[Drake] L75 power supply

[Rick Stealey]

The advice given by Hank is right on.  You should not be using the amp under your circumstances.  You need to replace that  resistor and install the glitch resistor that he suggested.  Then if your tube arcs again the big R will limit the current.  If the amp sat for a long time the tube probably has a little gas and the arc (hopefully) was just a one time event.  This may explain why the second tube didn't cause the problem. 
I'm not real sure without digging up a schematic but you don't want to take the chance of driving the tube with RF if you have no B+.  Get the power supply fixed, and you might as well plan on replacing the filter caps as it is just a matter of time till they are going to fail.

Now............. when you get the power supply updated as suggested, here's what you do -
Key up the transceiver, but do not talk.  Watch the plate of the tube get slightly red (maybe VERY red).  Let it run like this for awhile till you get too squeamish about keeping it going!  Maybe 5 minutes at a time, maybe 10 or 15.  This will "getter" the tube, cause it to absorb the little bit of gas that's in there and keep it from arcing again like it did.  While you're doing this, there should not be a bunch of blue gas glowing inside the tube.  The total time it takes to getter the tube is something someone else might have a better opinion of than I do, but of the order of a few hours is not unreasonable.  You CANNOT getter a 3-500 by just running the filaments, the plate has to get red hot.  Lots of info on W8JI's web site, w8ji.com.
By the way, I have an L4B, and I've been down this road before.

Rick  K2XT