{Collins} Re: 30L-1 & (shorted) 811-A questions



The tube with the "cooked" resistor is probably the
bad one.

You can do a check with that tube removed and not hurt
anything.

If the other tubes are in good shape you can just
replace the bad tube.  If you purchase new tubes
remember that Collins Radio NEVER installed "matched
sets".  Basically all that matching generally does for
radios in which the tubes are in parallel is to
increase the profit margin of the supplier.  Now there
are exceptions, especially where sweep tubes are
involved.  However, that has to do with the very broad
tolerances of the very few manufacturers of sweep
tubes today.

Unfortunately, especially the Chinese manufactured
811A tubes often develope shorts.  The fact that the
30L-1 has the tubes mounted horizontally results in an
even greater tube failure.  The old U.S.A.
manufactured 811A tubes do not usually have any
problems.  But, those tubes that are manufactured "off
shore" often do fail and many times after only a
relatively short period of time.  The Russian tubes
(primarily Svetlana) have a much lower failure rate
than the Chinese tubes.

Those tubes sold under the Taylor brand are now of
Chinese manufacturer.  The stateside supplier
supposedly "tests" the Chinese tubes and "culls" the
best to be stamped with the Taylor brand.  The rest
are sold as "Chinese" tubes.  However, the quality of
even the Taylor tubes is not generally anywhere near
as good as the original Taylor, RCA, and other U.S.
manufactured tubes.  Now the supplier "claims" that
the new Taylor tubes are "just as good" as the older
U.S. tubes.  However, I am definitely not convinced
that this is true.

Glen, K9STH


--- Al Schapira <a.d.schapira@xxxxxxxxxxxxxxxx> wrote:

Just after a QSO in which I touted my KWM-2 and 30L-1,
the 30L-1 let out some smoke after showing the classic
symptoms of a grid-filament short in (one of) the
811A's -- the 30L-1 drew too much plate current at
idle, and the relay pulled in even in RX mode.
 
I opened her up and found *one* of the 47 ohm grid
resistors cooked, the others being okay (two were
within 5% and the third was 10% high).  All other
resistors/capacitors/diodes that I measured were
within spec.
 
Before I proceed with the repair, I would like to get
the opinion of the list experts on the following
questions:
 
  - Is it correct to conclude that only the one 811A
with the cooked grid resistor has the short?
 
  - Is is okay to replace only that one 811A, or do
they have to be replaced in sets? 
 
  - Is there any way to test for a grid-filament
short?  I do not have a tube tester that will do an
811A. (DC ohmic tests reveal nothing unusual.)
 
  - Is it okay to power up the 30L-1 (on the test
bench) with only one 811A installed?   What about with
only three 811A's installed? Will this damage
anything?  (Maybe I can confirm that I have three good
811A's that way.)









Glen, K9STH

Website:  http://k9sth.com


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