[Collins] R-388 second mixer weirdness

[Craig Roberts]

Here's a baffling problem that's been plaguing me for months.  It's one 
of those "every time I think it's fixed, it comes back" weirdnesses:

After my R-388 has been playing for awhile (anywhere from a couple of 
hours to a couple of days), the sensitivity drops off dramatically.  
Replacing the second mixer tube (V-106 -- a 6BE6) solves the problem.  
The strange thing is, the tube doesn't have to be a new one.  It can be 
a used one, a JAN tube, a civilian tube, or even a tube that's been used 
in the same position before -- as long as it's different than the one 
currently employed.  This sounds heat related, but V-106 doesn't get 
unduly warm and the priblem may not show up for many, many hours and -- 
apparently -- the tube's not being destroyed.  Voltages seem okay.

Upon the advice of an old Collins factory tech, I replaced all the 100 
pf micas in the set -- but still... 

Anyone else run into this?

Thanks and 73,

Craig