[AMPS] More on the SB220

[Ian White, G3SEK]

Carl wrote:
>
>
>
>On Thu, 18 Mar 1999 15:48:44 -0000 Peter Chadwick
><Peter_Chadwick@mitel.com> writes:
>>
>>Could it be that you have a 'flash arc' or 'Rocky Point effect'? 
>>Although
>>the voltage may be considered low for a glass tube, the effects you 
>>describe
>>fit the description.

>An arc in a vacuum would not go BANG.
>Neither would the opening of a 2W resistor.
>
>Sounds like a HV breakdown to me that is external to the tube.

There could have been a silent arc inside the tube, with the bang coming
from the resistor. IMO a 2W resistor standing in the way of a near-short
across the HV supply is very likely to exit with a bang!

Very small "bubbles" of gas that is loosely combined within the
structural metal of the tube can take months or even years to diffuse to
the surface and be released, which would explain why these events happen
at very infrequent intervals. (With good manufacturing processing and a
bit of luck, they may never happen at all within the working life of the
tube.) Heating accelerates diffusion, so the event is more likely to
happen when the tube is working; but it could also happen when the tube
is cut off, because the majority current carriers don't actually come
from the cathode. 

It's quite possible to have a sudden local release of gas that raises
the pressure for the few microseconds needed to trigger an arc, and then
the gas could be re-absorbed over a timescale of seconds to minutes so
that it doesn't show by the time you get a high-pot tester on the tube. 

Bent filament helix? The authors of the paper that Peter cited noted
that arcs to/from the anode of a triode will easily divide themselves
between the grid and filament, and also that the currents in parallel
conductors can be large enough to force the conductors together by
magnetic attraction.

As regards the other damage such as an arced bandswitch, it's possibly
due to shock excitation by the extremely fast falling edge of anode
voltage - I'm guessing that this is probably much faster than the
circuit ever sees in normal operation. It really needs a SPICE
simulation to put some numbers on this effect, to see if we're in the
right ballpark.

A lot of this is speculation, but I'm trying to find an explanation that
fits *all* the reliably observed facts, not just a convenient sub-set.

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.demon.co.uk/g3sek

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