[Drake] Drake L7 Amp problem

[HENRY PFIZENMAYER]

L7/L4 gurus ... I zigged when I shoulda zagged and blew something in my L7
last Sat. I was checking antenna tuning for SSB portion of 75 meters , and
moved far enough in freq that SWR was something like 4 or 5 to 1 to  the L7
and forgot to retune the ant coupler. When I  started to peak the final
output power with low drive something flashed over big time , bright flash
(which may have been the 4 meter bulbs blowing ) opened one of the PS
circuit breakers and destroyed the sacrifical 0.82 ohm resistor in series
with the B+ in the PS.

I opened the L7 and looked as hard as I could to find any place where there
had been a flash over , everything looks fine , bandswitch both loading and
plate side looks clean as a whistle- no signs of arcing. RF chokes look like
new , all blocking caps are ok DC wise , and the extra plate capacitor that
gets switched in for lower bands is not shorted , and looks fine. Thinking
there may have been a flashover in one of the 3-500's , I checked for
grid-cathode shorts , or plate -grid shorts(on dead tubes , havent put
filament volts back on, will disable the HV and check that later). Checked
both the RFC that take the grids to DC ground ,they are OK and show no signs
of stress which I would think would appear if there had been a plate -grid
flashover. B+ circuitry shows up in the 15 - 18 megohm range to ground (with
the HV shorting interlock switch held open).

NOW , before I fire this thing up again (after replacing the 0.82 ohm
resistor and checking the diode stacks (which look ok visually) - the weird
thing is that the surge blew all 4 of the 1850 5 volt meter lamps that are
hard wired across the filament transformer. Has anyone seen this ?? I am
having a hard time figuring how the filament voltage got high enough to blow
these bulbs. There is no sign whatever of any flashover to the meter shield
plate that holds the 1850 sockets , they are pristine.

Any Ideas ?

Thanks ... Hank K7HP