[Roger Jones]

Hi, Vacuumlanders,
A true tale, minimally abridged!
I had my recently restored Eico HF12 integrated amplifier running the
left channel of my "vinyl system" (the other channel is a Leak TL/25
with a Point-One-Plus pre amp) when, BAAAAA.... it emitted a very loud
60 Hz hum! I hit the power bar switch in two seconds flat!
On the bench... check for good OPT... relief, both halves of primary
still good. Pull the EZ81 rectifier (don't like them, but I haven't
gotten around to putting in 1N4007 diodes yet!)... CT HV secondary
good. So, all iron is good, feeling better! With EZ81 still out,
power up slowly on the variac... no smoke, flame or sizzle, heaters
glowing, looking good... wait a minute, one 6BQ5 has no heater glow,
other one is OK... Hmm, Oh, well, need to replace... Grasp the dead
one to pull it out and, OMG, it's sizzling hot! No heater, no plate
power (no B+) but still hotter 'n hell! WTF is going on? Pull it out
with a rag, let it cool, then put it in my Heathkit TC2 tube checker
but on very low heater voltage (1.4 volts) as smelling a rat, then
slowly ramp it up to 6.3 volts. No glow but getting very hot again!
Line balance control is a bit cranked up so heater current must be a
bit high. Pulled it out and tested for HK short (my first guess) -
none, nor to any other electrode. Put it in a tube base and connect
up a heater supply with AC ammeter and AC voltmeter. At 6.3 volts,
current is ~0.7 amps (only ~4.5 watts), but no heater glow. Watch it
for 30 seconds then, poof, heater goes open circuit. Notice for first
time that tube shows a milky film on the glass top... air?
Pretty much end of story... EZ81 back in (for now.) I replaced both
6BQ5's with two from stock, selecting best static current match as
I've not yet put in a cathode current balance pot, even though I have
replaced the original single cathode bias resistor (165R) with two,
bypassed 330R's. All tests were again good. Total cost: one 6BQ5 and
my time.
Zipped up amp. and it's back on the LH channel sounding fine. I still
don't know what caused the burst of 60 Hz hum. Surely the dead heater
must be the culprit? But, by what mechanism I can't fathom.
Cheers,
Roger