[R-390] RE: 390A relay problems

[2002tii]

Tim wrote:

>Of course, different people have different expectations. Some want
>to protect all the internal circuitry against any hazards from normal
>line voltages or rapid turn-on, whereas others like me are quite happy
>to blow out those ancient electrolytics if they aren't up to snuff :-).

I honestly do not see the need for "inrush limiters" with properly 
designed radios like 390s, SP-600s, etc.

The two primary concerns would be filaments and filter 
capacitors.  In 35 years of playing with tubes for a living 
(including 15 of repairing and modifying tube equipment and 15 of 
designing it), I saw very few filament failures in 6 and 12 volt 
tubes operating from transformer secondaries -- and the failures 
virtually always occurred either very late in the tube's life when 
transconductance was way below spec, or very soon after new in the 
case of manufacturing defects.  (Failures were more common in the 
high-voltage filaments of certain tubes designed for "series string" 
applications, where the tube filaments are connected in series 
directly across the AC line as in cheap TVs and radios.)  My 
conclusion is that filament failure is simply not a well-founded 
worry with modern (octal base and newer) receiving tubes with 
indirectly heated cathodes operating from transformer 
secondaries.  By the time the filament of any modern receiving tube 
fails, the tube should have been replaced long ago due to weak 
emissions or transconductance.  (Yes, I know, these radios work more 
or less normally with very flat tubes, so it is possible that one 
might experience a filament failure when the radio still appears to 
be working properly.  But that doesn't mean you should leave tubes in 
the radio that long as a matter of course.  If you re-tubed a radio, 
you wouldn't ask for tubes that all had 30% of their minimum rated 
transcondance, would you?)

Pre-octal tubes (and particularly filamentary tubes rather than tubes 
with indirectly heated cathodes), carbon-filament lamps, and other 
antiquities are another matter.

As for electrolytic filter capacitors, by the time they are ready to 
fail, they have long ago ceased working to specification.  I see no 
virtue in babying them to prevent outright failure when they are not 
really doing their job properly anymore.  Failure just makes it 
necessary to attend to something that, if one were doing proper 
preventive maintenance, would have been taken care of long before.

Best regards,

Don