[R-390] AGC voltage issue

[David Wise]

You have forgotten something.

Max current is less than 5mA due to R549.

["Best-case design" rant snipped]

I won't hunt you down for using film/foil!  <g>
I can appreciate the satisfaction of using the best
materials money can buy.

But only at a distance.
In this particular case, my personal cost-benefit meter is
firmly pegged at the Cheap end of the scale.  My C551 is,
and will remain, Metallized.  There's another satisfaction in
being Just Good Enough.

Best regards,
Dave

* Yes, I used 400V in my article.  After what I've
learned in the past few years, now I say 630.
If you want to edit the pdf at r390a.net, go for it.
Sometime I'll measure my peak voltage, and if it exceeds
400, I swear I'll change it out.  Not to would be a
disservice to the next guy.

________________________________________
From: R-390 [r-390-bounces at mailman.qth.net] On Behalf Of Charles Steinmetz [csteinmetz at yandex.com]
Sent: Tuesday, September 16, 2014 5:11 PM
To: 390 list
Subject: Re: [R-390] AGC voltage issue

David wrote:

>Self-heal events (characteristic of metallized film) are harmless in
>C551, the AGC Time Constant cap.

While the radio is operating normally, that may be so (although C551
can end up with >200v across it with strong signals, as you noted in
your AGC article, and my tests indicate that a metallized film cap of
any voltage rating has self-healing events above 200v).  But during
the startup condition I described in my message, C551 may have >400v
across it for some seconds every time you turn on the radio,
positively ensuring that there will be self-heal events.

If your point is that self-heal events at this location would not
matter even if they occurred, I disagree.  Any self-healing current
would flow through the grid of V506A.  Cumulative occurrences would
damage V506's cathode.

Best regards,

Charles