[AMPS] tube arcs - internal and external

[John Lyles]

Thanks for your information, Dick, on the ETO experience with tube arcs.

To shed a little more experience with internal/external tube arcs, I will
take you back to 1992. I became involved with a problem that was the blight
of the RF plant here. The super power triodes in three or our finals were
popping. By popping, I mean exploding open. They cost about $130,000 each,
so we were seriously concerned. Our tax dollars at work!

--------------

Once I was measuring the plate dissipation with thermometers in the water
lines, and i heard the infamous tink and saw the tell tale symptoms of
broken ceramic. The plate tripped off, and I watched the filament voltage
change as the tube went up to air. Nothing we could do in time to save it.
The dissipation was 270 KW. The tube was rated for 300KW on paper. It is
running at 200 MHz, about 50% efficiency, grounded grid, cathode driven
cicuit. Self bias was provided with a series R in the cathode to ground.
Cathode current was 400 Amperes, plate voltage at 21,000V DC.

I remember witnessing arcs (hearing them). It was merely a tink in the tube
envelopes. Our crowbar was functional, to limit the deposited energy to a
pop, should you short out the B+ from the plate modulators. Using series
tubes as a plate modulator, we have the luxary of being able to quench the
B+ within a pulse, usually in a few microseconds. We have a pulsed RF
system, so there is a lot of off time between HV pulses too.  After a
lengthy investigation (months), with more broken tubes and down time, we
believe we solved the problem -- well we did solve it, as we haven't broken
a triode in operation since, although I knock on ceramic when I say it. By
the way, the arcs went from plate to grid for the most part. Holes in the
grid wires were obvious when we autopsied the tubes. I don't recall cathode
arc marks, but the cathode was usually totally oxidized, having run
filaments while the tube was going up to air.

What was 'fixed' was:

1) RF triode was produced by company run with new management -  RCA had
sold to Burle Industries in Lancaster, PA. There were changes in the
production, although the factory and many people were the same. Some had
left or retired. There were changes to sources of material, QA steps,
processes were modernized. During this time, all tubes were questionable as
to their absolute maximum ratings.
Similarly, we don't know how clean the ceramic was, and how good the seals
were. Internal gas could have been a problem, as we did not have ion pumps
connected on the tubes at that time. This was a later modification that we
made, which is now standard in all of these tubes.

2) Modulator tubes (the series switch in the B+ line) were 4CW250,000B from
Eimac. All tubes in operation had around 20,000-30,000 hours on them. No
buys had been made in 4 years. Someone had forgot that they will wear out,
these tube things. As the tubes got older, they tended to have more
internal faults, due to gas, warped grids, electron cutting on the anode.
The cutoff bias supplies were inadequate, not insuring full cutoff between
pulses. As the beam cut the anode, a lot of copper ions were liberated
inside the tube. Xrays could be measured. When they faulted, usually plate
to screen arced in the tube, and the noise we heard was the external spark
gap (several Champion spark plugs with the electrodes modified, connected
from screen to deck). It sounded like a shotgun, especially if the crowbar
was marginal. Another problem, ignitron crowbar needed maintenance. When
the pass tubes arced, a lot of safety margin was lost, putting the B+ from
capacitor banks directly on the final plate connection, at about 10KV
higher than normal. All of this was basically DC breakdown problems, no
parasitics to blame.

3) We put hardware protection in place, where the dissipation is monitored
in real time, and the HV shuts off if it exceeds 260 KW average plate
dissipation (as seen by the cooling water).

Other minor fixes were incorporated, such as new processed (debarnicled)
4CW250,000B tetrodes,  higher cutoff bias on their control grids, and a
lengthy test 'burn-in' for the final PA tubes, where we ran them at full
power and duty factor into 50 Ohms, for hundreds of hours before ever
subjecting them to VSWR of the accelerator tanks. Plus the interlocked
Perkin Elmer ion pump appendages on the final tubes, that I mentioned last
week here. When gas exceeded 100 uA of ion pump current, we tripped the HV
off.

---------

What has this got to do with the discussion at hand, about the internal and
external arcs of amplifier tubes, such as 8877? I reinforce the idea that
internal tube arcs are quiet, and that circuit components go with a bang.
And that the series R of the anode line (or a crowbar AND series pass tube
in our example) can minimize the damage. And that these arcs and bangs are
related to tube condition, internally: increasing gas, warped grids, broken
filament wires, etc. Whether these experiences are applicable or related to
the smaller tubes, I cannot be absolutely sure. But one would believe that
the laws of scaling apply, and that it is trouble with smaller geometries
and element spacings. Whether gas, or rocky point, or some kind of VHF
parasitic oscillation is the cause, I cannot venture a resonable guess. But
it sure would be advisable to put the protection R in the plate B+, and
maybe a fuse or overcurrent relay to kill the main HV supply. Or live with
the occasional consequences of a broken tube, with spattered gold, broken
wires, blown grid connection to ground, destroyed meters, etc.

By the way, my SB220 was up on 20 Meters Saturday cutting through the fog,
and I decided to run the plate tune and load all over 180 degrees of the
capacitors, while driving high speed dits into a dummy load at about 1200
watts output. No problem, no arcing or funny stuff. I should go up to 10
Meters and try it I suppose. I did put Rich's nichrome gadgets in it (the
hairipin plate feed parasitic suppressor and the grid resistors in place of
the chokes to ground) 4 years ago, as I had to tear it open to fix the
dreaded wafer switch. Mine had mechanically become misaligned - got the amp
for about $150 at a hamfest cause it wouldn't play right. Those output
wafers don't allow for  much misalignment, should the screws come loose on
the long rods that hold the switch together. Could this be the source from
SOME of the failed wafers in SB220s, those cheap switches Heath used?

John
K5PRO









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