What would cause these grid resistors to fry?

[Bowie]

  A client of mine brought his vintage V76 preamp in for fresh tubes and though a couple were weak, there was something else causing a severe gain issue, only getting around 30db instead of 76.  I traced it down to two bad grid resistors on the first and 3rd stage EF804 tubes.  They are the 1M R59 and R73 on the schematic below.  The resistors did not fry open, they were closed/no continuity.  I replaced them and all is well but I'd like to know what might have caused this to happen. 

  Voltages all look good now.  Client had this preamp "repaired" by a tech that the client feels screwed him over.  The fact that one of the boards was not re-mounted correctly and lets the input transformer flop around tells me that the work was poorly done.  Client says preamp worked for a couple hours before the gain dropped (must be when the grid resistors died).  What should I be looking for to make sure this doesn't happen again?
Thanks!

 

[abbey road d enfer]

  A client of mine brought his vintage V76 preamp in for fresh tubes and though a couple were weak, there was something else causing a severe gain issue, only getting around 30db instead of 76.  I traced it down to two bad grid resistors

[veer]You mean the screen resistors... indeed, screens are grids, but they're always called screens (or screen-grids). [/veer]

The resistors did not fry open, they were closed/no continuity. 

I don't get it. No-continuity means open. Closed would mean continuity for most of EE's.

I replaced them and all is well but I'd like to know what might have caused this to happen. 

First thing that comes to mind is excessive dissipation, but, even if the tube was shorted, dissipation in 1 Megohm is less than 0.1W (assuming 300V B+). I would think overvoltage would have produced much more damage and left traces, so I'm conjecturing very poor quality resistors...

 

[Bowie]

The resistors did not fry open, they were closed/no continuity.  I don't get it. No-continuity means open. Closed would mean continuity for most of EE's.

Ah, I see.  For whatever reason, I always thought no resistance = "open".

  First thing that comes to mind is excessive dissipation, but, even if the tube was shorted, dissipation in 1 Megohm is less than 0.1W (assuming 300V B+). I would think overvoltage would have produced much more damage and left traces, so I'm conjecturing very poor quality resistors...

They were stock Siemens resistors, similar in appearance to the pic below.  The trace between these two bad resistors has some light striping down it's length that looks like soot (though everything is running ok and with low noise now).  Do screen resistors tend to fail with age?  Or, is the fact that 2 went out at once imply something else?

  B+ is 250v.  When the failure happened, the two tubes related to these resistors were in good shape.

Thanks!