[R-390] Comments on ER Issue 208 AGC modification

[Charles Steinmetz]

For those who would like to follow along with my analysis of the ER 
Issue 208 AGC modification (12/25/19), but who don't have a copy of the 
ER article, it is on ko4bb.com.

To download it, go to the "manuals" page of the site 
<http://www.ko4bb.com/getsimple/index.php?id=manuals> and enter "r390a 
agc" into the search box (without quotation marks).

Best regards,

Charles


> David identified a hopelessly muddled (and, thereby, mistaken) paragraph
> in my original post and offered a correction with specific instructions
> for rewiring the AGC switch (S107) to accomplish what he correctly
> inferred that I intended.  Responding to David's post, Larry offered an
> alternative and somewhat easier re-wiring proposal.  My abundant thanks
> to both of them.
>
> David and Larry both managed to understand what I meant.  The key point
> is to rewire the AGC switch to choose among three, grounded capacitors
> to prevent the large AGC voltage step on switching.  This is easy to do,
> and, as David and Larry show, there are several ways to organize it.  [I
> have not traced the switch connections for either of their proposed
> fixes, but they are both careful workers so I'm confident their
> proposals will work as advertised.  I also have not compared their
> suggestions to my notes ("They're around here somewhere..."), so I'm not
> sure which, if either, is exactly what I do.]
>
> In my original "Comments," I did not intend to provide step-by-step
> instructions for curing the "moment of silence."  In this vein, I am now
> drafting a revised, PDF version, and will replace the muddled paragraph
> with the following:
>
> "CURING THE MOMENT OF SILENCE:  To cure this -- and only this -- you can
> very easily modify the AGC circuit by removing the capacitance
> multiplier tube (V506A) from the circuit and re-wiring the switch to
> select three plastic film capacitors in sequence, each of them grounded.
>   To produce the same AGC release behavior as a stock R390A, one would
> use 0.1uF, 2uF, and 20uF for Fast, Medium, and Slow AGC, respectively.
> [In my view, the "Slow AGC" capacitor should really be much larger, from
> 50 to 100uF -- I frequently use 56 or 68uF as a practical compromise.]"
>
> While I did not originally intend to provide step-by-step instructions
> for curing the "moment of silence," I see no reason not to include
> David's and Larry's contributions in the PDF version with appropriate
> attributions and thanks, if I have their permission (please let me know
> offline).
>
> One further note:  While the "cure" prevents the massive DC transient on
> the AGC line and the associated "moment of silence," the "Medium AGC"
> and "Slow AGC" capacitors can still be left with residual charge (and,
> therefore, non-zero voltage) when switching to one of the other
> capacitors.  It is good engineering practice to include bleed resistors
> from each of these two capacitors to the AGC buss, so that the voltages
> on the capacitors are maintained near the current AGC voltage and even
> these much smaller voltage transients on the AGC line are minimized. The
> bleed resistors must be very large, with time constants of a minute or so.
>
> Larry asks if I know who originated the "two diode" AGC mod.  I do not
> know who thought of it or did it first.  It seemed obvious to me, and I
> have been doing it myself since the early 1960s.  I mentioned it
> subsequently to other R390A tinkerers and found that some of them had
> also thought of it on their own.  By the mid-1960s, quite a few folks
> were doing it.
>
> Best regards,
>
> Charles
>