[R-390] 3rd Mixer noise, STINKIN' COSMOS PTO 8^)

[Barry Hauser]

Phil wrote:

>Now, you can ask, why didn't I test the tube earlier?  I DID, before doing
>an alignment of the PTO and it tested good.  Checking it now shows a
shorted
>tube with VERY low transconductance!

I was going to suggest a bad tube as most likely, but you said that you
tested them all.  What probably happened is that the tube was good or
perhaps becoming borderline with no initial shorts indication.  But then,
you were repeatedly installing, removing, fussing with, reinstalling, etc.
that PTO.  Tubes sometimes don't tolerate shock very well and they're most
prone when plugged into a chassis that's being moved around -- especially if
the tube is still hot -- which I suspect happened as you were working on the
PTO from the way you described it at the time.

BTW-- how do you know the transconductance/quality was low?  If you get a
shorts indication on a tube tester, as a rule you should never go to the
quality test, as you risk damage to the tube tester.  An exception might be
if you can clear the short while in the shorts test by allowing the tube to
warm up and tapping on it.  However, that kind of fiddling is basically
pointless, as the tube should be discarded.  (The purpose of tapping on a
tube is to check for an intermittant short in a suspect tube -- i.e. to
force a short, not temporarily finesse it away.)

>THE SICKENING THING IS I AM PROBABLY NOW GOING TO HAVE TO GO THROUGH A
>SEVERAL HOUR PROCEDURE TO SET ALL THOSE 48 LINEARITY SCREWS again. . .

Go easy on the PTO as you do this and periodically check the tube if you are
going to be repeatedly removing/installing the unit. Put a piece of foam
down on the table where you are going to set the PTO.  When re-installing,
avoid bumping it. Another consideration -- short-proneness and other latent
defects tend to run in families -- production runs.  So, if your supply of
6BA6's all came from the same lot, it may well happen again.

Barry