Recently we had an SB-200 which suddenly refused to work. I'm not clear as
to the exact symptoms as I wasn't there at the time. Previously the unit
had performed flawlessly for some years after being upgraded with the entire
suite of "Harbach mods ".
While looking at it for signs of trouble I found one resistor (R7) on the
PW-200 power supply board that was definitely "charred" ... however after
removing it I found it still showed continuity with 900 Ohms resistance (R7
is a 910 Ohm, 2 W). I think that this resistor provides the ALC bias
voltage.
When I removed the 572 B tubes (both GE) I noticed that one of the tubes
had an "object" rattling around inside, completely unattached to any of the
tube's internal structure. This "thingee" looks like a small spiral sprint,
one end of which (i.e. a wire) is bent over and passes entirely thru the
spring from one end to the other and protrudes quite beyond the other end.
I'm guessing that the spring body is about 3/8" long and the wire protruding
out the other end is maybe another 1/4 " in length. At the end of this
"wire" is a "hook".
Looking at the tube from the top, there is a ceramic piece roughly in the
form of a cross. An identical "spring" is recessed in one arm with the
"wire" passsing through the ceramic down into the interior of the tube and
apparently hooked onto the wire mesh structure that I take to be the grid.
On the other side of the ceramic cross is a empty recess with a tiny slot in
the bottom that obviously once contained the "loose" spring/wire piece.
I know very little about the physical structlure of such tubes. Obviously
the fact the this item became unattached is a "failure" of some sort. (With
a replacement tube the amp's OK.)
But what is the function of this piece, what does it's loss do to the tube,
how could it just fall out ?
I'd appreciate any thoughts anyone might have. I'll probably never run into
it again, but after all these years, it's got my curiosity up.
Probably best to reply directly ..it's not a high profile issue.
Thanks,
Joe, W5ASP
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