As a 'daily' user of VHF tetrodes we have witnessed damage from plate
to screen arcs in tubes which are being processed to higher average
power, as the elements outgas, and the tube cleans up. Those who
commented on (kudos! Ian) the various techniques to prevent damage
are right on - we use the same circuitry here. When the plate power
supply (17 kV) output protection fails (series impedance plus a fast
ignitron crowbar system), the inevitable result is failed screen
bypass caps. These capacitors are INTERNAL to the RCA/Burle 4616V4
tetrode. Because of this, the mica is punched through, and thats the
end of the tube, until it is rebuilt. We have a spark gap (Seimens or
Claire) rated for about 2200 volts mounted right at the screen
connection of the tube, to cathode. And a small series R which blows
open should the gap fire, sort of a fusable R to protect the power
supply from follow on current into the shorted gap. If it opens, of
course the screen floats, and the tube will runaway (high plate I)
until the gap clamps. But we assume that the gap will clamp first,
when an arc over occurs, then the R blows open. The screen power
supply is a small switcher made by Glassman, which charges a big cap,
around 100 uF. This has series R between it and the screen modulator.
The modulator is a string of IR HEXFETs rated for about a KV each,
operated as a floating deck series switch. They have optically
coupled logic drive to the gates. We pulse the screen of the tetrode
on and off 120 times per second, about a millisecond long pulse. The
peak screen voltage during on time is 1500 V, and the current runs a
few amperes plus or minus, depending on the tube. A Micro-Semi
PIP-440 transorb is across the drain to source of each FET, to
prevent them from being damaged in a plate to screen arc. This works,
have never lost any since we built this circuit in 1995. The original
pulser was a 3-400Z driving a 4CX1500B, and it was a real pain.
Once we got a workable protection circuit, plate to screen arcs are a
no brainer. But when the protection breaks, look out tube. Without
the mica capacitor inside, the tube would easily have a hole in the
screen grid mesh after one of these arcs.
For superpower triodes (RCA/Burle 7835), the arcs are usually plate
to the grid. The grid then has holes blown through. I have photos of
0.25 inch holes in one of our tubes with 33,000 hours of life on it,
and it was still working when we finally took it down for rebuilding.
Perhaps these tubes are more atypical than the 'smaller' tubes with
handles, but I firmly believe that the principles of arcing inside
would apply to either. Protect your socket bypass, your tube, and
your screen power supply. The control grid supply might stand to have
a few protectors also for the breakdown that gets to and thru the
screen grid.
Spark gaps, series R, MOV/Transorb, clamp circuits, crowbars, etc are
the rule with deposited energies over a few Joules.
John
K5PRO
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