Bill, W6WRT wrote:
>If not overtemp, it's almost certainly arcing inside the tube. Even
>good tubes arc occasionally with no harm to the tube, but there can be
>harm to components in the HV- to HV+ path and that includes the bias
>zener(s).
>
>IMO, for a legal-limit amp a ten watt zener is marginally adequate.
>Even if it can dissipate the cathode current x zener voltage, there
>isn't much left to absorb arcs. I would recommend a 50 watt stud mount
>type.
>
>There is a certain amount of guessing in the rating because you never
>know how massive the arc is going to be.
Actually you *can* know something about the arc.
1. The peak current. The main function of a surge limiting resistor in
the B+ line is to limit the peak current to a known value, so that Ipk =
Vb/R. The actual arc current will be time-varying but it can never
exceed that value of Ipk, so you *do* have a known value to design
against.
2. The time it will take to shut the HV supply down (by means of a fuse,
breaker, relay, crowbar or whatever means you have chosen). It isn't
easy to get beyond an initial guesstimate of "a few milliseconds", but
further detail *is* knowable if you're prepared to drill deep enough.
The big unknown (to me, at least) is what fraction of the total arc
current will flow through the cathode zener or other bias circuit. To
trace the current flows, see Figure 2 of:
http://www.ifwtech.co.uk/g3sek/boards/triode/triode-manual.pdf
If the grid collects 100% of the arc current, then none of it will flow
through to the cathode. The current will be steered around that part of
the circuit, flowing through the B-minus clamping diodes instead, and
the cathode zener (or other bias circuit) will be unaffected.
At the other extreme, if the arc punches clear through the grid
structure then a large fraction of the current will flow through the
cathode, causing damage to the cathode and possibly/probably the bias
circuit. For examples of cathode damage, see:
http://www.ifwtech.co.uk/g3sek/misc/gs35-arcs.gif
The GS35b is a planar triode with a disk-shaped oxide cathode and a mesh
("tea strainer") grid. Comparing the left-hand and right-hand images,
you can match up the marks on the anode with the corresponding marks on
the grid. If you then zoom into the left-hand image of the grid, you can
see where the largest arc has punched through to leave a burn mark on
the cathode underneath.
These kinds of damage would be much harder to see in other types of tube
construction, but I'm fairly sure that similar effects will exist.
The key to damage limitation is ALWAYS to install that surge limiting
resistor in the B+ line! Eimac Bulletin 17 explains exactly why:
http://www.ifwtech.co.uk/g3sek/misc/bull17.pdf
--
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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