[Amps] Amp Supply LK500-zc tube asymmetry

[Brian Moran]

I have an Amp Supply LK500zc, which I purchased (used) about 3 years ago, used briefly, then put on the shelf until recently... when I powered it up, there was a nice small snap (not a big one, a small one, like a small bug zapper) on keydown, and I just turned it off and put it aside (this during a RTTY contest). 
   
  This design is similar to many of the dual 3-500z designs out there -- parallel filament,  3000 no load V, etc. 
   
  Digging into it, I find that in addition to the parasitic suppressor on each tube, there's a series R of 1 ohm, 3 watts to the top of the plate choke -- on one side (the rear side), this "R" is open, but arcs nicely when keyed. 
   
  Not sure why the R's are there, so I took them out, they look like they'd get brittle sitting near those nice hot tubes getting alternatively heated and cooled with way more than 3w of heat. Some nice teflon-coated 18g wires the same length as the removed resistors are now installed.
   
  Getting everything back together, I notice that one tube (the 'front' one) glows orange under keydown conditions, while the back one doesn't. Ah! Bad tube! I label them, swap them, and try again. (BTW, get about 900w, 650ma at 2800v, into a dummy load with 90w drive. Seems a bit low to me).
   
  The FRONT one again (remember, back tube in front now) glows more than the rear one. AhA! I'm getting old(er), I must have mis-remembered. Swap the tubes again (for sure), try again, and the FRONT one again glows more orange under keydown...
   
  So, the FRONT tube always glows 'hotter' than the rear one.  Anyone know WHY this might be? The RF compartment is not different from the front to the rear -- no extra metal near the envelope of the tube...
   
  One thing I haven't checked is the cathode-side of the circuit, but according to the schematic, the filaments are directly tied together. 
   
  Anyone have any experience or ideas, or have seen this before?
   
  I did a search on the Amp-Archives for 'unbalanced' or 'uneven' glow, and did come across a spirited exchange about the series-R's in an SB-220... but not really something like the situation is now...
   
  Thanks,
  Brian N9ADG