Here's a topic, based on the melt-thru discussion (nothing about parasites
here). Maybe some of you have witnessed similar "oops!".... I've been
through some interesting tube failures in my life worth mentioning, both in
glass and ceramic-sealed tubes. We've all been there i bet, but wanted to
share these gems:
#1
The setting:
A Gates AM radio transmitter with 833's in it, those lovable big globes
that I saw ya'll verbally abusing the other day, something about being the
worst tubes to stabilize in a PA. Year - 1977.
Location:
Pulsaki, VA daytime radio station, my buddy is the engineer.
The failure:
One tube turns really white in the center of the anode, and then there is
this sucking in noise, pop, and then light went away, along with the
filament.
The cause:
The DC bias goes away in the modulator, that sets cutoff for class B.
Probably a failure of the DC coupled driver tube (was it an 807?).
Malfunction of circuit without good interlock or protection.
The result:
Very interesting looking tube remained.
#2
The setting:
A rebuilt Machlett LPT44 glass triode switch tube, used to pulse the mod
anode on a 1250 KW peak 800 MHz klystron. Tube is BIG, about 4 times the
size of the 833. It has a big warp on the side, where the glass blowers
kind of had trouble spinning it, and making the glass reweld together after
cutting it open.
1995
Location:
Particle accelerator RF plant:
Tube mounted under oil in lead-lined modulator tank.
The failure:
Tube runs good for a few months, then shut it off to do other stuff. Later,
when switched it on, noted that it wouldn't hold off HV. Opened tank. Tube
looked strange. Like a BIG magnefying lens. Suddenly very heavy when trying
to remove from the socket. Upon closer inspection found the thing totally
full of oil, an oil-filled vaccum tube. Must have punched through a seal,
or a slow leaker.
The cause:
???
The result:
Very interesting while the oil was inside, but it poured out.
#3
The setting:
4CX3000A modulator driver tube. OK I lied, it's not all about glass tubes,
I slipped in one ya'll are all familiar with.
1992
Location:
same as #2, but different system
The failure:
Someone testing the unit, leaves bottom plate off chassis, the blower is
supposed to force air through the tube, which is now leaking out the bottom
on the test bench. Shortly, the fiberglass chimney begins to smoke, and the
tube gain suddenly shifts wayyyy over.
The cause:
Like I said, lack of adequate cooling on the anode while under high
dissipation. Tube still worked but was a bit weakened. Cockpit error.
The result:
Slightly crusty tube
#4
The setting:
4CX1500A tetrode in socket. Transmitter development lab
1982
Location:
Major broadcast TX manufacturer, small midwest town. Prototype amplifier
for 1500 Watts, before switching to 8877.
The failure:
Left a rag in the air outlet or inlet on the cavity, after cleaning up some
metal chips. Amplifier was turned on full power, CW, at 100 MHz. Started
noting that the tuning was changing as I watched. Grabbed the knob and
tried to find resonance, but it was moving quite quickly. Then noted the
ugly smoke above. Hit the off button, and found the little tube quite
tarnished, but still working. It was weak, and the capacitances were
abnormal, became another trophy from working in RF amps.
The cause:
Cockpit error.
The result:
Scolding from the boss, very tarnished tube.
#5
The setting:
7835 superpower triode. Operating at 3 MW peak, 200 MHz, with about 270 KW
plate dissipation measured.
Location:
same as #2, different system
The failure:
Watching the filament voltage and current (5 VDC at 7000 Amperes), suddenly
heard slight tink from cavity, and the regulated filament current jumped as
the voltage dropped way down. Lost output. Tube went up to air
catastropically (gads my spelling here).
The cause:
One of several failures over same, that plagued a particular batch of tubes
and this particular amplifier.
This one had a big hole in the ceramic seal between plate and grid
connections. One of the most demoralizing experiences is to break these
tubes, as they cost US $135,000 new. Later put dissipation limits on the
amplifier, to switch HT off if gets >255 KW. No more trouble. "Back off,
dude", I told the operator. Actually the solution is a bit more complex,
won't go into it here.
The result:
Cry in your beer, and call the big boss with the deep purse....
I've not wasted any ham tubes (knock on wood); hence the commercial
tribulations here. Anyone got any cool (or hot) stories of tube
catastrophies to share here? I've seen some wonderful photos in the free
brochure that Econco (the rebuilder) provides. Some very ugly tubes with
black burnt spots and nasty melted contacts.
John
K5PRO
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