On Fri, 19 Mar 1999 17:02:03 -0700 "Richard W. Ehrhorn" <w4eto@rmii.com>
writes:
>
>Hi Ian...
>
>I have no doubt that we saw many "Rocky Point effect" flashovers in
>the
>3CV1500A7 (vapor cooled 3CX1000A7) tubes we used in our first amps -
>the
>ALPHA SEVENTY c. 1970. The BANG occurred much more often in standby
>(tube
>biased beyond cutoff) than when drawing plate current, and it was
>LOUD.
>Reason? The arc was in a vacuum.
Are you saying that an arc in a vacuum will be loud? If so then I
certainly have to revise my thinking !
BTW, I need a SK-870 socket for a 3CV1500/3CX1000A7.
73 Carl KM1H
But until we got smart and installed
>
>surge-current suppression resistors ("glitch resistors") in the HV
>leads,
>the 25 uf filter cap charged to ~4 kV discharging through the tube arc
>
>often blew the plate choke into 200+ neat half-turns of magnet wire!
>
>Eimac engineers referred to the cause of the flashovers as
>"barnacles," but
>your description is much more descriptive. These flashovers usually
>occurred very early in tube life - more often than not during factory
>
>checkout of the amplifiers - and were self-clearing after two or three
>
>flashes. We also experienced frequent new-tubes-in standby flashovers
>in
>early 8877s and 8874s through the late 70's, but in the last 15 years
>or so
>they seem much less common.
>
>But the BANG from those RF chokes disintegrating during a Rocky Point
>arc
>during checkout of early '70V made several of us pretty jittery about
>
>pushing the power-on button the first time! Proper fault current
>limiting
>resistors tame the phenomenon to the point that many times the plate
>overcurrent relay or breaker doesn't even trip.
>
>73, Dick W0ID
>
>-----Original Message-----
>From: Ian White, G3SEK [SMTP:G3SEK@ifwtech.demon.co.uk]
>Sent: Thursday, March 18, 1999 5:36 PM
>To: amps@contesting.com
>Subject: Re: [AMPS] More on the SB220
>
>
>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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>
>
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>
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