedance. This negative feedback reducesthe voltage gain considerably and in order to bring it back up to areasonable level a small amount of positive feedback is provided bymaking a fraction of the cathode resistors of the half 12AT7 and 6AK6common to both tubes. The effect of the positive feedback is to cancel the negative current feedback developed across the cathode of thehalf 12AT7 and so raise its gain."


The Navy EIB 895 was an option, not a requirement so I have no idea how often this modification was actually incorporated into the R-390A.  Still think it is poorly thought out and that perhaps the addition of a screen grid resistor might have been more effective and much easier to implement.  But this would again affect gain and further complicate matters.

Regards,
Jim

Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy

-----Original Message-----
From: Larry H <larry41gm at gmail.com>
To: Jim Whartenby <old_radio at aol.com>
Cc: r-390 at mailman.qth.net <r-390 at mailman.qth.net>
Sent: Fri, Oct 14, 2022 2:21 am
Subject: Re: [R-390] R-390A Use of Stand By on Function switch

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